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10/13/2012
Design Philosophy of Concrete Structures & Lessons Learnt
from Failures of Structures
Waleed A. Thanoon,
Professor
Civil Engineering Department
College of Engineering& Architecture
University of Nizwa
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1. Introduction
Reinforced concrete is a composite material consists from concrete and steel reinforcement. A full bond between the two materials is the main assumption used in designing reinforced concrete members.
Concrete has many advantages such as its cheapness, durability, versatility and high compressive strength but it has great disadvantage of being weak in tension.
Steel has considerably higher tensile strength but tends to be more expensive per unit weight and less durable. Combing both materials will result in a reinforced concrete material where the concrete resist the compressive force and the steel resist the tensile forces.
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Figure 1: Advantages and Disadvantages of concrete and steel Reinforcement
Property Concrete Steel
Strength in tension Poor Good
Strength in compression Good Good-but slender bars will buckle
Strength in shear Fair Good
Durability Good Fair-corrodes if unprotected
Resistance to fire Good Poor-suffers rapid loss of strength at
high temperatures
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Figure 2: Typical RC Section
Combing both materials will
result in a reinforced concrete
material where the concrete
resist the compressive force
and the steel resist the tensile
forces.
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2. Structural Design
Design is a creative process to satisfy specified
requirement. It requires a wide experience and
skill. Any design is a trial and error process.
In structural engineering design, the design
process may be divided into three stages:
a) Conceptual Design
b) Preliminary analysis and design
c) Detailed analysis and design
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a) Conceptual Design:
It consists of the preparing different drawing for the a few
possible structural systems which are safe, buildable,
economical and robust.
This stage of design, involves the identification of design
constrains and the synthesis of the structural systems which
comply with these constrains.
Some of these constrains, are the budget, site and size
restrictions, provisions for safe access, environmental
requirement and utility. These constrains are specified by
client’s and local planning and environmental council.
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b) Preliminary analysis and design:
This stage involves performing initial calculations to determine
whether the proposed structural systems are structurally
feasible.
Rules of thumb are used to determine preliminary sizes for the
various members and approximate methods are used to check
these sizes and to estimate the quantities of reinforcement
required.
This stage requires the experience of the structural engineer to
decide the sizes of different members.
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Text Book & References
W. H Mosley, RC Design
MacGinley & BS Choo, RC design Theory
and Examples
McKenzie, Design of Structural Elements
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Structural Members
A complete structure is essentially a combination of members which can be categorized by their main function. A typical reinforced concrete structure consists from floor slabs which mainly carry transverse load (perpendicular to its plane). Slabs transfer the load either to the beams or directly to the columns (called flat slab).
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Beams, which carry the load from the slabs and transfer it through bending and shear to the columns.
Columns are the vertical members which carries the load from the beam (or directly from the slab in case of flat slab). A typical column will be subjected to axial load, and bending moments. The ground columns will transfer the load to the
footings, which in turn transfer the load to the ground soil.
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Retaining Walls
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Reinforced
Concrete
Pre-Stressed
Concrete
Composite
Construction
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FLAT SLAB
FLAT
PLATE
FLOOR SLAB
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ONE WAY SLAB
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TWO-WAY SLAB
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RIBBED SLAB
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WAFFLE SLAB (TWO WAY RIBBED SLAB)
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BRIDGE
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BOX-GIRDER BRIDGE
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Tension The force is applied
parallel to the longitudinal axis of the member trying to stretch it.
It produces a uniform tension stress. In r.c. members
once the concrete cracks under the smallest tension force, the tensile force is carried solely by steel reinforcement .
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Compression
The force is applied parallel to the longitudinal axis of the member trying to squeeze it. It produces a uniform compressive stress.
Steel reinforcement will carries a greater proportion of the stress due to its high stiffness (steel has higher modulus of elasticity than concrete).
The compression member must have some flexural rigidity to prevent failure through buckling.
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Shear
The force is applied perpendicular to the axis of the member (parallel to the cross section of the member).
The shear force tried to cut the member.
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Torsion
Torsion occurs in members when transverse external force acts out side the plane containing the axis of the member.
It cause a twisting action in the loaded member
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Bending (Flexure)
Transversely loaded members transmit their load by bending action (flexure).
The bending moment will case sagging of the member.
Sagging will cause shortening in one side and elongation in the other opposite side.
This will crate compression and tension forces in both side respectively.
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SHEAR FAILURE
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FLEXURE FAILURE
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PUCHING FAILURE
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COLUMNS FAILURE -CRUSHING
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COLUMNS FAILURE -Crushing
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BRIDGE FAILURE
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SHEAR FLAILURE
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Failure of Airport Terminal
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JOINT FAILURE
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BAD DETAILING-COLUMN
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BAD DETAILING-BEAM
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CORROSION OF STEEL
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EXCESSIVE DEFLECTION
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SLOPE FAILURE
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Design Philosophy
Safety and economy play a significant role in any structural design. The
structure must be designed to carry the applied load with any type of
material failure (ultimate limit state). At the same time the structure
must be fit for use for its design life (serviceability limit state).
Limit states are defined as states beyond which the structure no longer
satisfies the performance requirements of the design.
In general the design requirements are:
• Strength (ultimate limit state): Material failure, structural
collapse
• Serviceability limit: Cracking, deflection, corrosion,
vibration….etc
• Stability: Buckling, Overturning, Sway etc.
• Structural Integrity: robustness, jointing….etc.
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Serviceability Limit States
Ultimate Limit States
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Of course the cost is another important factor which will determine the
structural system, material, finishing etc. In addition before doing
any analysis or design for any structure, the design criteria must be
defined. These are:
1. Type of use:
dictates the general value of the imposed loading, the fire resistance
requirements, internal exposure conditions and serviceability
criteria.
2. Location:
dictates the value of the wind loading and the external exposure
conditions
3. Finishes, Cladding: dictate the values of the permanent loading
4. Special requirements:
Limits on deflection, drift: dictates the structural system and/or
material.
Design Requirements
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Figure 32: Formwork
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Figure 33 Steel Reinforcement
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Figure 34: Preparation for casting
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Figure 35: Concreting & Finishing
