Lecture 3 Fundamentals

1
Lecture 3 - Fundamentals

• September 6, 2002
• CVEN 444

2
Lecture Goals

• Advantages and disadvantages of concrete
  structures
• Design Process
• Limit states
• Design Philosophy
• Loading

3
Advantages of Concrete Structures

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

4
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

5
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)

6
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

7
Disadvantages of Concrete Structures

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

8
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

10
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.

11
Design Process

• Phase 1 Definition of clients needs and
  priorities.
• Functional requirements
• Aesthetic requirements
• Budgetary requirements

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Design Process

• Phase 2 Development of project concept
• Develop possible layouts
• Approximate analysis preliminary members
  sizes/cost for each arrangement

13
Design Process

• Phase 2 Development of project concept
• Selection most desirable structural system
• Appropriateness
• Economical/Cost
• Maintainability

14
Design Process

• Phase 3 Design of individual system
• Structural analysis (based on preliminary design)
• Moments
• Shear forces
• Axial forces

15
Design Process

• Phase 3 Design of individual system(cont.)
• Member design
• Prepare construction days and specifications.
• Proportion members to resist forces
• aesthetics
• constructability
• maintainability

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Limit States and Design

• Limit State
• Condition in which a structure or structural
  element is no longer acceptable for its intended
  use.
• Major groups for RC structural limit states
• Ultimate
• Serviceability
• Special

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Ultimate Limit State

• Ultimate limit state
• structural collapse of all or part of the
  structure ( very low probability of occurrence)
  and loss of life can occur.
• Loss of equilibrium of a part or all of a
  structure as a rigid body (tipping, sliding of
  structure).

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Ultimate Limit States

• Ultimate limit state
• Rupture of critical components causing partial or
  complete collapse. (flexural, shear failure).

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Ultimate Limit States

• Progressive Collapse
• Minor local failure overloads causing adjacent
  members to failure entire structure collapses.
• Structural integrity is provided by tying the
  structure together with correct detailing of
  reinforcement provides alternative load paths in
  case of localized failure

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Ultimate Limit States

• Formation of a plastic mechanism - yielding of
  reinforced forms plastic hinges at enough
  sections to make structure unstable.
• Instability cased by deformations of structure
  causing buckling of members.
• Fatigue - members can fracture under repeated
  stress cycles of service loads (may cause
  collapse).

21
Serviceability Limit States

• Functional use of structure is disrupted, but
  collapse is not expected
• More often tolerated than an an ultimate limit
  state since less danger of loss of life.
• Excessive crack width leakage
  corrosion of reinforcement gradual
  deterioration of structure.

22
Serviceability Limit States

• More often tolerated than an an ultimate limit
  state since less danger of loss of life.
• Excessive deflections for normal service caused
  by possible effects
• malfunction of machinery
• visually unacceptable

23
Serviceability Limit States

• More often tolerated than an an ultimate limit
  state since less danger of loss of life.
• Excessive deflections for normal service caused
  by possible effects
• damage of nonstructural elements
• changes in force distributions
• ponding on roofs collapse of roof

24
Serviceability Limit States

• More often tolerated than an an ultimate limit
  state since less danger of loss of life.
• Undesirable vibrations
• vertical floors/ bridges
• lateral/torsional tall buildings
• Change in the loading

25
Special Limit States

• Damage/failure caused by abnormal conditions or
  loading.
• Extreme earthquakes damage/collapse
• Floods damage/collapse

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Special Limit States

• Damage/failure caused by abnormal conditions or
  loading.
• Effects of fire,explosions, or vehicular
  collisions.
• Effects of corrosion, deterioration
• Long-term physical or chemical instability

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Limit States Design

• Identify all potential modes of failure.
• Determine acceptable safety levels for normal
  structures building codes load
  combination/factors.

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Limit States Design

• Consider the significant limits states.
• Members are designed for ultimate limit states
• Serviceability is checked.
• Exceptions may include
• water tanks (crack width)
• monorails (deflection)

29
ACI Building Codes
Whenever two different materials , such as steel
and concrete, acting together, it is
understandable that the analysis for strength of
a reinforced concrete member has to be partial
empirical although rational. These semi-rational
principles and methods are being constant revised
and improved as a result of theoretical and
experimental research accumulate. The American
Concrete Institute (ACI), serves as clearing
house for these changes, issues building code
requirements.
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Design Philosophy

• Two philosophies of design have long prevalent.
• Working stress method focuses on conditions
  at service loads.
• Strength of design method focusing on
  conditions at loads greater than the service
  loads when failure may be imminent.
• The strength design method is deemed conceptually
  more realistic to establish structural safety.

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Strength Design Method
In the strength method, the service loads are
increased sufficiently by factors to obtain the
load at which failure is considered to be
imminent. This load is called the factored
load or factored service load.
32
Strength Design Method
Strength provide is computed in accordance with
rules and assumptions of behavior prescribed by
the building code and the strength required is
obtained by performing a structural analysis
using factored loads. The strength provided has
commonly referred to as ultimate strength.
However, it is a code defined value for strength
and not necessarily ultimate. The ACI Code
uses a conservative definition of strength.
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Safety Provisions
Structures and structural members must always be
designed to carry some reserve load above what is
expected under normal use.
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Safety Provisions
There are three main reasons why some sort of
safety factor are necessary in structural
design. 1 Variability in resistance. 2
Variability in loading. 3 Consequences of
failure.
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Variability in Resistance

• Variability of the strengths of concrete and
  reinforcement.
• Differences between the as-built dimensions and
  those found in structural drawings.
• Effects of simplification made in the derivation
  of the members resistance.

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Variability in Resistance
Comparison of measured and computed failure
moments based on all data for reinforced concrete
beams with fc gt 2000 psi.
37
Variability in Loading
Frequency distribution of sustained component of
live loads in offices.
38
Consequences of Failure
A number of subjective factors must be considered
in determining an acceptable level of safety.

• Potential loss of life.
• Cost of clearing the debris and replacement of
  the structure and its contents.
• Cost to society.
• Type of failure warning of failure, existence of
  alternative load paths.

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Margin of Safety
The distributions of the resistance and the
loading are used to get a probability of failure
of the structure.
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Margin of Safety
The term Y R - S is called the safety margin.
The probability of failure is defined as and
the safety index is
41
Loading

• SPECIFICATIONS
• Cities in the U.S. generally base their building
  code on one of the three model codes
• Uniform Building Code
• Basic Building Code (BOCA)
• Standard Building Code

42
Loading
These codes have been consolidated in the 2000
International Building Code. Loadings in these
codes are mainly based on ASCE Minimum Design
Loads for Buildings and Other Structures (ASCE
7-95) has been updated to ASCE 7-98.
43
Dead Loading

• Weight of all permanent construction
• Constant magnitude and fixed location

44
Dead Loads

• Examples
• Weight of the Structure
• (Walls, Floors, Roofs, Ceilings, Stairways)
• Fixed Service Equipment
• (HVAC, Piping Weights, Cable Tray, Etc.)
• Can Be Uncertain.
• pavement thickness
• earth fill over underground structure

45
Live Loads

• Loads produced by use and occupancy of the
  structure.
• Maximum loads likely to be produced by the
  intended use.
• Not less than the minimum uniformly distributed
  load given by Code.

46
Live Loads
See Table 2-1 from ASCE 7-95 Stairs and
exitways 100 psf Storage warehouses 125 psf
(light) 250 psf (heavy) Minimum
concentrated loads are also given in the codes.
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Live Loads
48
Live Loads
ASCE 7-95 allows reduced live loads for members
with influence area (AI) of 400 sq. ft. or
more where Lo ? 0.50 Lo for members
supporting one floor ?
0.40 Lo otherwise
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Live Loads
AI determined by raising member to be designed
by a unit amount. Portion of loaded area that is
raised AI Beam AI 2 tributary
area Column AI 4 tributary area Two-Way
Slab AI panel area
(see Fig. 2-10, text)
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Load Reduction
51
Environmental Loads

• Snow Loads
• Earthquake
• Wind
• Soil Pressure
• Ponding of Rainwater
• Temperature Differentials

52
Classification of Buildings for Wind, Snow and
Earthquake Loads
Based on Use Categories (I through IV)
Buildings and other structures that represent a
low hazard to human life in the event of a
failure (such as agricultural facilities) Buildin
gs/structures not in categories I, III, and IV
I II
53
Classification of Buildings for Wind, Snow and
Earthquake Loads
Based on Use Categories (I through IV)
Buildings/structures that represent a substantial
hazard to human life in the event of a failure
(assembly halls, schools, colleges, jails,
buildings containing toxic/explosive
substances)
III
54
Classification of Buildings for Wind, Snow and
Earthquake Loads
Based on Use Categories (I through IV)
Buildings/structures designated essential
facilities (hospitals, fire and police stations,
communication centers, power-generating stations)
IV
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Snow Loads
The coefficients of snow loads are defined in
weight.
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Snow Loads

• Ground Snow Loads (Map in Fig. 6, ASCE 7)
• Based on historical data (not always the maximum
  values)
• Basic equation in codes is for flat roof snow
  loads
• Additional equations for drifting effects, sloped
  roofs, etc.
• Use ACI live load factor
• No LL reduction factor allowed

57
Wind Loads

• Wind pressure is proportional to velocity squared
  (v2 )
• Wind velocity pressure qz

58
Wind Loads
where 0.00256 reflects mass density of air and
unit conversions. V Basic 3-second gust wind
speed (mph) at a height of 33 ft. above the
ground in open terrain. (150 chance of
exceedance in 1 year) Kz Exposure
coefficient (bldg. ht., roughness of terrain) kzt
Coefficient accounting for wind speed up over
hills I Importance factor
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Wind Loads
Design wind pressure, p qz G Cp G Gust
Response Factor Cp External pressure
coefficients (accounts for pressure directions
on building)
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Earthquake Loads

• Inertia forces caused by earthquake motion
• F m a
• Distribution of forces can be found using
  equivalent static force procedure (code, not
  allowed for every building) or using dynamic
  analysis procedures

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Earthquake Loads
Inertia forces caused by earthquake motion.
Equivalent Static Force Procedure for example, in
ASCE 7-95 V Cs W where V Total lateral
base shear Cs Seismic response
coefficient W Total dead load
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Earthquake Loads
Total Dead Load, W 1.0 Dead Load 0.25
Storage Loads larger of partition loads or 10
psf Weight of permanent equipment contents
of vessels 20 or more of snow load
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Earthquake Loads
where Cv Seismic coefficient based on soil
profile and Av Ca Seismic coefficient based on
soil profiled and Aa R Response modification
factor (ability to deform in inelastic range) T
Fundamental period of the structure
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Earthquake Loads
where T Fundamental period of the
structure T CT hn 3/4 where CT 0.030 for
MRF of concrete 0.020 for other concrete
buildings. hn Building height
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Earthquake Map
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Roof Loads

• Ponding of rainwater
• Roof must be able to support all rainwater that
  could accumulate in an area if primary drains
  were blocked.
• Ponding Failure
• ? Rain water ponds in area of maximum
  deflection
• ? increases deflection
• ? allows more accumulation of water ? cycle
  continues? potential failure

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Roof Loads

• Roof loads are in addition to snow loads
• Minimum loads for workers and construction
  materials during erection and repair

68
Construction Loads

• Construction materials
• Weight of formwork supporting weight of fresh
  concrete