Lecture 2 Structural System Overview

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Lecture 2Structural System Overview

• CVEN 444 -Structural Concrete Design
• January 15, 2003

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Presentation Overview

• RC structural systems
• RC structural members
• Advantages and disadvantages of concrete
  structures

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One-Way Joist Floor System
Rib (joist) slab (One-way bending)
2D gravity or lateral frames
2D lateral frames
Floor joists, type
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One-Way Joist Floor System
Rib (joist) slab with beams (One-way bending)
Lateral space frame
Floor joists, type
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One-Way Joist Floor System
Typical Joist
Top of Slab
8-24 for 30 Modules 16-24 for 53
Modules 14-24 for 66 Modules .
112 Slope, type
Width varies 4, 6 or larger

• 2 or 3 cc. Joists
• 4 or 6 cc. Skip joists
• 5 or 6 cc Wide-module joists

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One-Way Joist Floor System

• Advantages
• Longer spans with heavy loads
• Reduced dead load due to voids
• Electrical, mechanical etc. can be placed between
  voids
• Good vibration resistance
• Typical Applications
• Medium-to-long spans with heavy loading
• For 30 modules, 35 40 spans
• For 53 66 modules, 35 50 spans

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Two-Way Joist Floor System
Waffle slab (Two-way bending)
2D lateral frames
Waffle pans, type
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Two-Way Joist Floor System

• Advantages
• Longer spans with heavy loads
• Reduced dead load due to voids
• Electrical, mechanical etc. can be placed in
  voids
• Good vibration resistance
• Attractive Ceiling
• Typical Applications
• Long spans with heavy loading
• For 3, 4, and 5 modules, 40 50 spans and
  beyond

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Floor System Effective Cost(PCA 2000)
100
One-way joist
Flat Slab
Flat Plate
Live Load, psf
50
25
30
35
50
Bay Spacing, ft
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B. Lateral Load Systems

• Frame Overview
• Flat plate ( slab)-column (w/ and w/o drop
  panels and/or capitals) frame systems
• Beam-column frame systems
• Shear wall systems (building frame and bearing
  wall)
• Dual systems (frames and shear walls)

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Frame Coplanar system of beam (or slab) and
column elements dominated by flexural deformation
Planar (2D)
Space (3D)
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Basic Behavior
Gravity Load
Lateral Loading
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2D vs. 3D Frames (Plan)
2 or 4 frames , 2 frames
4 frames , 4 frames
Floor joists, type
Planar
Space
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Frame Advantages

• Optimum use of floor space, ie. optimal for
  office buildings, 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.

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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 frames are about 8
  times stiffer than steel frames of the same
  strength.
• Span lengths are limited when using normal
  reinforced concrete (generally less than about 40
  ft, but up to about 50 ft). Span lengths can be
  increased by using pre-stressed concrete.

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Frame Lateral Load Systems
Flat plate-column frame
Effective slab width
Elevation
Plan
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Frame Lateral Load Systems
Beam-column frame
Elevation
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Frame Lateral Load Systems
Diaphragm (shear) element Carries lateral
loading to the lateral load resisting system
Lateral load frame, type.
Plate element
Deformed shape -Lateral load distributes to
frames proportional to tributary area
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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
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Shear Wall Lateral Load Systems
Shear deformations generally govern
Shear wall
Edge column
Elevation
Interior gravity frames
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Shear Wall Lateral Load Systems
Elevator shaft configuration
Gravity frames
Shear walls
Hole
Coupling beams
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Dual Lateral Load Systems
Wall-Frame Dual System
Lateral frames 25 of lateral load, minimum
Hole
Shear walls
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4. Structural Members

• Beams
• Columns
• Slabs/plates/shells/folded plates
• Walls/diaphragms

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Beam Elements

• Defn Members subject to bending and shear

d2,Q2
d1,Q1

• Elastic Properties
• kb f ( EI/Ln) (bending) s My/I (normal
  stress)
• ks GA/L (shear) v VQ/Ib (shear stress)
• db f (load, support conditions, L, E, I)
  (bending)

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Column Elements

• Defn Members subject to bending, shear, and
  axial

V
L
d3
F
F
V
E,I,A
d2,Q2
d1,Q1
M
M

• Elastic Properties
• ka EA/L (axial) sa F/A (normal stress)
• kb f ( EI/Ln) (bending) sb My/I (normal
  stress)
• ks GA/L (shear) v VQ/Ib (shear stress)
• db f (load, support conditions, L, E, I, A)
  (normal)

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Slab/Plate Elements

• Defn Members subject to bi-directional bending
  shear

z
y
Mx, My, and Vz Qx, Qy, and dz
x
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Wall/Diaphragm Elements

• Defn Members subject to shear

y
Vx and Vx dx and dy
x
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Advantages of Concrete Structures

• Economical
• Thinner floor systems Reduced Building
  Height Lower wind loads (lt A)
  Saving in Cladding
• Materials widely available

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Advantages of Concrete Structures

• Suitability of material for architectural and
  structural function
• Concrete place in plastic condition - desired
  shape texture can be obtained with forms and
  finishing techniques
• Designer can choose shape and size

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Advantages of Concrete Structures

• Fire Resistance
• Concrete building have 1-3 hour fire rating with
  no fire proofing (steel and timber require
  fireproofing to obtain this rating)
• Rigidity
• Greater stiffness mass reduces oscillations
  (wind), floor vibrations (walking)

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Advantages of Concrete Structures

• Low Maintenance
• Availability of Materials
• Sand, gravel, cement, H20 concrete mixing
  facilities widely available
• Reinforcement - easy to transport as compared
  to structural steel

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Disadvantages of Concrete Structures

• Low tensile strength - 0.1 fc
  cracking if not properly reinforced

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Disadvantages of Concrete Structures

• Forms and Shoring (additional steps)
• Construction of forms
• Removal of forms
• Prepping (or shoring) the new concrete to support
  weight until strength is adequate.
• Labor/Materials cost not required for other types
  of materials

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Disadvantages of Concrete Structures

• Strength per unit volume is relatively low.
• fc (5-10 of steel)
• greater volume required
• long spans typical built with steel

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Disadvantages of Concrete Structures

• Time-dependent volume changes
• Concrete steel undergo similar expansion and
  contraction.
• Concrete undergoes drying shrinkage, which may
  cause deflections and cracking.
• Creep of concrete under sustained loads causes an
  increase in deflection with time.