AASHTO LRFD GuideSpecForDesignOf Pedestrian Bridges December2009

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PEDE,8TRIAN BRIDGES

© 2009 by the American Association of State Highway and Transportation Officials. All rights reserved. Duplication is a violation of applicable law.

11

Officers:

EXECUTIVE COMMITTEE

2008-2009

Voting Members

President: Larry L. "Butch" Brown, Sr., Mississippi

Vice President: Susan Martinovich, Nevada

Secretary-Treasurer: Carlos Braceras, Utah

Regional Representatives:

REGION I:

REGION II:

Joseph Marie, Connecticut, One-Year Term

Gabe Klein, District of Columbia, Two-Year Term

Dan Flowers, Arkansas, One-Year Term

Mike Hancock, Kentucky, Two-Year Tenn

REGION III: Nancy J. Richardson, Iowa, One-Year Term

Thomas K. Sorel, Minnesota, Two-Year Term

REGION IV: Paula Hammond, Washington, One-Year Term

Amadeo Saenz, Jr., Texas, Two-Year Term

Nonvoting Members

Immediate Past President: Allen Biehler, Pennsylvania

AASHTO Executive Director: John Horsley, Washington, DC

111

HIGHWAYS SUBCOMMITTEE ON BRIDGES AND STRUCTURES, 2009

MALCOLM T. KERLEY, Chair KEVIN THOMPSON, Vice Chair

M. MYINT LWIN, Federal Highway Administration, Secretary RAJ AILANEY, Federal Highway Administration, Assistant Secretary

KEN KOBETSKY, AASHTO Liaison KELLEY REHM, AASHTO Liaison

ALABAMA, John F. "Buddy" Black, William "Tim" Colquett, George H. Conner

ALASKA, Richard A. Pratt

ARIZONA, Jean A. Nehme

ARKANSAS, Phil Brand

CALIFORNIA, Kevin Thompson, Susan Hida, Barton 1. Newton

COLORADO, Mark A. Leonard, Michael G. Salamon

CONNECTICUT, Julie F. Georges

DELAWARE, Jiten K. Soneji, Barry A. Benton

DISTRICT OF COLUMBIA, Nicolas Galdos, L. Donald Cooney, Konjit "Connie" Eskender

FLORIDA, Marcus Ansley, Sam Fallaha, Jeff Pouliotte

GEORGIA, Paul V. Liles, Jr.

HAWAII, Paul T. Santo

IDAHO, Matthew M. Farrar

ILLINOIS, Ralph E. Anderson, Thomas J. Domagalski

INDIANA, Anne M. Rearick

IOWA, Norman L. McDonald

KANSAS, Kenneth F. Hurst, James 1. Brennan, Loren R. Risch

KENTUCKY, Mark Hite

LOUISIANA, Hossein Ghara, Arthur D'Andrea, Paul Fossier

MAINE, David B. Sherlock, Jeffrey S. Folsom

MARYLAND, Earle S. Freedman, Robert J. Healy

MASSACHUSETTS, Alexander K. Bardow, Shirley Eslinger

MICHIGAN, Steven P. Beck, David Juntunen

MINNESOTA, Daniel L. Dorgan, Kevin Western

MISSISSIPPI, Mitchell K. Carr, B. Keith Carr

MISSOURI, Dennis Heckman, Michael Harms

MONT ANA, Kent M. Barnes

NEBRASKA, Mark 1. Traynowicz, Mark Ahlman, Fouad Jaber

NEVADA, Mark P. Elicegui, Todd Stefonowicz

NEW HAMPSHIRE, Mark W. Richardson, David L. Scott

NEW JERSEY, Richard W. Dunne

NEW MEXICO, Raymond M. Trujillo, Jimmy D. Camp

NEW YORK, George A. Christian, Donald F. Dwyer, Arthur P. Yannotti

NORTH CAROLINA, Greg R. Perfetti

NORTH DAKOTA, Terrence R. Udland

iv

OHIO, Timothy 1. Keller, Jawdat Siddiqi

OKLAHOMA, Robert 1. Rusch, Gregory D. Allen, John A. Schmiedel

OREGON, Bruce V. Johnson, Hormoz Seradj

PENNSYLVANIA, Thomas P. Macioce, Harold C. "Hal" Rogers, Jr., Lou Ruzzi

PUERTO RICO, (Vacant)

RHODE ISLAND, David Fish

SOUTH CAROLINA, Barry W. Bowers, Jeff Sizemore

SOUTH DAKOTA, Kevin Goeden

TENNESSEE, Edward P. Wasserman

TEXAS, David P. Hohmann, Keith L. Ramsey

U.S. DOT, M. Myint Lwin, Firas 1. Sheikh Ibrahim

UTAH, (Vacant)

VERMONT, Wayne B. Symonds

VIRGINIA, Malcolm T. Kerley, Kendal Walus, Prasad L. Nallapaneni, Julius F. J. Volgyi, Jr.

WASHINGTON, Jugesh Kapur, Tony M. Allen, Bijan Khaleghi

WEST VIRGINIA, Gregory Bailey, James D. Shook

WISCONSIN, Scot Becker, Beth A. Cannestra, William Dreher

WYOMING, Gregg C. Fredrick, Keith R. Fulton

GOLDEN GATE BRIDGE, Kary H. Witt

N.J. TURNPIKE AUTHORITY, Richard J. Raczynski

N.Y. STATE BRIDGE AUTHORITY, William J. Moreau

PENN. TURNPIKE COMMISSION, James L. Stump

U.S. ARMY CORPS OF ENGINEERSDEPARTMENT OF THE ARMY, Christopher H. Westbrook

U.S. COAST GUARD, Hala Elgaaly

U.S. DEPARTMENT OF AGRICULTURE-FOREST SERVICE, John R. Kattell, Scott F. Mitchell

ALBERTA, Tom Loo

NEW BRUNSWICK, Doug Noble

NOV A SCOTIA, Mark Pertus

ONT ARlO, Bala Tharmabala

SASKA TCHEWAN, Howard Yea

TRANSPORTATION RESEARCH BOARD-Waseem Dekelbab

LRFD GUIDE SPECIFICATIONS FOR THE DESIGN OF PEDESTRIAN BRIDGES

Table of Contents

I--GENERAL ................................................................................................................................................................. 1

1.1-Scope ................................................................................................................................................................ 1

1.2-Manufacturer-Designed Systems ...................................................................................................................... 1

1.3-Collision Mitigation ......................................................................................................................................... 1

2-PHILOSOPHY ........................................................................................................................................................... 2

3-LOADS ....................................................................................................................................................................... 2

3. I-Pedestrian Loading (PL) ................................................................................................................................... 2

3.2-Vehicle Load (LL) ............................................................................................................................................ 4

3.3-Equestrian Load (LL) ....................................................................................................................................... 5

3.4-Wind Load (WS) .............................................................................................................................................. 5

3.5-Fatigue Load (LL) ............................................................................................................................................ 6

3.6-Application of Loads ........................................................................................................................................ 6

3.7 -Combination of Loads ...................................................................................................................................... 6

4--FATIGUE ................................................................................................................................................................... 7

4.1-Resistance ......................................................................................................................................................... 7

4.2-Fracture ............................................................................................................................................................. 7

5-DEFLECTIONS ......................................................................................................................................................... 7

6-VIBRATIONS ............................................................................................................................................................ 7

7---STABILITY ............................................................................................................................................................... 8

7 . I-Half-Through Trusses ....................................................................................................................................... 8

7.1. I-Lateral Frame Design Force ................................................................................................................... 8

7.1.2-Top Chord Stability ................................................................................................................................ 9

7.1.3-Altemative Analysis Procedures .......................................................................................................... 11

7.2-Steel Twin I-Girder and Single Tub Girder Systems ...................................................................................... 12

7.2.I-General ................................................................................................................................................. 12

7 .2.2-Lateral-Torsional Buckling Resistance-Twin I-Girder ...................................................................... 12

7 .2.3-Lateral-Torsional Buckling Resistance-Singly Symmetric Sections ................................................. 13

8-TYPE SPECIFIC PROVISIONS .............................................................................................................................. 13

8. I-Arches .. , ............................................................................................................... , ......................................... 13

8.2-Steel HSS Members ........................................................................................................................................ 13

8.2.I-General ................................................................................................................................................. 13

8.2.2-Detailing ..................................... , ............................... , ......................................................................... 14

8.2.3-Tubular Fracture Critical Members ...................................................................................................... 14

8.3-Fiber Reinforced Polymer (FRP) Members .................................................................................................... 15

9-DESIGN EXAMPLE ................................................................................................................................................ 16

Illustrative Example of Key Provisions of Guide Specifications ............................................................................ 16

General Information ................................................................................................................................................ 16

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Truss Members: All Rectangular HSS .................................................................................................................... 17

Floorbeams .............................................................................................................................................................. 17

Dead Load ............................................................................................................................................................... 18

Pedestrian Live Load .............................................................................................................................................. 18

Vehicle Load ........................................................................................................................................................... 18

Wind Load .............................................................................................................................................................. 19

Total Vertical Loads per Truss ................................................................................................................................ 21

Truss Member Design Loads .................................................................................................................................. 21

Truss Top Chord Lateral Support ........................................................................................................................... 21

Top Chord Compressive Resistance ....................................................................................................................... 23

Lateral Force to Be Resisted by Verticals ............................................................................................................... 24

End Posts ................................................................................................................................................................. 25

Deflection ................................................................................................................................................................ 25

Vibrations ................................................................................................................................................................ 25

Vertical Direction ............................................................................................................................................ 25

Lateral Direction .............................................................................................................................................. 26

Fatigue .................................................................................................................................................................... 27

REFERENCES ............................................................................................................................................................... 29

VI


1-GENERAL

l.l-SCOPE

These Guide Specifications address the design and construction of typical pedestrian bridges which are designed for and intended to carry primarily pedestrians, bicyclists, equestrian riders, and light maintenance vehicles, but not designed for and intended to carry typical highway traffic. Pedestrian bridges with cable supports or atypical structural systems are not specifically addressed.

These Guide Specifications provide additional guidance on the design and construction of pedestrian bridges in supplement to that available in the AASHTO LRFD Bridge Design Specifications (AASHTO LRFD). Only those issues requiring additional or different treatment due to the nature of pedestrian bridges and their loadings are addressed. In Article 3 of this document, the load definitions and abbreviations are taken from AASHTO LRFD. Aluminum and wood structures are adequately covered in AASHTO LRFD, and as such are not specifically addressed herein.

Implementation of the wind loading and fatigue loading provisions require reference to the AASHTO Standard Specifications for Structural Supports for Highway Signs, Luminaires, and Traffic Signals (AASHTO Signs).

1.2-MANUFACTURER-DESIGNED SYSTEMS

Where manufacturer-designed systems are used for a pedestrian bridge crossing, the engineer responsible for the design of the system shall submit sealed calculations prepared by a licensed Professional Engineer for that system.

1.3-COLLISION MITIGATION

AASHTO LRFD Article 2.3.3.2 specifies an increased vertical clearance for pedestrian bridges 1.0 ft higher than for highway bridges, in order to

C1.l

This edition of the Guide Specifications was developed from the previous Allowable Stress Design (ASD)- and Load Factor Design (LFD)-based first edition (AASHTO 1997). An evaluation of available foreign specifications covering pedestrian bridges, and failure investigation reports, as well as research results related to the behavior and performance of pedestrian bridges was performed during the development of the LRFD Guide Specifications.

Cl.2

It is important to clearly delineate the responsibilities of each party when proprietary bridge systems are used. All portions of the design must be supported by sealed calculations, whether from the bridge manufacturer, or the specifying engineer. The interface between the manufacturer-designed system and the project-specific substructures and foundations needs careful attention.

Cl.3

In most cases, increasing vertical clearance IS the most cost-effective method of risk mitigation.

2 LRFD GUIDE SPECIFICATIONS FOR THE DESIGl'I OF PEDESTRIAN BRIDGES

mitigate the risk from vehicle collisions with the superstructure. Should the owner desire additional mitigation, the following steps may be taken:

• Increasing vertical clearance in addition to that contained in AASHTO LRFD

• Providing structural continuity of the superstructure, either between spans or with the substructure

• Increasing the mass of the superstructure

• Increasing the lateral resistance of the superstructure

2-PHILOSOPHY

Pedestrian bridges shall be designed for specified limit states to achieve the objectives of safety; serviceability, including comfort of the pedestrian user (vibration); and constructability with due regard to issues of inspectability, economy, and aesthetics, as specified in AASHTO LRFD. These Guide Specifications are based on the LRFD philosophy. Mixing provisions from specifications other than those referenced herein, even if LRFD based, should be avoided.

3-LoADS

3.l-PEDESTRIAN LOADING (PL)

Pedestrian bridges shall be designed for a uniform pedestrian loading of 90 psf. This loading shall be patterned to produce the maximum load effects. Consideration of dynamic load allowance is not required with this loading.

C3.l

This article modifies the pedestrian loading provisions of the Fourth Edition of AASHTO LRFD, through the 2009 Interim. The previous edition of these Guide Specifications used a base nominal loading of 85 psf, reducible to 65 psf based on influence area for the pedestrian load. With the LFD load factors, this results in factored loads of 2.17(85) = 184 psf and 2.17(65) = 141 psf. The Fourth Edition of AASHTO LRFD specified a constant 85 psf regardless of influence area. Multiplying by the load factor, this results in 1.75(85) = 149 psf. This falls within the range of the previous factored loading, albeit toward the lower end.

European codes appear to start with a higher nominal load (approx 105 psf), but then allow reductions based on loaded length. Additionally, the load factor applied is 1.5, resulting in a maximum factored load of ( 1. 5) 1 05 = 158 psf. For a long loaded length, this load can be reduced to as low as 50 psf, resulting in a factored load of (1.5)50 = 75 psf. The effect of resistance factors has not been accounted for in the above discussion of the European codes. There are,


however, warnings to the designer that a reduction in the load based on loaded length may not be appropriate for structures likely to see significant crowd loadings, such as bridges near stadiums.

Consideration might be given to the maximum credible pedestrian loading. There is a physical limit on how much load can be applied to a bridge from the static weight of pedestrians. It appears that this load is around 150 psf, based on work done by Nowak (2000) from where Figures C 1 through C3 were taken. Although there does not appear to be any available information relating to the probabilistic distribution of pedestrian live loading, knowing the maximum credible load helps to define the limits of the upper tail of the distribution of load. The use of a 90 psf nominal live load in combination with a load factor of 1.75 results in a loading of 158 psf, which provides a marginal, but sufficient, reserve compared with the maximum credible load of 150 psf.

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4 LRFD GUIDE SPECIFICATIONS FOR THE DESIGN OF PEDESTRIAN BRIDGES

3.2-VEHICLE LOAD (LL)

Where vehicular access is not prevented by pennanent physical methods, pedestrian bridges shall be designed for a maintenance vehicle load specified in Figure I and Table I for the Strength I Load Combination unless otherwise specified by the Owner.

Figure C3.1-1-Live Load of 50 psf



C3.2

The vehicle loading specified is equivalent to the Htrucks shown in Article 3.6.1.6 of AASHTO LRFD 2009 Interim and contained in previous versions of the AASHTO Standard Specifications jor Highway Bridges.

LRfD GUIDE SPECIFICATIONS FOR THE DESIGN OF PEDESTRIAN BRIDGES

truck shall be placed to produce the maximum effects and shall not be placed in combinations

with the pedestrian load. The dynamic load allowance need not be considered for this loading.

Table 3.2-1-Design Vehicle

Clear Deck Width 7 to 10 ft

Over 10 ft

4.0 kips 2.0 kips

14 ft 0 in.

Design Vehicle H5 HlO

Figure 3.2-1-Maintenance Vehicle Configurations

3.3-EQUESTRIAN LOAD (LL)

Decks intended to carry equestrian loading shall be designed for a patch load of 1.00 kip over a square area measuring 4.0 in. on a side.

3.4-WIND LOAD (WS)

Pedestrian bridges shall be designed for wind loads as specified in AASHTO Signs, Articles 3.8 and 3.9. Unless otherwise directed by the Owner, the Wind Importance Factor, Jr, shall be taken as l.l5. The loading shall be applied over the exposed area in front

C3.3

The equestrian load is a live load and intended to ensure adequate punching shear capacity of pedestrian bridge decks where horses are expected. The loading was derived from hoof pressure measurements reported in Roland et. al. (2005). The worst loading occurs during a canter where the loading on one hoof approaches 100 percent of the total weight of the horse. The total factored load of 1.75 kips is approximately the maximum credible weight of a draft horse. This loading is expected to control only deck design.

C3.4

The wind loading is taken from AASHTO Signs specification rather than from AASHTO LRFD due to the potentially flexible nature of pedestrian bridges, and also due to the potential for traffic signs to be mounted on them.

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elevation including enclosures. Wind load on signs supported by the pedestrian bridge shall be included.

In addition to the wind load specified above, a vertical uplift line load as specified in AASHTO LRFD Article 3.8.2 and determined as the force caused by a pressure of 0.020 ksf over the full deck width, shall be applied concurrently. This loading shall be applied at the windward quarter point of the deck width.

3.S-FATIGUE LOAD (LL)

The fatigue loading used for the fatigue and fracture limit state (Fatigue I) shall be as specified in Section 11 of AASHTO Signs. The Natural Wind Gust specified in Article 11.7.3 and the Truck-Induced Gust specified in Article 11.7.4 of that specification need only be considered, as appropriate.

3.6-APPLlCATlON OF LOADS

When determining the application of the pedestrian live loading which maximizes or minimizes the load effect on a given member, the least dimension of the loaded area shall be greater than or equal to 2.0 ft.

3.7-COMBINATlON OF LOADS

The types of bridges identified in Article 1.1 shall be designed for the load combinations and load factors specified in AASHTO LRFD Table 3.4.1-1, with the following exceptions:

• Load combinations Strength II, Strength IV, and Strength V need not be considered.

• The load factor for the Fatigue I load combination shall be taken as 1.0, and the Fatigue II load combination need not be considered.

Where main gravity load carrying elements also form part of the railing system, railing loads as specified in Article 13.8.2 of AASHTO LRFD shall be applied concurrently with all other live loads for the Strength Limit States.

C3.S

Wind loads are not part of the Fatigue I load combination for vehicular bridges. This article designates wind as a live load for pedestrian bridges, via the designation LL. Wind should be considered a fatigue live load for pedestrian bridges.

Neither the pedestrian live load nor the maintenance vehicle load used for strength and serviceability is appropriate as a fatigue design loading due to the very infrequent nature of this loading. The fatigue loading specified is consistent with the treatment of sign support structures. For bridges crossing roadways, the truck-induced gust loading should be considered. The other loadings specified in AASHTO Signs are not applicable to pedestrian bridges due to their decreased susceptibility to galloping or vortex shedding vibrations.

C3.6

The dimension given is meant to represent a single line of pedestrians; any width less than this would not represent a practical loading scenario.

C3.7

Load combination Strength II is meant for special permit trucks, which is not applicable to pedestrian bridges. Strength IV is for dead load dominant structures such as long span trusses, and would not likely apply to pedestrian bridges. Strength V addresses the case of strong wind combined with reduced live loading, which is not likely to occur for pedestrian bridges. For unusual cases where the excluded load combinations have a reasonable chance of occurring, they should be considered in the design. The fatigue loading specified in AASHTO Signs and referenced herein was calibrated for a load factor of 1.0 and the design condition of infinite life.

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4-FATlGUE

4.1-RESIST ANCE

The fatigue resistance for steel components and details shall be as specified in AASHTO LRFD, Article 6.6.1.2.5 for the Fatigue I load combination. For round HSS components and details not covered in AASHTO LRFD, the nominal fatigue resistance may be taken from Table 11.3 of AASHTO Signs or Figure 2.13 of A WS D 1.1 Structural Welding Code-Steel. For square and rectangular HSS components and details, the nominal fatigue resistance may be taken from the provisions of the Design Guide 8 of the International Committee for the Development and Study of Tubular Structures (CIDECT).

The fatigue resistance for steel reinforcement in concrete structures shall be as specified in AASHTO LRFD Article 5.5.3.

4.2-FRACTURE

Except as specified herein, all of the proViSIOns specified in Article 6.6.2 of AASHTO LRFD relating to Charpy V-notch (CVN) fracture toughness requirements, including Fracture Critical Member (FCM) and Main Member designation, shall apply to steel pedestrian bridges. Design of tubular members shall also satisfy the provisions of Article 8.2. If supported by the characteristics of the site and application, the Owner may waive the FCM requirements, including Article 8.2.3 of these specifications.

5-DEFLECTIONS

Deflections should be investigated at the service limit state using load combination Service I in Table 3.4.1-1 of AASHTO LRFD. For spans other than cantilever arms, the deflection of the bridge due to the unfactored pedestrian live loading shall not exceed lI360 of the span length. Deflection in cantilever arms due to the pedestrian live loading shall not exceed 1/220 of the cantilever length. Horizontal deflections tmder unfactored wind loading shall not exceed 11360 of the span length.

6--VIBRATIONS

Unless waived by the Owner, vibrations shall be investigated as a service limit state using load combination Service I in Table 3.4.1-1 of AASHTO LRFD. Vibration of the structure shall not cause

C4.1

CIDECT Design Guides are a foreign specification available through the AISC. See Zhao et aI., 2001.

C4.2

For pedestrian bridges crossing low-volume traffic waterways and roads, or areas not accessible to the general public, FCM treatment may not be appropriate.

C5

Table 2.5.2.6.3-1 of AASHTO LRFD contains guidance on span-to-depth ratios that may be invoked by an Owner.

The 11360 criteria IS consistent with the increased pedestrian loading compared to previous versions of AASHTO pedestrian bridge guidance. See also Article 2.5.2.6.2 of AASHTO LRFD for bridges carrying both roadway and pedestrian traffic.

C6

Due to the vibration problems experienced in London on the Millennium bridge, there have been many publications in the technical literature, primarily in Europe, on this topic. Despite this large body of

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discomfort or concern to users of a pedestrian bridge. Except as specified herein, the fundamental frequency in a vertical mode of the pedestrian bridge without live load shall be greater than 3.0 hertz (Hz) to avoid the first harmonic. In the lateral direction, the fundamental frequency of the pedestrian bridge shall be greater than 1.3 Hz. If the fundamental frequency cannot satisfy these limitations or if the second hannonic is a concern, an evaluation of the dynamic performance shall be made. This evaluation shall consider:

• The frequency and magnitude of pedestrian footfall loadings

• The phasing of loading from multiple pedestrians on the bridge at the same time, including the "lock-in" phenomena

• Appropriate estimation of structural damping

• Frequency-dependent limits on acceleration and/or velocity

In lieu of such evaluation in the vertical direction, the bridge may be proportioned such that either of the following criteria are satisfied:

.r? 2.861n C!O) (6-1)

or

W ? 180e(-O.35f) (6-2)

where:

W = the weight of the supported structure, including only dead load (kips)

f the fundamental frequency in the vertical direction (Hz)

7-STABILlTY

7.1-HALF-THROUGH TRUSSES

7.1.1-Lateral Frame Design Force

The vertical truss members, the floorbeams, and their connections shall be proportioned to resist a lateral force applied at the top of the truss verticals. The lateral force shall not be less than O.OI/K times the average factored design compressive force in the two adjacent top chord members, where K is the design effective length factor for the individual top chord members supported between the truss verticals. In no case shall the value for 0.0 11K be less than 0.003 when

knowledge, it does not appear there has been convergence toward one method of evaluation, or development of any specification that adequately covers this issue.

These provisions address the issue of vibration from two directions: maintaining a minimum natural vibration frequency above those induced by pedestrians, and specifying a minimum weight to limit vibration amplitudes if the frequency limits are not met. Although somewhat outdated, both of these approaches are still viable and have the great advantage of simplicity.

The technical guide published by SETRA (Service d'Etudes Techniques des Routes et Autoroutes) (2006) appears to present a relatively straightforward method for addressing vibration issues when the frequencies of the bridge fall within the pacing frequencies of pedestrians.

The "lock-in" phenomenon refers to the tendency of people to synchronize their pacing frequency to the lateral frequency of the bridge when the lateral displacements begin to grow. In other words, instead of random frequencies and phasing among the loading from pedestrians on the bridge, the frequencies and phases becomes fully correlated with the bridge motion.

C7.1.1

This article modifies the proVIsions of AASHTO LRFD by replacing the 300 pounds per linear foot design requirement for truss verticals with provisions based on research reported in Galambos (1998). These provisions establish the minimum lateral strength of the verticals based on the degree of lateral support necessary for the top chord to resist the maximum design compressive force.


determining the minimum lateral force, regardless of the K-value used to determine the compressive capacity of the top chord. The lateral frame design force shall be applied concurrently with the loading used to determine the average compressive force above.

End posts shall be designed as a simple cantilever to carry their applied axial load combined with a lateral load of 1.0 percent of the end post axial load, applied laterally at the upper end.

7.1.2-Top Chord Stability

The top chord shall be considered as a column with elastic lateral supports at the panel points. Except as noted herein, the contribution of the connection stiffness between the floorbeam and the vertical member shall be considered in determining the stiffness of the elastic lateral supports.

When the following criteria are satisfied, the connection may be assumed rigid for the purpose of U-frame stiffness calculation:

Matched member widths in simple unreinforced HSS connections of the floorbeam to vertical as applicable (no deformation allowed due to tube wall plastification of member faces at service loads).

.. The connection of the floorbeam to the vertical shall not include the HSS chord member, i.e., the vertical and floorbeam shall not be connected to different sides of a HSS chord.

III Fixed end moments in the floorbeams and verticals due to floorbeam rotations in addition to the loads derived from a U-frame analysis have been accounted for in strength design of the connections.

The proposed design and details for the connection of the floorbeam demonstrating satisfaction of the criteria specified herein shall be submitted to the Owner for review. The Owner's decision as to acceptability shall

final.

procedure for determining the resistance of a compression member in AASHTO LRFD may be used

determine the resistance of the compression chord a value for the effective length factor, K, based on

a lateral U-frame and obtained from Table 1. In this

C lateral stiffness of the U-frame made of the truss verticals and the floorbeam taken as PIt, (kiplin.)

C7.1.2

The use of the 1.33 factor applied to the factored compression load to determine Pc is in recognition that for uniformly loaded structures there is a higher probability of the maximum compression force occurring simultaneously in adjacent truss panels. For further discussion, refer to Galambos (1998). To utilize the procedures given here for top chord stability design without modification for cross-frame connection stiffuess, proper detailing must be used to ensure that the connection is "fully rigid" at service loads; if not, the stiffness of the connections must be incorporated into the overall stiffness of the U-frames in order to correctly predict top chord buckling.

It has been seen that if a stepped connection is used, the stiffness of U-frames in pony truss bridges are less than that predicted when using member stiffness only. Additionally, welding to two faces of an HSS member that are at 90 degrees to each other causes the chord to go into the shape of a parallelogram or "ovalize" when trying to pass moment between the members. This too affects the cross-frame stiffness.

See AISC guidance for the strength design of HSS connections.

Interpolation of values between those given in the table is acceptable.

9

10

P


arbitrary lateral load as shown schematically in Figure 1 (kips)

lateral deflection resulting from lateral load P and shown schematically in Figure 1 (in.)

L length ofthe chord between panel points (in.)

Pc desired critical buckling load (kips) of the truss chord member, which shall be taken as 1.33 times the factored compressive load

n number of panels in the truss

p p

I I

I \ j \ I r---------------------------~I L ~

'~-- ~--~

Figure 7.1.2-1-Lateral V-Frame

t

LRFD GUIDE SPECIFICATIONS FOR THE DESIGN OF PEDESTRIAN BRIDGES 11

Table 7.1.2-1-Values of 11K for Various Values of CLIPc and n

11K n =4 n = 6 n = 8 n = 10 1.000 3.686 3.616 3.660 3.714 0.980 3.284 2.944 2.806 0.960 3.000 2.665 2.542 0.950 2.595 0.940 2.754 2.303 0.920 2.643 2.146 0.900 3.352 2.593 2.263 2.045 0.850 2.460 2.013 1.794 0.800 2.961 2.313 1.889 1.629 0.750 2.147 1.750 1.501 0.700 2.448 1.955 1.595 1.359 0.650 1.739 1.442 1.236 0.600 2.035 1.639 1.338 1.133 0.550 1.517 1.211 1.007 0.500 1.750 1.362 1.047 0.847 0.450 1.158 0.829 0.714 0.400 1.232 0.886 0.627 0.555 0.350 0.530 0.434 0.352 0.300 0.121 0.187 0.249 0.170 0.293 0 0.259 0 0.250 0.135 0.107 0.200 0.045 0.068 0.180 0 0.150 0.017 0.139 0 0.114 0.100 0.097 0.085

7.1.3-Alternative Analysis Procedures

The use of a second-order numerical analysis procedure to evaluate the stability of the top chord of a half-through truss is acceptable in lieu of the procedure above, provided the following aspects are included in the model:

• Effects of initial out-of-straightness, both between panel points and across the entire length of the compression chord

• Effects of residual stresses in compression members due to fabrication and construction

• Effects of the stiffness of vertical to floorbeam connections

n = 12 n = 14 n = 16 3.754 3.785 3.809 2.787 2.771 2.774 2.456 2.454 2.479

2.252 2.254 2.282 2.094 2.101 2.121 1.951 1.968 1.981 1.709 1.681 1.694 1.480 1.456 1.465 1.344 1.273 1.262 1.200 1.111 1.088 1.087 0.988 0.940 0.985 0.878 0.808 0.860 0.768 0.708 0.750 0.668 0.600 0.624 0.537 0.500 0.454 0.428 0.383 0.323 0.292 0.280 0.203 0.183 0.187

0.103 0.121 0.112 0.055 0.053 0.070

0.031 0.029 0.025

0 0.003 0.010

0 0

C7.1.3

Given the increasing availability of software that is capable of second order analyses, such an analysis is a practical alternative to the method given in Article 7.1.2. However, the design equations in AASHTO LRFD account for the issues identified, and any alternative method should also address these. One method that might be followed would be to use the second order elastic analysis to determine the K factor for a given chord size and panel point frame stiffness, and then the design equations of AASHTO LRFD to determine the corresponding resistance.


7.2-STEEL TWIN I-GIRDER AND SINGLE TUB GIRDER SYSTEMS

7.2.1-General

F or potentially torsionally flexible systems such as twin I-girder and single tub girder structural systems, the designer shall consider:

• The out-of-plane stiffness of twin I-girders, prior to becoming composite with a concrete deck, can be significantly smaller than the in-plane, or vertical, stiffness. This can lead to a lateral-torsional buckling instability during construction.

• Single tub girders, prior to becoming composite with a concrete deck, behave as singly symmetric sections with a shear center below the bottom flange. AASHTO LRFD Article 6.7.5.3 requires top lateral bracing in tub section members to prevent lateral torsional buckling of these sections.

• Prior to becoming composite with a concrete deck, twin I-girders with bottom flange bracing will behave as a tub girder and will exhibit the same tendencies toward lateral-torsional buckling. Top lateral bracing shall be provided as for tub sections, or the stability shall be checked as a singly symmetric member.

7.2.2-Lateral-Torsional Buckling ResistanceTwin I-Girder

For evaluating the stability of twin I-girder systems without a composite deck or lateral bracing, the equation given by Yura and Widianto (2005) may be used:

(7.2.2-1)

where:

Mn = nominal in-plane flexural resistance of one girder (kip-in.)

Mer = critical elastic lateral-torsional buckling moment of one girder (kip-in.)

s spacing between girders (in.)

E modulus of elasticity of steel (ksi)

L effective buckling length for lateral-torsional buckling (ft)

C7.2.1

Several incidents have highlighted the need for a careful evaluation of the stability of pedestrian bridges, especially during the construction stages. Structural systems consisting of two parallel girders can exhibit very different behavior during construction, depending on the bracing systems used. Lateral bracing contributes significantly to the lateral-torsional buckling capacity of the beam. For girders without lateral bracing during construction, lateral-torsional buckling capacity should be carefully evaluated. After the deck is cast, the section is effectively a "C" shape, which is singly symmetrical. Use of the appropriate lateral-torsional buckling equation is critical, and reference should be made to Galambos (1998). Further information is contained in Y ura and Widianto (2005), as well as Kozy and Tunstall (2007).


Iyo =

Ixo =

out-of-plane moment of inertia of one girder (in.4)

in-plane moment of inertia of one girder (in.4)

in-plane plastic moment of one girder (kip-in.)

Where a concrete deck is used, continuous twin I-girder systems shall be made composite with the deck for the entire length of the bridge.

7.2.3-Lateral-Torsional Buckling ResistanceSingly Symmetric Sections

The lateral-torsional stability of singly symmetric sections not covered in Article 7.2.2 shall be investigated usmg information available m the literature.

8-TYPE SPECIFIC PROVISIONS

S.l-ARCHES

Arches shall be designed in accordance with the provisions of AASHTO LRFD with guidance from Nettleton (1977).

S.2-STEEL HSS MEMBERS

S.2.I-General

The capacities or resistances of connections for steel HSS members shall be in accordance with the Chapter K of the specifications and commentary of AISC (2005) or AASHTO Signs. Resistances for fatigue design shall be in accordance with Section 2.20.6 of Structural Welding Code-Steel ANSI! A WS D 1.1 or Section 11 of AASHTO Signs. All loads, load factors, and resistance factors shall be as specified by AASHTO LRFD and these Guide Specifications. For member design other than connections:

• Flexure resistance of steel HSS members shall be according to AASHTO LRFD Article 6.12 as box sections.

• Shear resistance of steel HSS members shall be according to AASHTO LRFD Article 6.11.9 as box sections.

C7.2.3

Equations for the determination of the lateral-torsional buckling moment in singly symmetric sections are given in the Guide to Stability Design Criteria for Metal Structures by Galambos (1998), specifically in Chapter 5. Equation 5.10 of that chapter presents the general formula for singly symmetric members where bending is in the plane of symmetry. Methods for accounting for location of loading with respect to the shear center are provided, as well as for determining the appropriate buckling lengths considering rotational restraints.

CS.2.I

AISC has partnered with ClDECT to publish a set of HSS Design Guides. These guides are published internationally and have not been reviewed by AISC and are not necessarily in accordance with the AISC Specifications. However, the documents are a good resource on HSS connections and systems.


• Tension and compression resistance shall be according to AASHTO LRFD Articles 6.8.2 and 6.9.2, respectively.

For electric-resistance-welded HSS members, the design wall thickness shall be taken as 0.93 times the nominal wall thickness.

8.2.2-Detailing

The minimum nominal metal thickness of closed structural tubular members shall be 0.25 in. Corrosion mitigation shall be considered.

8.2.3-Tubular Fracture Critical Members

The AASHTO/A WS Fracture Control Plan for N onredundant Members contained in AASHTOI A WS D1.5, Section 12, shall be applied to tubular members (HSS members), where required by AASHTO LRFD Articles 6.6.2 and C6.6.2, with the following modifications:

• ASTM A500, A501, A847, and A618 shall be added to those standards listed in Article 12.4.1 of AASHTO/AWS Dl.5.

• F or the purposes of determining preheat and interpass temperatures, the values for AASHTO M 270MlM 270 or ASTM A 709 Grade 50 shall be used.

C8.2.2

Different philosophies exist on how best to protect tubular members from corrosion. One method of corrosion protection is to vent the interior of the tube adequately and to provide some form of surface treatment, often a galvanized finish, to prevent corrosion. Issues to consider include the size of the field pieces to be galvanized, the size of local galvanizing kettles, and the service environment ofthe bridge.

Another method is to use a weathering steel material to provide corrosion protection. Adequate drainage and ventilation of the interior must be provided, and consideration of the suitability of the operating environment must be made. FHW A Technical Advisory T 5140.22 (1989) provides guidance in the use of weathering steels. An increase in the minimum thickness, when relying on weathering steel as the corrosion protection, should be considered.

A third, and often less successful, method is to completely seal the interior of the member from the atmosphere. This requires careful detailing of the connections, as even a small opening will allow moisture-laden air into the interior, and over time this can result in a large accumulation of water. Box members in a large truss that were supposedly sealed to the atmosphere have been fOlmd to contain several feet of water.

C8.2.3

No current specification adequately covers the use of tubular members in a fracture critical capacity. AASHTOI A WS D 1.5 specifically excludes tubular members. It appears significant research is required to address the unique aspects of both the longitudinal weld used to create the closed shape, as well as the one-sided groove welds without backing bars used in the connections of HSS. Until such time as this research is performed, the procedure specified herein represents the best available method for addressing fracture critical issues in HSS construction.

, LRFD GUIDE SPECIFICATIONS FOR THE DESIGN OF PEDESTRIAN BRIDGES 15

• Steel for tubular sections (HSS) shall conform to the Charpy V-notch requirements defined in A709-07. Filler metal shall be treated as A709 and conform to the requirements of A WS D 1.5 Table 12.1.

• Welding details for cyclically loaded tubular members specified by AASHTO/ A WS D 1.1 shall be used.

• All welds require qualification using A WS D 1.1 Article 4.S.

8.3-FIBER REINFORCED POLYMER (FRP) MEMBERS

The minimum thickness of closed structural FRP members (such as tubes) shall be 0.25 in. The minimum thickness of open structural FRP members (such as channels), including connection plates, shall be 0.375 in.

C8.3

For design of FRP members in pedestrian bridges, reference may be made to the AASHTO Guide Specifications for Design of FRP Pedestrian Bridges (200S). Little information is currently available regarding resistance equations or resistance factors for this material used in bridge structures. Several design specifications covering FRP pultruded shapes are currently under development by the American Society of Civil Engineers and may be of use in the future for the design of FRP pedestrian bridges.

-16 LRFD GUIDE SPECIFICATIONS FOR THE DESIGN OF PEDESTRIAN BRIDGES

9-DESIGN EXAMPLE

HALF-THROUGH TRUSS BRIDGE WITH HSS MEMBERS

ILLUSTRATIVE EXAMPLE OF KEY PROVISIONS OF GUIDE SPECIFICATIONS

LOAD AND RESISTANCE FACTOR DESIGN

GENERAL INFORMA nON

Specifications Used:

• AASHTO LRFD Bridge Design Specifications, 2007 with 2008 Interims (AASHTO LRFD)

• AASHTO Standard Specifications for Structural Supports for Highway Signs, Luminaires, and Traffic Signals, 2008 (AASHTO Signs)

• AASHTO LRFD Guide Specifications for the Design of Pedestrian Bridges (Specification)

Geometry:

Span = 72 ft

Deck width, Wdeck = 10ft

CL-CL trusses = 10.5 ft

A500, Gr. B, Fy = 46 ksi

12 Panels @ 6 It 0 in. ~ 72 ft 0 in. Span

-,.-

10 ft 0 in. Deck Width~

I I I I I I

T ~Floorbeam

10ft 6 In. if -if Trusses

J J

!. Symm @

: <t Span

i

,.--,.-

.S: <.0 <t=

'"

--- -

J

'" .S:

1:' 0

0 ..c: <t=

()

l!) G-' I

G-'

LRFD GUIDE SPECU'ICATlONS FOR THE DESIGN OF PEDESTRIAN BRIDGES

TRUSS MEMBERS: ALL RECTANGULAR HSS

Top and Bottom Chords:

Section: HSS6 x 3 x 5116

A 4.68 in."

w 16.93 plf

End Posts:

Section: HSS6 x 3 x 5/16

A 4.68 in.2

w 16.93plf

Vertical Posts:


A 4.1 in.2

w 14.8 plf

Ix Ie = 12.6 in.4

Diagonals:


A 2.91 in?

w 10.48 plf

FLOORBEAMS:

Section: W8 x 10

Ix

Sx

Spacing

Ib = 30.8 in.4

7.81 in. 3

6 ft at each panel point

17

18 LRFD GUIDE SPECIFICATIONS FOR THE DESIGN Of' PEDESTRIAN BRIDGES

DEAD LOAD:

Weight of each truss

Assumed deck loading

Weight of deck and floor system

Total dead load

62 plf per truss

25 psf

25 psfx 10.50 ft/2

132 plf per truss

62 plf + 132 plf

194 plf

PEDESTRIAN LIVE LOAD (SPECIFICATION, ARTICLE 3.1):

For Design of Main Truss Members:

Use 200 plf

The deck area may be used to compute design pedestrian live load for all main member components (truss members). The deck area is the non-zero influence surface for all such components.

Use 90 psfwithout impact.

Live load per truss pedestrian loading x deck widthl2

90 psf x 10.0 ft/2

450 plf

For Design of Secondary Members-Deck, Stringers, Floorbeams:

Use 90 psfwithout impact.

VEHICLE LOAD (SPECIFICATION, ARTICLE 3.2):

Vehicular access is not prevented by fixed physical methods; therefore, the pedestrian bridge should be designed for an occasional single maintenance vehicle load.

Use Table 3.2-1 for Minimum Axle Loads and Spacings.

The vehicular load shall not be placed in combination with the pedestrian load. Consideration of impact is not included with this vehicular loading.

Use the following vehicle for a clear deck width between 7 ft and 10 ft:

Front axle 2 kips

Rear axle 8 kips

Axle spacing 14 ft


Wheel spacing 6 ft

Note: For this example, the pedestrian load controls for the truss design; however, the vehicle load will control for the floor system design.

WIND LOAD (SPECIFICATION, ARTICLE 3.4):

Assume 100 mph design wind.

Use wind load as specified in AASHTO Signs, Articles 3.8 and 3.9.

Neglect wind load on the live load vehicle.

The design life shall be taken as 50 years for the purpose of calculating the wind loading.

Horizontal Wind Loading:

Apply the design horizontal wind pressure on the truss components.

Pz design wind pressure on superstructure using AASHTO Signs, Eq. 3-1 or Table 3-7 (pst)

(AASHTO Signs, Eq. 3-1)

where:

Kz height and exposure factor from AASHTO Signs, Eq. C3-1 or Table 3-5

1.00 (conservatively taken from Table 3-5 for a height of 32.8 ft)

G gust effect factor

1.14 (minimum)

V basic wind velocity

100 mph

lr wind importance factor from AASHTO Signs, Table 3-2

1.00

Cd wind drag coefficient from AASHTO Signs, Table 3-6

2.00

Pz 58.4 psf (Alternatively, AASHTO Signs, Table 3-7 may be used with a Ccl value of2.0 applied.)

20

Projected vertical area per linear foot:

Chords: 2 @ 3 in.ll2 x 6 ftJ6 ft Verticals: 3 in.l12 x 4.75 ft long/6 ft Diagonals: 3 in.l12 x 7.81 ft long/6 ft Total per Tmss:

Deck + Stringers: 10 in.l12

0.50 ft2 0.20 ft2

0.33 ft2

1.03 ft2

0.83 ft2

LRFD GUIDE SPECIFICA nONS FOR THE DESIGN OF PEDESTRIAN BRIDGES

total horizontal wind on superstmcture (plf)

(2 tmsses x 1.03 fe + 0.83 ft2) x 58.4 psf

169 plf

Note: The full lateral wind loads must be resisted by the entire superstructure. Appropriate portions of the design wind loads must also be distributed to the tmss top chord for design lateral forces on the tmss verticals.

Vertical Wind Loading:

Apply a vertical pressure of 0.020 ksf over the full deck width concurrently with the horizontal loading. This loading shall be applied at the windward quarter point of the deck width.

WSv vertical wind load on the full projected area of the superstmcture applied at the windward quarter point (plf)

Pv(Wdeck)

where:

Pv vertical wind loading on superstmcture (ksf)

0.020 ksf

Wdeck total deck width (ft)

10.0 ft

Therefore,

WSv 0.020 ksfx 1000 x lO.OO ft

200 plf

V erticalload on leeward tmss

V erticalload on windward tmss

200 plfx (7.5 ft + (0.5 in. + 2.5 in.)/12 in.lft)110.50 ft

147.6 plf

200 plfx (2.5 ft + (0.5 in. + 2.5 in.)/12 in.ft)/10.50 ft

52.4 plf(uplift)


TOTAL VERTICAL LOADS PER TRUSS (SPECIFICATION, ARTICLE 3.7):

DEAD LOAD (DC l +DC2): 200 plf

LIVE LOAD (Pedestrian, PL): 450 plf

WIND (Overturning, WS): 148 plf

Load Factors (AASHTO LRFD, Table 3.4.1-1):

Limit State: DCl &DC2 PL WS Str I 1.25 1.75 0 Str III 1.25 0 1.40 SerI 1.00 1.00 0.30

TRUSS MEMBER DESIGN LOADS:

Panel point load from controlling load comb. = 1.038 kIf x 6.0 ft panel = 6.23 kip/panel

Maximum Truss Member Axial Loads (from separate truss analysis):

Chord (U05-U06) 134.57 kips (compression)

End Post (UOO-LOO) 34.27 kips ( compression)

Diagonal (UOO-LOl) 53.52 kips (tension)

Vertical (UOl-LOl) 28.04 kips (compression)

TRUSS TOP CHORD LATERAL SUPPORT (SPECIFICATION, ARTICLE 7.1):

Assume the truss verticals are adequate to resist the lateral force per Specification, Article 7.1.1. (Must verify assumption; see Article 7.1.1, Lateral Frame Design Force.)

Lateral support is provided by a transverse U-frame consisting of the floorbeam and truss verticals.

Determine the design effective length factor, K, for the individual top chord members supported between the truss verticals using Specification, Table 7.1.2-1.

Compute CL/Pc for use in Table 7.1.2-1.


C transverse frame spring constant, can be fOlmd from separate computer analysis, or from equation below

E

h2 [ ( h / 3Ic ) + ( b / 2Ib ) ]

(from Guide to Stability Design Criteria for Metal Structures, edited by T. V. Galambos, 1968)

where:

E modulus of elasticity (ksi)

29,000 ksi

Floorbeam span: b = 126.0 in.

Effective height of vertical: h 54.0 in.

Floorbeam: W8 x lO

Vertical: HSS5 x 3 x 5116 Ie = 12.6 in.4

C 2.863 kiplin.

Alternatively, perform a 2-D computer analysis of the U-Frame

where:

C PI!!..

2.917 kip/in. (from a separate 2D analysis)

Use C 2.863.

L unbraced length of the chord in compression (i.e., length between panel points) (in.)

72 in.

desired critical buckling load (i.e., factored compressive force) multiplied by 1.33 (kips) (Specification, Article 7.1.2)

178.98 kips

CLIPe 1.15

n number of panels

12

Therefore,

11K 0.678 (Specification, by interpolation of Table 7.1.2-1)

K 1.47


TOP CHORD COMPRESSIVE RESISTANCE (AASHTO LRFD, ARTICLE 6.9.2):

Check the slenderness ratio against the limiting value.

For main members: KLlr :::;; 120

For bracing members: KLlr:::;; 140

Section: HSS6 x 3 x 5/16

A 4.68 in?

radius of gyration about the x-axis (in.)

2.07 in.

radius of gyration about the y-axis (in.)

1.19 in.

K 1.47

L 72 in.

KLlrx (1.47 x 72 in.)/2.07 in.

51.3<120

KLlry (1.00 x 72 in.)/1.l9 in.

60.5 < 120

OK

OK

P r factored resistance of components in compression (kips)

(AASHTO LRFD, Eq. 6.9.2.1-1)

where:

resistance factor for compressive per AASHTO LRFD, Article 6.5.4.2

0.9

nominal compressive resistance per AASHTO LRFD, Article 6.9.4 (kips)

Determine the nominal compressive resistance, Pn

If A :::;; 2.25, then:

Pn = A

0.66 F.vAs (AASHTOLRFD, Eq. 6.9.4.l-1)

If A > 2.25, then:

23

G



A=(KL)2 F;! 'Slt E


= 0.59

where:

As gross cross-sectional area (in.2)

4.68 in.2

Fy specified minimum yield strength (ksi)

46 ksi

E modulus of elasticity (ksi)

29,000 ksi

KLlrs Maximum of KLlrx,KLlry

61

Therefore, the top chord factored resistance is:

Pn 0.660.59 x 46 ksi x 4.68 in.2

168 kips

151 kips> Pchord = 134.57 kips OK

LATERAL FORCE TO BE RESISTED BY VERTICALS (SPECIFICATION, ARTICLE 7.1.1):

HI minimum lateral force (kips)

O.OllK(Pavg)

where:

K 1.47

Pavg average design compressive force in adjacent chord members (kips)

134.57 kips

Verify limit 0.0111.47 0.007 > 0.003 OK

LRFD GUIDE SPECIFICA nONS FOR THE DESIGN OF PEDESTRIAN BRIDGES 25

Therefore,

HI 0.0111.47 x 134.57 kips

0.91 kip

Apply HI as the lateral force at the top of the Truss Verticals. Apply HI concurrently with other primary forces in the Verticals (combined compression plus bending analysis). Include lateral wind forces for AASHTO LRFD Load Combination Strength Ill.

Length of vertical 54.0 in.

Lateral Moment in Vertical 0.91 kip x 54.0 in. = 49.27 kip-in.

Only load development is shown, check of capacity follows typical procedures.

END POSTS (SPECIFICATION, ARTICLE 7.1.1):

Apply the lateral force, C, at the top end of post and design as a cantilever combined with axial load. The lateral force, C, is taken as 1.0 percent of the end post axial load.

Lateral Force: C = 0.01 x 34.27 kips = 0.34 kip

Note: All other truss members are analyzed using conventional methods per AASHTO LRFD.

Only load development is shown, check of capacity follows typical procedures.

DEFLECTION (SPECIFICATION, ARTICLE 5):

Maximum pedestrian LL Deflection = 1/360 of the span length = 72.00 ft x 12/360 = 2.40 in.

From Truss Analysis, LL Deflection (WLL = 0.450 kip/ft) = 1.20 in. < L/360 OK

VIBRATIONS (SPECIFICATION, ARTICLE 6):

Vertical Direction:

Estimate the fundamental frequency in the vertical direction, f, by approximating the truss as a simply supported tmiform beam.

The fundamental frequency in a vertical mode without consideration of live load should be greater than 3.0 Hz to avoid the first harmonic.

f=0.18~ g t.DL

where:


g acceleration due to gravity (ft/s2)

32.2 ft/s2

~DL maximum vertical deflection ofthe truss due to the dead load (ft)

0.0444 ft (from a separate analysis with w = 0.20 kif per truss)

f = O.lS~ 32.2 = 4.S5 Hz > 3.0 Hz minimum desirable 0.0444

OK

For illustration purposes, assume higher harmonics (second, third, etc.) are a concern. The bridge should be proportioned such that the following criteria are satisfied:

f 2.S6ln (lSOIW)

where:

w full weight of the supported structure including dead load (kips)

2 trusses x 0.20 klfx 72.00 ft

2S.S kips (Dead Load Only)

2.S6 In (ISO/2S.S0) 5.24 Hz

f 4.S5 Hz is not greater than 5.24 Hz. Modifications required.

Possible modifications include using a heavier deck system or increasing the depth of the truss.

Lateral Direction:

Estimate the flmdamental frequency in the lateral direction, fiat, by approximating the truss as a simply supported uniform beam rotated 90 degrees.

The fundamental frequency in a lateral mode without consideration of live load should be greater than 1.3 Hz to avoid the first harmonic.

Assume the lateral wind bracing is 3 x 3 x 1I4 in. structural tubing.

f=O.lS~ g ~DLlat

where:

g

~DLlat

acceleration due to gravity (ft/s2)

32.2 ft/s2

maximum lateral deflection of the truss due to the dead load (ft)

0.OS44 ft (from a separate analysis) •


• , ~2.2 3 H 3 H ., d' b j = 0.18 --- = .52 z> 1. z mmnllum eSlra Ie 0.0844

OK

FATIGUE (SPECIFICATION, ARTICLE 3.5):

Use AASHTO Signs, Article 11.7.3.

AASHTO Signs, Article 11.7.4-Not used as it is assumed that the pedestrian bridge is not over a highway.

Cd wind drag coefficient per AASHTO Signs, Table 3-6

2.00

wind importance factor per AASHTO Signs, Table 3-2

1.00

10.4 psf

total horizontal wind on superstructure for fatigue (pIt)

(2 trusses x 1.03 fe + 0.83 ft) x 10.4 psf

31 plf

Maximum Member Force from Wind:

Bottom Chord, Member L05-L06 = 5.6 kips (from a separate analysis)

/}.f Stress Range

(5.6 kips!4.68 in.2)

1.20 ksi

y(/}.f) ::; (M)n (AASHTO LRFD, Eq. 6.6.1.2.2-1)

where:

y 1.0 (Specification, Article 3.7)

N 1.20 ksi

(M)n (Specification, Article 4.1)

where:

(M)n 16 ksi (Category B-base metal) (AASHTO Signs, Table 11-3)


(1.0)( 1.20) :s; 16

1.20 < 16 OK

Welded member connections and fracture toughness requirements are outside the limits of this pedestrian bridge design example. They will be the responsibility of the Designer.

;1


REFERENCES

AASHTO. 1997. Guide Specifications for Design of Pedestrian Bridges. American Association of State Highway and Transportation Officials, Washington, DC.

AASHTO. 2001. Standard Specifications for Structural Supportsfor Highway Signs, Luminaires, and Trajfic Signals, 4th Edition. American Association of State Highway and Transportation Officials, Washington, DC.

AASHTO. 2002. Standard Specifications for Highway Bridges, 17th Edition. American Association of State Highway and Transportation Officials, Washington, DC.

AASHTO. 2007. AASHTO LRFD Bridge Design Specifications, 4th Edition, 2008 and 2009 Interim. American Association of State Highway and Transportation Officials, Washington, DC.

AASHTO. 2008. AASHTO Guide Specifications for Design of FRP Pedestrian Bridges, 1st Edition. American Association of State Highway and Transportation Officials, Washington, DC.

AISC. 2005. Specification jar Structural Steel Buildings, ANSIIAISC 360-05. American Institute of Steel Construction, Chicago, IL.

Allen, D. E. and T. M. Murray. 1993. Design Criterion for Vibrations Due to Walking. In AISC Journal, 4th Quarter. American Institute of Steel Construction, Chicago, IL., pp. 117-129.

A WS. 2008. Bridge Welding Code, AASHTOI A WS D 1. 5 MID 1.5 :2008. American Welding Society, Miami, FL.

A WS. 2006. Stnlctural Welding Code-Steel, AASHTOI A WS D 1.1M/D 1.lM:2006. American Welding Society, Miami, FL.

Bachmann, H. 2002. Lively footbridges-a real challenge. Proceedings of the International Conference on the Design and Dynamic Behavior of Footbridges, November 20-22, 2002, Paris, France, pp.18-30.

Blekhennan, A. N. 2007. Autoparametric Resonance in a Pedestrian Steel Arch Bridge: Solferino Bridge, Paris. In Journal of Bridge Engineering, Volume 12, Issue 6. American Society of Civil Engineers, Reston, V A, pp. 669-676.

Dallard, P., T. Fitzpatrick, A. Flint, Low A., R. R. Smith, M. Willford, and M. Roche. 2001. London Millennium Bridge: Pedestrian-Induced Lateral Vibration. In Journal of Bridge Engineering, Volume 6, Issue 6. American Society of Civil Engineers, Reston, VA, pp. 412-417.

Dallard, P., et al. 2001. The London Millennium Footbridge. In The Structural Engineer, 79(22). The Institute of Structural Engineers, London, pp. 17-33.

FHWA. 1989. Uncoated Weathering Steel in Structures, Technical Advisory T 5140.22. Federal Highway Administration, US Department of Transportation, Washington, DC.

Galambos, T. V. 1998. Guide to Stability Design Criteria jar Metal Structures, 5th Edition. John Wiley & Sons, Inc., New York, NY.

Kozy, B. and S. Tunstall. 2007. Stability Analysis and Bracing for System Buckling in Twin I-Girder Bridges. In Bridge Structures: Assessment, Design and Construction, Volume 3, No. 3-4. Routledge Journals, Florence, KY, pp 149-163.

Nettleton, D. A. 1977. Arch Bridges. Bridge Division, Office of Engineering, Federal Highway Administration, U.S. Department of Transportation, Washington, DC.

Nowak, A. S. and K. R. Collins. 2000. Reliability ofStnlctures, McGraw-Hill International Editions, Civil Engineering Series. The McGraw-Hill Companies, Singapore.

29


Poston, Randall W. and Jeffery S. West. 200S.Investigation of the Charlotte Motor Speedway Bridge Collapse, Metropolis & Beyond 2005. Proceedings of ASCE's 2005 Structures Congress and the 2005 Forensic Engineering Symposium, April 20-24, 2005, New York, NY.

Roberts, T. M. 2005. Lateral Pedestrian Excitation of Footbridges. In Journal of Bridge Engineering, Volume 10, Issue 1. American Society of Civil Engineers, Reston, V A, pp. 107-112.

Roland, E. S., M. L.Hull, and S. M. Stover. 2005. Design and Demonstration of a Dynamometric Horseshoe for Measuring Ground Reaction Loads of Horses during Racing Conditions. In Journal of Biomechanics, Volume 38, No. 10. Elsevier Inc., The Nerherlands, pp. 2102-2112.

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Willford, M. 2002. Dynamic actions and reactions of pedestrians. Proceedings of the International Conference on the Design and Dynamic Behavior of Footbridges, November 20-22, 2002, Paris, France.

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Zhao, X.-L., S. Herion, J. A. Packer, R. S. Puthli, G. Sedlacek, J. Wardenier, K. Weynand, A. M. van Wingerde, and N. Yoemans. 2001. Design Guide 8-for CHS and RHS Welded Joints Under Fatigue Loading. ClDECT, TOV Verlag, Germany.

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Documents

AASHTO LRFD GuideSpecForDesignOf Pedestrian Bridges December2009