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CVEN 483

Structural System Overview

Dr. J. BracciFall 2001 Semester

CVEN 483 Structural Systems 2

Presentation Overview

1. Building system primary function2. Types of load3. Building materials4. Structural members5. Structural systems

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1. Basic Building System Functions

Support gravity loads for strength and serviceability during:

1. Normal use (service) conditions2. Maximum considered use conditions3. Environmental loading of varying

intensities


Lateral deflection (sway)

Wind or earthquakes

Vertical deflection (sag)

Dead, Live, etc.

Performance-Based Design: Control displacements within acceptable limits during service loading, factored loaded, and varying intensities of environmental loading

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2. Types of Load

Gravity:DeadLiveImpactSnowRain/flood

LateralWindEarthquakeSoil lateral pressureThermalCentrifugal


3. Building Materials

• Reinforced Concrete• Structural Steel• Reinforced Masonry• Wood• Aluminum• Metal Structures• Fiber Reinforced Polymers

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Important Material Characteristics

• Modulus of Elasticity, E• Yield point• Strain hardening• Ductility• Toughness, hardness• Creep, shrinkage, thermal, relaxation• High-cycle fatigue


Reinforced Concrete(Reference ACI 318-99)

(a) Unconfined Concrete

0.003~ 0.002

0.3 f’c

0.85 f’c

f’c

~0.1 f’c

E = 57000 (f’c)0.5

Strain, in/in

Stre

ss

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(a) Confined Concrete

0.003 ~> 0.03

f’c

f’cc

Strain, in/in

Stre

ss



(b) Normal reinforcing SteelASTM A615,A617 [40,60 ksi]

(c) Prestressing bars & strands - A421,A416 [250,

270 ksi]

~0.002 ~ 0.16

fu

Strain, in/in

Stre

ss fy

~ 0.04

fpu

Strain, in/in

Stre

ss

fpy

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Structural Steel(Reference AISC-LRFD 1998 or AISC-ASD 19??)

• ASTM A36, 572, 615

~0.002 ~ 0.16

Strain, in/in

Stre

ss


Reinforced Masonry(Reference ACI 530-99/ASCE 5-99/TMS 402-99)

• ASTM C34, C56, C62, C126 (Clay or Shale)• ASTM C55, C73, C90, C129, C744 (Concrete)

0.33 f’m

f’m

Em = 700-900 f’m

Strain, in/in

Stre

ss

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Wood(Reference NDS 1997)

• Historical design approach was based on allowable stresses (ASD). LRFD approach is currently available.

• Many grades of wood – Southern Pine dominant in TX• Many types of failure mechanism, ie. various forms of

crushing and splitting (parallel or perpendicular to the grain)

• Bolted/nailed connections • Plywood – bonded sheets of wood (improves directionality

properties of wood)


Aluminum(Reference Metals Handbook)

Advantages:1. High strength/weight ratio2. Minimal maintenance due to stability in most

atmospheric environments 3. Fatigue advantages ??Applications:1. Aircraft

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Metal Structures

Advantages:• Warehouse type structures


Fiber Reinforced Polymers(ACI Committee 440, Trejo et al [2000])

Advantages are realized:• Due to the high strength• Strength/weight ratio• Corrosive resistance• Non-magnetic characteristics

Disadvantages:• High temperatures• Brittle behavior

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4. Structural Members

• Truss elements, including cables (tension only)• Beams• Columns• Slabs/plates/shells/folded plates• Walls/diaphragms


Truss ElementsDefn: Two force members, ie axial loads at nodes

only

F FL

A,E

Elastic Properties:

ka = EA/L (axial stiffness)

σ = F/A (normal stress)

δ = FL / EA (deflection)

δ

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Beam ElementsDefn: Members subject to bending and shear

Elastic Properties:

kb = f ( EI/Ln) (bending) σ = My/I (normal stress)

ks = GA/L (shear) v = VQ/Ib (shear stress)

δb = f (load, support conditions, L, E, I) (bending)

V

VL

E,I,AMM

δ2,Θ2δ1,Θ1


Column ElementsDefn: Members subject to bending, shear, and axial

Elastic Properties:

ka = EA/L (axial) σa = F/A (normal stress)

kb = f ( EI/Ln) (bending) σb = My/I (normal stress)

ks = GA/L (shear) v = VQ/Ib (shear stress)

δb = f (load, support conditions, L, E, I, A) (normal)

V

VL

E,I,A MMF F

δ2,Θ2δ1,Θ1

δ3

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Slab/Plate ElementsDefn: Members subject to bi-directional bending & shear

x

yz

Mx, My, and Vz

Θx, Θy, and δz


Wall/Diaphragm ElementsDefn: Members subject to shear

x

y

Vx and Vy

δx and δy

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5. Structural Systems

Gravity:• Trusses• Frames• Walls• Dual systems• Plates

Lateral:• Trusses• Frames• Braced Frames• Walls• Dual systems• Diaphragms


Truss: Coplanar system of truss elements governed by axial deformations

Planar (2D) Space (3D)

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Basic Truss Unit

Stable Unstable


Truss Types

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Truss Behavior

• Act as long, deep beams with cutout webs• Resistance increases when upper and lower chords

are spaced further apart. Bottom, corner elements are critical


Truss Connections

• Modeled as pinned• Gusset plates used to connect members at

nodal points• Riveted (past practice), high strength bolt,

or welded connections

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Truss Advantages

• Optimum use of material properties (entire section acts in tension or compression)

• Optimal for high strength, lightweight materials, ie steel, aluminum, FRP

• Ideal for long spans, ie. roofs and bridges• Constructability is efficient, ie. build on ground or

in fab shop and lift into place


Truss Disadvantages

• Requires significant total depth, which increases nonstructural cladding.

• With long spans, vibrations tend to be a problem in terms of both magnitude and frequency of vibration.

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Steel Bar Joists

• Steel bar joists can be economical in some buildings, ie. roofs and floors in any Walmart, sporting complex, or newer office buildings


Frame SystemsIBC 2000

• Building Frame– Complete space frame systems providing support for gravity loads

and seismic resistance is provided by shear walls or braced frames

• Dual Frame– Complete space frame systems providing support for gravity loads

and seismic resistance is provided by the space frame and shear walls or braced frames

• Space Frame– Members that are capable of supporting gravity loads and also

provide resistance to seismic forces

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Moment Frame SystemsIBC 2000

• Ordinary (OMF)– Members and joints are capable of resisting forces by flexure as

well as along the member axis

• Intermediate (IMF)– Members and joints are capable of resisting forces by flexure as

well as along the member axis with some extra detailing requirements for ductility

• Special (SMF)– Members and joints are capable of resisting forces by flexure as

well as along the member axis with special detailing requirement for ductility


Frame: Coplanar system of beam and column elements dominated by flexural deformation


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Basic Behavior

Gravity Load Lateral Loading


2D vs. 3D Frames (Plan)

Planar Space

Gravity Frame Lateral Frame

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Frame Advantages

• Optimum use of floor space, ie. optimal for office bldgs, retail, parking structures where open space is required.

• Relatively simple and experienced construction process• Generally economical for low-to mid-rise construction

(less than about 20 stories)• In Houston, most frames are made of reinforced concrete.


Frame Disadvantages

• Generally, frames are flexible structures and lateral deflections generally control the design process for buildings with greater than about 4 stories. Note that concrete frame are about 8 times stiffer than steel frames of the same strength.

• Span lengths are limited with using normal reinforced concrete (generally less than about 40 ft, but up to about 50 ft). Span lengths can be increased by using prestressed concrete.

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Frame Lateral Load SystemsFlat plate-column frame:

Plan Elevation

Effective slab width


Frame Lateral Load SystemsBeam-column frame:

Elevation

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Frame Lateral Load SystemsDiaphragm (shear) element: Carries lateral loading to the lateral load resisting system

Lateral load frame, typ.

Plate element

Deformed shape -Lateral load distributes to frames proportional to tributary area


Frame Lateral Load SystemsFor relatively square plans, diaphragms are generally considered rigid

Space frame with square plan

Deformed shape has constant lateral displacement - No diaphragm flexibility, ie. lateral load distributes to frame proportional to frame stiffness

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Braced Frame: Coplanar system of beam and column elements dominated by flexural deformation and truss elements dominated by axial deformation



Braced FramesConcentric

Elevation

Eccentric

Link elements

Truss elements

Elevation

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Concentrically Braced FramesBeam-column frame:

ElevationNote: Deformations are a function of axial stiffness in truss elements


Eccentrically Braced FramesBeam-column frame:

ElevationNote: Deformations are a function of shear stiffness in link elements

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Frame Lateral Load SystemsDiaphragm (shear) element: Carries lateral loading to the lateral load resisting system

Lateral load frame, typ.

Plate element

Deformed shape -Lateral load distributes to frames proportional to tributary area


Braced Frame Advantages

• Much stiffer and stronger lateral load system when compared with frame systems

• Optimum use of floor space. Most braced frames are on the building perimeter or near elevator stairwell

• Generally economical for low-rise construction (less than about 5 stories)

• Eccentricity braced frames have superior seismic resistance due to very ductile link elements (fuses)

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Frame Disadvantages

• Architectural constraints. Sometimes braces must be hidden and other times can be visualized as part of the architectural scheme.


Shear Wall Lateral Load SystemsShear wall

Elevation

Edge column

Interior gravity frames

Shear deformations generally govern

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Shear Wall Lateral Load Systems

Gravity frames

Shear walls

Coupling beams

Elevator shaft configuration

Hole


Dual Lateral Load Systems

Lateral frames –25% of lateral load, minimum

Shear walls

Wall-Frame Dual System:

Hole

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Non-Structural System

• Cladding – concrete, masonry, glass, etc• Electrical, mechanical, HVAC, etc.• Ceilings, partition walls, book cases, filing cabinets,

elevated computer floors, etc.


Floor Diaphragm Flexibility

• Can be a concern with exterior braced frames or shear wall systems that have a rectangular floor plan

• Special design considerations must be followed• According to IBC 2000, lateral forces get distributed to the

lateral force resisting system– Proportional to the frame stiffness for rigid diaphragms– Proportional to the tributary mass that each frame carries for

flexible diaphragms

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