Structural Design and Limit States
Aims and methods of design
Codes state that the aim of design is the achievement of an acceptable probability that the structure will perform satisfactorily during its life. It must carry the loads safely, not deform excessively and have adequate durability and resistance to effects of misuse and fire. Codes recognize that no structure can be made completely safe and that it is only possible to reduce the probability of failure to an acceptably low level.
The method recommended in the code is limit state design where account is taken of theory, experiment and experience. It adds that calculations alone are not sufficient to produce a safe, serviceable and durable structure. Correct selection of materials, quality control and supervision of construction are equally important.
Criteria for a safe design—limit states
The criterion for a safe design is that the structure should not become unfit for use, i.e. that it should not reach a limit state during its design life. This is achieved, in particular, by designing the structure to ensure that it does not reach
1. the ultimate limit state—the whole structure or its elements should not collapse, overturn or buckle when subjected to the design loads
2. serviceability limit states—the structure should not become unfit for use due to excessive deflection, cracking or vibration
The structure must also be durable, i.e. it must not deteriorate or be damaged excessively by the action of substances coming into contact with it. The code places particular emphasis on durability.
For reinforced concrete structures the normal practice is to design for the ultimate limit state, check for serviceability and take all necessary precautions to ensure durability.
Ultimate limit state
(a) Strength
The structure must be designed to carry the most severe combination of loads to which it is subjected. The sections of the elements must be capable of resisting the axial loads, shears and moments derived from the analysis.
The design is made for ultimate loads and design strengths of materials with partial safety factors applied to loads and material strengths. This permits uncertainties in the estimation of loads and in the performance of materials to be assessed separately.
The section strength is determined using plastic analysis based on the short-term design stress-strain curves for concrete and reinforcing steel.
(b) Stability
The layout should be such as to give a stable and robust structure. It stresses that the engineer responsible for overall stability should ensure compatibility of design and details of parts and components.
Overall stability of a structure is provided by shear walls, lift shafts, staircases and rigid frame action or a combination of these means. The structure should be such as to transmit all loads, dead, imposed and wind, safely to the foundations.
(c) Robustness
Code states that the planning and design should be such that damage to a small area or failure of a single element should not cause collapse of a major part of a structure. This means that the design should be resistant to progressive collapse. The code specifies that this type of failure can be avoided by taking the following precautions.
1. The structure should be capable of resisting notional horizontal loads applied at roof level and at each floor level. The loads are 1.5% of the characteristic dead weight of the structure between mid-height of the storey below and either midheight of the storey above or the roof surface. The wind load is not to be taken as less than the notional horizontal load.
2. All structures are to be provided with effective horizontal ties. These are
(a) peripheral ties
(b) internal ties
(c) horizontal ties to column and walls
3. For buildings of five or more storeys, key elements are to be identified, failure of which would cause more than a limited amount of damage. These key elements must be designed for a specially heavy ultimate load of 34 kN/m2 applied in any direction on the area supported by the member.
4. For buildings of five or more storeys it must be possible to remove any vertical loadbearing element other than a key element without causing more than a limited amount of damage. This requirement is generally achieved by the inclusion of vertical ties in addition to the other provisions noted above.
Serviceability limit states
The code states that account is to be taken of temperature, creep, shrinkage, sway and settlement.
The main serviceability limit states and code provisions are as follows.
(a) Deflection
The deformation of the structure should not adversely affect its efficiency or
appearance. Deflections may be calculated, but in normal cases span-to-effective depth ratios can be used to check compliance with requirements.
(b) Cracking
Cracking should be kept within reasonable limits by correct detailing. Crack widths can be calculated, but in normal cases cracking can be controlled by adhering to detailing rules with regard to bar spacing in zones where the concrete is in tension.
In analysing a section for the serviceability limit states the behavior is assessed assuming a linear elastic relationship for steel and concrete stresses. Allowance is made for the stiffening effect of concrete in the tension zone and for creep and shrinkage.
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