AECT210-Lecture 41.pdf

7/29/2019 AECT210-Lecture 41.pdf

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Lecture 41 - Page 1 of 14

Lecture 41 Structural Systems & Engineering Practice

Long Span One Way Structural Systems

Long span structures typically refer to spans exceeding 60 feet. A one-way structural system is characterized by relatively large linear spanningelements in one direction. Smaller spanning members are used to carryloads to the primary members. Typically, one-way structural systems areused in rectangular framing bays. Most of our discussion of structuralmembers has been one-way systems.

Below is a summary of typical long span one-way systems:

Long Span One-Way Structural SystemsSystem: Typical Spans, feet Typ. Depth-to-

Span Ratio:

Steel beams & girders 10 72 1/20Steel rigid frame 30 150 1/20Flat steel truss 40 300 1/10Pitched steel truss 40 150 1/8Steel arch 50 500 1/100Steel bar joists 20 144 1/24Steel joist girders 20 100 1/12Prestressed concrete single T beam 20 120 1/25Prestressed concrete double T beam 20 60 1/25Concrete arch 40 300 1/50Wood glulam beams 10 60 1/24

Flat wood trusses 40 120 1/10Pitched wood trusses 40 100 1/6Wood arch 50 250 1/40

Design Considerations for One-Way Systems:

1) Lack of redundancy A serious problem with one-way systems is thepossibility of catastrophic collapse if one of the primary members wereto fail. Redundancy means having a secondary load path in case ofsuch failures.

2) Stability During Construction Temporary shoring and other methodsare often necessary to prevent toppling of large structural membersduring construction.

3) Stability After Construction Additional lateral bracing, bridging, shearwalls or other permanent methods of maintaining stability is typicallynecessary.


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4) Connections Most failures of one-way systems occur at connections.Special attention is required to shop drawings for fabrication ofconnections (typically performed by fabricator NOT the engineer).

5) Ponding A real problem with flat roofs, ponding occurs when waterhas no place to go. The weight of water creates deflection which in

turn allows more water to collect. This cycle continues until failureoccurs.

6) Temperature Movement A relatively short structural member of 25feet subjected to temperature variations will have very negligibletemperature movement. However, long span members must be ableto flex by a significant amount that must be accounted for.

7) Tolerances Long span members often require some tolerances toallow for fit. These tolerances are accomplished by making slotted boltholes, shims, additional seat angles, etc.

8) Shipping One-way structural members tend to be very big andheavy. Delivering these large members to the construction site can bevery problematic.


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Long Span Two-Way Structural Systems

As its name implies, a two-way system distributes load across two or moremembers. All members in a two-way system are considered to be primary

members. A two-way system is most efficient when the shape is squareso that there is equal distribution along the supporting members. In arectangular shape, the members spanning the short path carry more of theload. Two-way structures have MUCH more redundancy than one-waystructures. They are also MUCH more difficult to analyze and designbecause of their static indeterminacy.

P/4 P/4

P

P/2

P/2

P/4 P/4

P

Simple one-way system

Simple two-way system


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Types of two-way structural systems:

1) Space Frame Basically, it is a 3-dimensional truss. Lots ofredundancy built into this type of truss system. Very difficult to erectsince there are many members framing into a single point.

2) Dome Probably the most efficient structural system. A circular dome(as shown below) has vertical meridian lines that act like verticalarches in compression and horizontal hoops that act in tension.

Load

Meridians

Hoops

Circular dome


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Geodesic Dome Computer model

Geodesic Dome


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3) Thin-Shell Structures Carries shear, compression and tension in theplane of the shell. These structures are deformation resistant basedon their shape. Examples of thin-shell structures are vaults, hyperbolicparaboloids and folded plates.

Computer generated model of barrel vault structure

Groin vault


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Computer generated 3D plot of hyperbolic paraboloid

Hyperbolic paraboloid membrane structure roof


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Folded plate structure

Thin shell concrete structure Empire State Plaza The EGG


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4) Membrane structures Similar to thin shell structures, membranestructures are also considered to be form resistant. However, thesefabric-like membranes can carry tension ONLY. They are extremelylightweight. Their biggest disadvantage is that they change shapebased on loading and can also flutter in the wind. Membranestructures can come in various forms, including tents, air-supported

structures, domes, etc.

Membrane roof


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Membrane roof

Air-supported membrane roof Carrier Dome


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Structural Engineering Practice

1) Architect Structural Engineer Relationship A structural engineerworks in a team-like environment with the architect, owner, contractor,subcontractors, consultants and vendors. He (or she) will generallywork for the architect, but may also be hired directly by the owner.

Normally, the architect will select the structural system to be used, butwill confer with the structural engineer before committing to theconcept. Sometimes the structural engineer will select the structuralsystem especially for complex or large projects.

2) Coordination and Cooperation The structural engineer usually worksin SUPPORT of the functional relationships of the building. Theseinclude architectural, mechanical, electrical, and all other functions. Assuch, the structural engineer must coordinate the design of thestructure with any and all of these requirements and be willing tomake last-minute adjustments as necessary. Cooperation is expected

on the part of ALL construction team participants so that problemsolving can occur. Remember that it is rare (if ever) that every portionof the original design is carried completely through construction.

3) Preparation of Construction Documents The following structuralinformation shall be included on the construction documents (i.e.,drawings and specifications) as per IBC Section 1603:

a. Fully dimensioned plans, sections, elevations, details and otherinformation pertinent to the structural design of all elements

b. Floor live load

c. Roof line loadd. Roof snow loade. Wind design dataf. Earthquake design datag. Other loads as requiredh. Live loads conspicuously noted in areas within industrial &

commercial buildings where the live load exceeds 50 PSF

4) Shop Drawings Upon completion of design documents, thecontractor will hire manufacturers to prepare production drawings(shop drawings) that show exact cut lengths, product data, and details

not shown on the design documents. The engineer is responsible forchecking the design content but NOT the minute dimensional detailand quantities necessary for fabrication, and will sign-off that theinformation is approved or not.


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Shop Drawing Approval Stamp

5) Construction Inspections Depending on the scope of the project, astructural engineer will normally be required to perform at least someconstruction inspections. Poured-in-place concrete constructiongenerally requires a lot of inspection because of the on-site quality

control issues, while pre-engineered metal buildings generally requirea minimum of quality control inspections.

6) Engineering Licensure All 50 states require licensure before a personcan legally call himself Engineer. Reciprocity is usually allowedbetween states. Generally, licensure involves the following sequence:

a. Education 4-year B.S. degree from an accredited ABETengineering program. Under the recently approved ASCEPolicy 465, the educational requirements will be increased to aMasters degree gradually being grandfathered into law by

2020.


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b. EIT Exam This 8-hour Engineer In Training is the Part A of 2exams that must be passed and is sometimes called theFundamentals of Engineering exam. It includes 120 generalengineering questions including math, physics, chemistry,thermodynamics, statics, dynamics, fluid dynamics, electrical

theory, computer programming, engineering economics andother subjects typically during 4 years in an engineeringprogram. It may be taken during the last semester of senioryear in college.

c. Apprenticeship After graduation from college, a minimum of 4years of documented experience working under the supervisionof a PE.

d. Principles & Practice Exam This is the 8-hour Part B exam. Itis taken ONLY after required apprenticeship is approved. It

consists of 8 problems relating to specific field of engineering,i.e., civil, electrical, mechanical, chemical, etc. Upon passingthis exam, a PE license is granted.

e. Continuing Education Many, if not most states now requireprofessional engineers to participate in about 10 12 hours ofcontinuing education per year to maintain licensure. Thiseducation is usually in the form of attending seminars,workshops, college classes, etc.


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Below is a list of licensure requirements in New York.

A total of 6 years of credit is required for admission to the Fundamentals ofEngineering examination (Part A)

A total of 12 years of credit is required for admission to the Principles andPractice examination (Part B) for licensure.