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 if 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 : Clause 2.2.2.1 of the code states that 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 load and wind , safely to the foundations.
(c) Robustness : Clause 2.2.2.2 of the 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 clause specifies that this type of failure can be avoided by taking the following precautions.
- The structure should be capable of resisting notional horizontal loads applied at roof level and at each floor level. The load are 1.5% of the characteristic dead weight of the structure between mid-height of the storey above or the roof surface. The wind load is not to be taken as less than the notional horizontal load.
- 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 building 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 34kN/m square applied in any direction on the area supported by the member. Provisions regarding the application of this load are set out in BS8110: Part 2, section 2.6.
4. For buildings of five or more storeys it must be possible to remove any vertical load bearing 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.