Structural analysis of the repaired structural frame should be based on methods of analysis and load arrangements as set out in current codes of practice. In addition to normal
assumptions, the analysis should take due account of any dimensional changes, lack of verticality and residual forces which could have resulted from the elevated temperatures during the fire. As discussed in Section 2.2.5, a column with a large out of plumb will be required to carry substantially greater bending moments than would be assumed in normal design.
3.2.2 Element design
The design of the reinstated and repaired concrete elements should be based on design methods as set out in current codes of practice.
Repaired concrete elements will comprise a combination of the remaining section of the existing member and the repair materials. Modified material parameters may have been established in the fire damage assessment process and must be taken into account in the redesign of the repaired members. The strength properties of the repair material should be used where appropriate.
3.3 REPAIR CRITERIA
3.3.1 Reduced material strengths
In the design of the repaired section of the structure it is necessary to take account of the reduced strength of the remaining concrete and reinforcement which may have suffered from the elevated temperatures during the fire. Ideally this reduction in strength should be based on the results of tests on samples taken from the sections of the fire damaged zone of the
structure which are to remain after the repairs are complete. The concrete strength should be based on compression tests on concrete cores, see Section 2.3.5. Similarly the design strength of the steel reinforcement should be based on tensile tests on reinforcement samples taken from the fire zone, see Section 2.3.6.
Due account may taken of the fact that the test results give an indication of the actual strength of the concrete and steel by making appropriate modifications to the material strength factors. Further information may be found in the Institution of Structural Engineers document
Appraisal of existing structures(35) and the Highways Agency Assessment of concrete highway bridges and structures(36).
As an alternative to the above, where there is some knowledge of the fire temperature, residual strength factors obtained from Figures 3 and 10 may be used. The factors are to be applied to the design stresses in the residual concrete and reinforcement. An average factor for the whole member may be obtained by considering separate layers in a cross-section. Stresses in compression, tension, shear, torsion and bond are to be reduced in this way. If no data exists regarding the original strength of the concrete it should be determined from cores taken from existing undamaged concrete. The strength of the reinforcement should be determined from samples taken from sections of the undamaged concrete structure.
3.3.3 Bond strength
Provided the full cover to the existing reinforcement, along with any shear reinforcement, is reinstated during the repair, full bond between the existing reinforcement and surrounding repair material may be assumed and hence lap and anchorage lengths will not be
compromised. Where existing bars have buckled (see Section 2.2.7 and Figure 11) they will no longer be adequately bonded to the concrete. In this situation, it will be necessary to remove sufficient concrete from behind the bars so that the repair material fully surrounds the reinforcement, to ensure full composite action.
3.3.4 Bar size and spacing
Bar spacing should be sufficient to ensure full compaction of the repair material, see Section 4.6.1. Bar spacing should also consider the direction from which the sprayed concrete or other material is to be applied. Some reduction in the spacing may be considered where there is access from several faces. Adequate compaction is vital to successful repair, and any deviations from the recommended minimum should be discussed with the repair contractor. 3.3.5 Shear reinforcement
Additional shear reinforcement should be anchored in such a way to enable it to function properly with the undamaged portions of the member concerned, see Section 4.6.2 and Figure B.10.
3.4 MEMBER DESIGN
3.4.1 General
Member design should take into account the stress history of the remaining concrete and steel. Beams and slabs may be propped during the repair process and the undamaged concrete may therefore be assumed to be unstressed at the time of the repair. However the remaining concrete in columns and walls may be highly stressed at the time of the repair by loads from the structure above. In these cases careful consideration needs to be given to the distribution of loads between the remaining concrete and steel and the new repair materials.
3.4.2 Beams and slabs – bending
Where appropriate, beams and slabs may be repaired and strengthened to resist the applied bending moments by adding repair concrete and tension reinforcement. The added
tensile force which can be resisted by the original tension reinforcement, taking into account the residual strength. The approach is illustrated by Examples 1 and 2 in Appendix B. 3.4.3 Beams – shear
Where appropriate, beams may be strengthened to resist the redesign shear forces by adding shear reinforcement. The added reinforcement must be sufficient to resist the redesign shear force minus the permissible shear forces, which can be resisted by the original shear
reinforcement taking into account the residual strength for both the steel and the concrete. The approach is illustrated by Example 2 in Appendix B.
3.4.4 Columns
Columns may be strengthened to resist the redesign loads by adding reinforcement and repair concrete. The resultant section comprising the repair materials and the original concrete and reinforcement, taking account of the residual strengths, must be sufficient to resist the
redesign axial load and bending moments. Example 3 in Appendix B shows the approach for an axially loaded column. Special consideration should be given to columns with a permanent lack of verticality and the resulting additional P-∆ effect should be included in the design based on measured imperfections. The design should also account of any potential permanent reduction of the stiffness of the concrete and reinforcement due to the fire. This becomes particularly important for a slender column, whose failure mode would be dominated by stability rather then strength.
Designers should note the possible difficulties in adding longitudinal reinforcement due to overhead obstructions from upper floor slabs, beams, etc, particularly where bending is a dominant influence in the column design and it is necessary to provide adequate anchorage beyond the point at which the additional steel is no longer required.
3.4.5 Walls
The general principles of column strengthening may be applied to walls but there may be difficulty in threading new bars through the remaining floors. It is recommended that vertical wall reinforcement should be therefore be nominal, terminating above and below the floor slabs, and that the original, undamaged concrete acting together with the repair should accordingly be capable of achieving the desired strength as an unreinforced wall wherever possible. This recommendation will not apply to shear walls which may be heavily reinforced to resist lateral forces and for which special treatment to ensure maintenance of continuity of the steel reinforcement may be required in designing the repair.
3.5 DESIGN OUTPUT
3.6.1 Demolition and construction sequence drawings
The designer should prepare drawings which clearly show the extent of demolition of the fire damaged structure and should include details of any associated temporary propping which may be required in the temporary condition to enable the repair works to be carried out safely.
In some instances it will be necessary to provide temporary propping beyond the extent of the damaged structure (e.g. where demolition could cause increased design forces in adjacent spans).
Where critical to the design assumptions the designer should prepare construction sequence drawings showing the timing of the installation and removal of any temporary propping and the sequence of the repair operation.
3.5.2 Key plans
Key plans should be prepared at each floor level showing the location of the repair work and the positioning of the detailed sections.
3.5.3 Design details
Design details are required at each of the repair locations. These will be determined on the basis of testing (as described in Chapter 2) along with practical considerations such as the method of breaking out the damaged concrete and the subsequent method of repair (see Chapter 4). The details should include the following information:
• The extent of breaking out of fire damaged concrete and removal of fire damaged reinforcement steel.
• Requirements for preparation of concrete surfaces that are to receive repair concrete including any special requirements to prevent feathered edges.
• Details of new steel reinforcement including lap length and splicing with original bars, mechanical anchorage, cover etc.
• Any fabric reinforcement that may be required to hold the repair concrete in place in the temporary condition, including means of supporting the fabric and the required concrete cover.
• The thickness and the properties of the repair concrete.
Some typical repair details, using sprayed concrete, are shown in Section 4.9.7. Similar details will be used for hand-applied repair materials or concrete cast in formwork.
3.5.4 Specifications
In addition to the design drawings and details the designer should prepare detailed material and workmanship specifications for the repair work. This should include full information on the repair materials and include means for ensuring quality control.
Information on suitable repair methods is given in Chapter 4. 3.5.5 Design calculations
The designer should prepare design calculations for the repair works. The calculations should clearly set out all assumptions used in preparing the repair information, including the
following:
• Material properties of the existing structure
• Residual strength of fire damaged concrete and steel to remain in the repaired building • Material properties of repair concrete and new steel reinforcement
• Design loadings
• Fire resistance requirements for the repaired structure • Durability requirements for the repaired structure
• Analysis method including any simplifying assumptions to be used for the frame analyses
• Load arrangements to be used in structural analyses
• Details of the frame analyses including design forces at the repair sections • Member design based on these design forces.
3.5.6 Method statements
Method statements giving full details of the proposed means of carrying out the repair works should be prepared by the repair sub-contractor and submitted to the design engineer for comment before commencing work on site.
3.6 LOAD TESTS
Quality control of the materials carried out in the repair will generally be by routine tests on the compressive strength of the repair concrete and tension tests on the steel reinforcement. Where there is any uncertainty about the material strength or the quality of the repairs a load test may be carried out on the repaired structure, although this is not generally considered to be routine.
Where the circumstances are such that load testing is deemed to be necessary it should be carried out in accordance with current codes of practice, such as Section 9 of BS 8110 Part 2(37). Some general guidance on load testing is included in Institution of Structural Engineers’ Appraisal of existing structures(35).