Thumb Rules Buildings

Rules of Thumbfor Steel Design

Socrates A. Ioannides

John L. Ruddy

Author

Socrates A. Ioannides, Ph.D.,P.E., president of Structural

Affiliates International, Inc. (SAI),has more than two decades ofstructural engineering experience,both practical and academic. Hereceived his bachelor's degreeunder a Fullbright Scholarshipfrom Vanderbilt University inNashville, Tennessee, in 1974and his master's and Ph.D.degrees from also from VanderbiltUniversity in 1978. In 1976, hejoined Stanley D. Lindsey andAssociates, Ltd, starting as astructural designer and was vicepresident when he left in 1986 tofound Structural AffiliatesInternational, Inc.

Dr. Ioannides has worked onprojects throughout the UnitedStates and abroad and is aLicensed Professional Engineer inmore than two-thirds of the 50states. He is also a LicensedStructural Engineer inWashington, Illinois and Cyprus.

Dr. Ioannides has built animpressive track record in seismicdesign including the retrofit of theSouth Carolina State House, aswell as other projects in theWestern States and in the NewMadrid and Charleston faultareas. The South Carolina StateHouse is the first historic renova-tion project to utilize seismic baseisolation in the Eastern UnitedStates.

Throughout his career, Dr.Ioannides has served as anadjunct professor at VanderbiltUniversity, teaching courses onthe graduate level. He hasauthored and presented morethan 30 papers on various topicsto produce effective and innova-tive structural solutions. Theseinclude seismic bridge design,finite element analysis and sever-al other topics dealing with practi-cal applications of theoreticalmaterials. Dr. Ioannides has alsoreceived numerous awards for hiswork both in practical applicationsand in academic activities at

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Vanderbilt University.Dr. Ioannides spearheaded

SAI's partnership with theMalaysia steel industry formingPerwaja Structures, International,Sdn Bhd (PSI). During the pasttwo years, he has spent consider-able time in Malaysia educatingthe construction community onthe benefits of structural steel aswell as structural steel design.PSI is involved in more than adozen steel projects ranging fromUniversity campuses to high-risebridges. Currently under designis a 50-story high-rise with the dis-tinction of being the first sky-scraper in Malaysia to utilize alllocally produced and fabricatesteel.

His involvement with fire pro-tection issues includes evaluationof existing fireproofed steel struc-tures built differently than original-ly designed, as well as assistingother design professionals toevaluate the fire protectionscheme for new steel structures.He currently is serving as a char-ter member of AISC's Task Forcefor Fire Engineering of StructuralSteel Buildings-TechnicalSolutions Task Group.

Dr. Ioannides expanded histeam in January of 1998 with theopening of a San Diego Officeand the addition of J. John Walsh,S.E. heading operations fromCalifornia. John L. Ruddy, P.E.joined the team as chief opera-tions officer, at the Nashvilleoffice, in February of 1998. Dr.Ioannides and his Vice President,Robert P. Beall, P.E. have alwaysbelieved in brainstorming an aneffective means of creativity, inno-vation and design. The expan-sion of their teams has thisprocess even richer.

Author

Mr. John Ruddy joinedStructural Affiliates

International, Inc. (SAI) as chiefoperating officer and principal inJanuary of 1998. Mr. Ruddy hasover 34 years of experience with

© 2003 by American Institute of Steel Construction, Inc. All rights reserved.This publication or any part thereof must not be reproduced in any form without permission of the publisher.

analysis, systems selection, struc-tural design, cost evaluation,development of computer applica-tions in structural design, specifi-cations, supervision of contractdocuments, engineering teamcoordination, and client liaison forboth new and renovated projectswith both structural consultingfirms and architectural and engi-neering practices.

A graduate of the University ofDayton with a bachelor of sciencein civil engineering and ofCleveland State University with amaster of science in civil engi-neering, he holds professionalengineering registration in tenstates.

A professional member as wellas a frequent lecturer for theAmerican Institute of SteelConstruction (AISC), he has pre-sented programs on "Design ofLight and Heavy IndustrialBuildings" and the compositedesign session of "Steel DesignCurrent Practices." He is also theauthor of "Economics of Low-riseSteel-framed Structures", pub-lished in the AISC EngineeringJournal, Third Quarter, 1983. He

currently is serving as a chartermember of AISC's Task Force forFire Engineering of StructuralSteel Buildings.

John Ruddy is an active mem-ber of the American Society ofCivil Engineers (ASCE), servingon the Committee on Design ofSteel-framed Buildings. He hasserved as a director ofConnecticut Engineers in PrivatePractice and was a foundingmember of the StructuralEngineers Coalition ofConnecticut.

Summary

In earlier times when computerswere neither available nor

essential, one objective of thestructural design process was todiscover a computational method,which was elegant, simple andappropriately accurate. Whensuch a process was identified itwas recorded as an expedientapproach to solving a recurringstructural design problem. Thisquick, "Rules of Thumb" becameessential resources for the struc-tural engineer. As computer soft-ware has proliferated, become

very comprehensive, and beenmade very user friendly, theimportance of rules of thumb andapproximate methods has beendiminished. It has been arguedthat, with the computational speedand ease of application of com-puter methods, the need forapproximations and "Rules ofThumb" no longer exists.However, equally imposing argu-ments can be made for the valueof these quick approachessuch as:

• The structural engineershould have tools to make on-the-spot intelligent decisions.

• A reasonable solution isoften required as computer input.

• The validity of the computeroutput should be verified withrational approximations.

So, with the objective of foster-ing continued development, useand enthusiasm for "Rules ofThumb" and approximate meth-ods, several steel framing "Rulesof Thumb" are presented in thispaper. Because a majority ofthese have been developed overthe past they are based onAllowable Stress Design (ASD).Future development will be basedon Load and Resistance Factor

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RULES OF THUMB FOR STEEL DESIGN

John L. Ruddy, P.E., Member ASCE1

Socrates A. Ioannides, Ph.D., S.E., Member ASCE2

ABSTRACT: In earlier times when computers were neither available nor essential, oneobjective of the structural design process was to discover a computational method, which waselegant, simple and appropriately accurate. When such a process was identified it was recordedas an expedient approach to solving a recurring structural design problem. Thus, quick "Rules ofThumb" became essential resources for the structural engineer. As computer software hasproliferated, become very comprehensive, and been made very user friendly, the importance ofrules of thumb and approximate methods has been diminished. It has been argued that, with thecomputational speed and ease of application of computer methods, the need for approximationsand "Rules of Thumb" no longer exists. However, equally imposing arguments can be made forthe value of these quick approaches such as:

• The structural engineer should have tools to make on-the-spot intelligent decisions,

• A reasonable solution is often required as computer input,

• The validity of the computer output should be verified with rational approximations.

So, with the objective of fostering continued development, use and enthusiasm for "Rules ofThumb" and approximate methods, several steel framing "Rules of Thumb" are presented in thispaper. Because a majority of these have been developed over the past they are based onAllowable Stress Design (ASD). Future development will be based on Load and ResistanceFactor Design (LRFD).

STRUCTURAL DEPTHS; Inevitably, a question raised in a project concept meeting is whatwill be the structural depth? Regularly, the participants are impressed by the response of thestructural engineer and that positive impression lasts if the actual depths designed fall within therange of these early predictions. Therefore, it is important to have established rules of thumb,which allow structural depth predictions. The depth of the structural system is influenced by thespan of the elements as well as such variables as the spacing of elements, loads and loadingconditions, continuity, etc. Nonetheless, ratios of span to depth can often be relied upon toprovide a guide and a starting point from which further refinement can be made. With thecaution that variables other than span need to be considered, the following guide is presented:

SYSTEMSteel BeamSteel JoistFloor MemberRoof MemberPlate GirderJoist GirderSteel TrussSpace Frame

20 to 28

202415121212 to 20

SPAN RANGE0' to 75'

8' to 144'

40' to 100'20' to 100'40' to 300'80' to 300'

1 Chief Operating Officer, SAI; e-mail: [email protected] President, Structural Affiliates International, Inc. (SAI), (http://www.saii.com)2424 Hillsboro Rd, Nashville, TN 37212, (615) 269-0069; e-mail: [email protected]

17-3© 2003 by American Institute of Steel Construction, Inc. All rights reserved.

This publication or any part thereof must not be reproduced in any form without permission of the publisher.

It is convenient to remember that serviceable steel section depths are in the range of ½" of depthfor each foot of span (L/24). Some people might find it easier to remember the followingsimplified rule where the length is expressed in feet and the depth of the member in inches:

Depth of Roof Beams, Roof Joists = 0.5*LengthDepth of Floor Beams, Floor Joists = 0.6*LengthDepth of Composite Beams = 0.55*Length

BEAMS: The rapid determination of a steel section size can be made without reference to a steelmanual using a very simple equation. If the moment capacity, depth and foot weight of theeconomy steel beams listed in the AISC Specification are tabulated with moment divided by thedepth as the independent variable and foot weight as the dependent variable, a linear regressionanalysis results in a rather simple equation for Fy=36 ksi.

The closest economy section of the depth used in the equation that has a foot weight greater thanpredicted by the equation indicates the beam that will sustain the moment. This equation wasconfirmed by the author using an alternate approach, coined "Visual Semi-rigorous CurveFitting"3. If all the beam sections are included, a slope value in the linear equation of 5.2 yieldscloser approximations for Fy=36 ksi.

Consider a beam spanning 30 feet supporting a 10 foot width of floor with a total supported loadof 140 psf, resulting in a moment of 157.5 foot-kips. For an 18" deep beam, the equation yields43.75 pounds per foot. A W18x50 is the predicted section and the actual moment capacity is 176foot-kips. If a beam depth of 21" is assumed, the equation yields 37.5 suggesting a W21x44,which has a moment capacity of 162 foot-kips.

A similar formulation for steel having Fy = 50 ksi produces:

For an 18" deep beam, the equation yields 30.6 pounds per foot, therefore, a W18x35 ispredicted. The actual capacity of a W18x35 beam with Fy=50 ksi is 158 foot kips.

For common composite beam floor systems (e.g. 5½" slabs with 3" composite deck, 4½"slab with 2" composite deck, etc.), the simplified equations yield relatively accurate foot weightsif 70% to 75% of the simple span moment is used for M. Following are two more "Rules ofThumb" relating to composite construction and Fy=36:

In ASD Number of shear studs required for Full Composite Action = 1.1*WtIn LRFD Number of shear studs required for Full Composite Action = 1.25*Wt

ROOF SYSTEMS: A common approach to economy in steel roof systems of single storybuildings is to cantilever girders over the columns. The ends of the cantilever support a reducedspan beam. When this system is subjected to a uniform load and multiple equal spans areavailable, a cantilever length approximately equal to 15% (0.146) of the span length will result in

3 Ioannides, S. A. (1996). "Restraining Effects of Simple Steel Connections on Beam and Column Design,"National Steel Construction Conference Proceedings.

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the maximum moment in any span being equal to 1/16 wL2. For end spans, negative andpositive moments can be balanced using a cantilever length equal to 25% of the first interiorspan.

Another approach to economical roof systems is the use of plastic analysis. Although not ascritical for this system, splice locations in the plastically designed continuous beams are usuallychosen so that they are close to the point of zero moment.

Hinge or splice location for cantilever or continuous roof systems is 15% to25% of span length

TRUSSES; The foot weight of trusses utilizing Fy=36 ksi steel can be calculated by assumingFa=22 ksi. The Chord Force is then equal to the moment (M) in foot-kips divided by(the effective depth - lever arm) in feet, resulting in a chord area of By recognizing thatWt =A*3.4, converting to inches and assuming that and that the total truss weight isequal to 3.5 times the chord weight then:

This includes connection material. Substitute 4.5 for 6 in the above equation when Fy=50 ksi.

RIGID FRAME ANALYSIS APPROXIMATIONS; The following "Rules of Thumb" areuseful in determining preliminary sizes for Rigid Moment Frames resisting Lateral loads. Theyare based on the traditional "Portal Frame" approach modified from the authors' experienceswith "real" frames.

Interior Columns at Roof

Interior Columns Not at RoofThe moments in beams framing into exterior columns are half of the abovevalues

STEEL WEIGHT ESTIMATES; Cost is generally the basis for confirming a structural systemsince safety and function are essential for any options considered. Economy is related to theweight of the structural steel although costs are influenced by many other parameters. Yet,weight can be a valuable indicator of cost and Rules of Thumb are useful in establishing anexpectation for steel weight. A quick assessment of anticipated weight serves as a check of thereliability of the weight determined by more involved investigations.

Bracing is a cost-effective means of providing lateral load resistance for low to medium risebuildings. As the building height increases, the unit steel weight increases since columns aresubjected to larger loading at the lower floors and lateral load resisting components are subjectedto greater loads for greater heights. Thus, one parameter influencing the steel weight is buildingheight. A rough approximation for steel weight per square foot in a braced building using steelwith Fy = 50 ksi is:

Wt(psf) = stories/3 + 7

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A three-story building would have a steel weight in the range of 8 psf and a 27-story buildingwould require 16 psf. Certainly, this relationship is an over simplification. Yet, it provides avalue, which can be used to confirm that the results of a more detailed analysis are reasonable.

TALL BUILDING STRUCTURAL SYSTEMS: The late Fazlur Khan hypothesized that theappropriate structural system to resist lateral loads was directly related to building height. Hepredicted that structural economy could be realized using the appropriate system as follows:

STORIES<3030 to 4041 to 6061 to 8081 to 100101 to 110111 to 140

LATERAL LOAD RESISTING SYSTEMRigid frameFrame - shear trussBelt trussFramed tubeTruss - tube w/ interior columnsBundled tubeTruss - tube without interior columns

MISCELLANEOUS:

A = wt/3.4S = D*Wt/10 (same as Wt = 5M/D)I = D2*Wt/20End rotation of a simple beam = 0.2 radiansDeflection of simple span beam (reduction due to connections) = 80% of calculatedRoof Framing Systems

For Cantilevered or continuous roof beams:• Run beams in short direction• Optimum bay size is 30' x 40'For Truss Joist and Joist roof systems:• Run Girders in Long direction• Optimum bay size is 40' x 40'

NOMENCLATURE;

Area (in2)Nominal member depth (inches)System depth (ft)Yield strength of steelStory HeightMoment of Inertia (in4)Length (ft)Bending moment (foot-kips)Design Moment for BeamDesign Moment for ColumnNumber of Columns (not bays) in the story of the FrameElastic Section Modulus (in3)Total Story Shear for the FrameFoot weight of the steel beam (pounds per foot)Weight of steel structure (psf)

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