CVEN 483. Structural System Overview

Transcription

1 CVEN 483 Structural System Overview Dr. J. Bracci Fall 2001 Semester Presentation Overview 1. Building system primary function 2. Types of load 3. Building materials 4. Structural members 5. Structural systems CVEN 483 Structural Systems 2 1

2 1. Basic Building System Functions Support gravity loads for strength and serviceability during: 1. Normal use (service) conditions 2. Maximum considered use conditions 3. Environmental loading of varying intensities CVEN 483 Structural Systems 3 Vertical deflection (sag) Lateral deflection (sway) Dead, Live, etc. Wind or earthquakes Performance-Based Design: Control displacements within acceptable limits during service loading, factored loaded, and varying intensities of environmental loading CVEN 483 Structural Systems 4 2

3 2. Types of Load Gravity: Dead Live Impact Snow Rain/flood Lateral Wind Earthquake Soil lateral pressure Thermal Centrifugal CVEN 483 Structural Systems 5 3. Building Materials Reinforced Concrete Structural Steel Reinforced Masonry Wood Aluminum Metal Structures Fiber Reinforced Polymers CVEN 483 Structural Systems 6 3

4 Important Material Characteristics Modulus of Elasticity, E Yield point Strain hardening Ductility Toughness, hardness Creep, shrinkage, thermal, relaxation High-cycle fatigue CVEN 483 Structural Systems 7 Reinforced Concrete (Reference ACI ) (a) Unconfined Concrete f c 0.85 f c Stress 0.3 f c E = (f c) 0.5 ~0.1 f c ~ Strain, in/in CVEN 483 Structural Systems 8 4

5 Reinforced Concrete (Reference ACI ) (a) Confined Concrete f cc Stress f c ~> 0.03 Strain, in/in CVEN 483 Structural Systems 9 Reinforced Concrete (Reference ACI ) (b) Normal reinforcing Steel ASTM A615,A617 [40,60 ksi] (c) Prestressing bars & strands - A421,A416 [250, 270 ksi] Stress fu fy Stress fpu fpy ~0.002 ~ 0.16 Strain, in/in ~ 0.04 Strain, in/in CVEN 483 Structural Systems 10 5

6 Structural Steel (Reference AISC-LRFD 1998 or AISC-ASD 19??) ASTM A36, 572, 615 Stress ~0.002 ~ 0.16 Strain, in/in CVEN 483 Structural Systems 11 Reinforced Masonry (Reference ACI /ASCE 5-99/TMS ) ASTM C34, C56, C62, C126 (Clay or Shale) ASTM C55, C73, C90, C129, C744 (Concrete) f m Stress 0.33 f m Em = f m Strain, in/in CVEN 483 Structural Systems 12 6

7 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) CVEN 483 Structural Systems 13 Aluminum (Reference Metals Handbook) Advantages: 1. High strength/weight ratio 2. Minimal maintenance due to stability in most atmospheric environments 3. Fatigue advantages?? Applications: 1. Aircraft CVEN 483 Structural Systems 14 7

8 Metal Structures Advantages: Warehouse type structures CVEN 483 Structural Systems 15 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 CVEN 483 Structural Systems 16 8

9 4. Structural Members Truss elements, including cables (tension only) Beams Columns Slabs/plates/shells/folded plates Walls/diaphragms CVEN 483 Structural Systems 17 Truss Elements Defn: Two force members, ie axial loads at nodes only L F A,E F δ Elastic Properties: k a = EA/L (axial stiffness) σ = F/A (normal stress) δ = FL / EA (deflection) CVEN 483 Structural Systems 18 9

10 Beam Elements Defn: Members subject to bending and shear M V L E,I,A V M δ 1,Θ 1 δ 2,Θ 2 Elastic Properties: k b = f ( EI/L n ) (bending) σ = My/I (normal stress) k s = GA/L (shear) v = VQ/Ib (shear stress) δ b = f (load, support conditions, L, E, I) (bending) CVEN 483 Structural Systems 19 Column Elements δ 3 Defn: Members subject to bending, shear, and axial F L V F M V E,I,A M δ 1,Θ 1 δ 2,Θ 2 Elastic Properties: k a = EA/L (axial) σ a = F/A (normal stress) k b = f ( EI/L n ) (bending) σ b = My/I (normal stress) k s = GA/L (shear) v = VQ/Ib (shear stress) δ b = f (load, support conditions, L, E, I, A) (normal) CVEN 483 Structural Systems 20 10

11 Slab/Plate Elements Defn: Members subject to bi-directional bending & shear z y x M x, M y, and V z Θ x, Θ y, and δ z CVEN 483 Structural Systems 21 Wall/Diaphragm Elements Defn: Members subject to shear y x V x and V y δ x and δ y CVEN 483 Structural Systems 22 11

12 5. Structural Systems Gravity: Trusses Frames Walls Dual systems Plates Lateral: Trusses Frames Braced Frames Walls Dual systems Diaphragms CVEN 483 Structural Systems 23 Truss: Coplanar system of truss elements governed by axial deformations Planar (2D) Space (3D) CVEN 483 Structural Systems 24 12

13 Basic Truss Unit Stable Unstable CVEN 483 Structural Systems 25 Truss Types CVEN 483 Structural Systems 26 13

14 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 CVEN 483 Structural Systems 27 Truss Connections Modeled as pinned Gusset plates used to connect members at nodal points Riveted (past practice), high strength bolt, or welded connections CVEN 483 Structural Systems 28 14

15 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 CVEN 483 Structural Systems 29 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. CVEN 483 Structural Systems 30 15

16 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 CVEN 483 Structural Systems 31 Frame Systems IBC 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 CVEN 483 Structural Systems 32 16

17 Moment Frame Systems IBC 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 CVEN 483 Structural Systems 33 Frame: Coplanar system of beam and column elements dominated by flexural deformation Planar (2D) Space (3D) CVEN 483 Structural Systems 34 17

18 Basic Behavior Gravity Load Lateral Loading CVEN 483 Structural Systems 35 2D vs. 3D Frames (Plan) Gravity Frame Lateral Frame Planar Space CVEN 483 Structural Systems 36 18

19 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. CVEN 483 Structural Systems 37 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. CVEN 483 Structural Systems 38 19

20 Frame Lateral Load Systems Flat plate-column frame: Effective slab width Plan Elevation CVEN 483 Structural Systems 39 Frame Lateral Load Systems Beam-column frame: Elevation CVEN 483 Structural Systems 40 20

21 Frame Lateral Load Systems Diaphragm (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 CVEN 483 Structural Systems 41 Frame Lateral Load Systems For 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 CVEN 483 Structural Systems 42 21

22 Braced Frame: Coplanar system of beam and column elements dominated by flexural deformation and truss elements dominated by axial deformation Planar (2D) Space (3D) CVEN 483 Structural Systems 43 Concentric Braced Frames Eccentric Link elements Truss elements Elevation Elevation CVEN 483 Structural Systems 44 22

23 Concentrically Braced Frames Beam-column frame: Note: Deformations are a function of axial stiffness in truss elements Elevation CVEN 483 Structural Systems 45 Eccentrically Braced Frames Beam-column frame: Note: Deformations are a function of shear stiffness in link elements Elevation CVEN 483 Structural Systems 46 23

24 Frame Lateral Load Systems Diaphragm (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 CVEN 483 Structural Systems 47 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) CVEN 483 Structural Systems 48 24

25 Frame Disadvantages Architectural constraints. Sometimes braces must be hidden and other times can be visualized as part of the architectural scheme. CVEN 483 Structural Systems 49 Shear Wall Lateral Load Systems Shear wall Edge column Shear deformations generally govern Interior gravity frames Elevation CVEN 483 Structural Systems 50 25

26 Shear Wall Lateral Load Systems Elevator shaft configuration Gravity frames Hole Shear walls Coupling beams CVEN 483 Structural Systems 51 Dual Lateral Load Systems Wall-Frame Dual System: Lateral frames 25% of lateral load, minimum Hole Shear walls CVEN 483 Structural Systems 52 26

27 Non-Structural System Cladding concrete, masonry, glass, etc Electrical, mechanical, HVAC, etc. Ceilings, partition walls, book cases, filing cabinets, elevated computer floors, etc. CVEN 483 Structural Systems 53 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 CVEN 483 Structural Systems 54 27