EARTHQUAKE RESISTANT ANALYSIS AND DESIGN OF RC BUILDING WITH BRACES

Abstract

Reinforced concrete (RC) buildings are most often designed as moment resisting frames with brick-masonary infills serving as partitions. In some cases they are also designed with RC shear walls. Accordingly IS 1893 (Part 1): 2002 Table 7 gives the values of Response Reduction Factors, R, for the different types of RC buildings as i) R = 3 for ordinary RC moment-resisting frame (OMRF), (ii) R = 5 for special RC moment-resisting frame (SMRF), (iii) R = 3 for ordinary reinforced concrete shear walls, (iv) R = 4 for ductile shear walls, (v) R = 3 for ordinary shear wall with OMRF, (vi) R = 4 for ordinary shear wall with SMRF, (vii) R = 4.5 for ductile shear wall with OMRF, (viii) R = 5 for ductile shear wall with SMRF. For steel buildings the values of R are: (i) R = 4 for steel frame with concentric braces, (ii) R = 5 for steel frame with eccentric braced frames. Buildings are usually vulnerable to lateral loads caused by earthquakes. Trussed frames expected to be more efficient in resisting lateral loads than moment resisting frames. Therefore, RC braced frame building should also be explored for its structural economy in resisting earthquake loads. IS 1893 (Part 1) does not specify that RC buildings could also be designed as braced frames. The concentric braced frames are more efficient in resisting lateral loads. Therefore, in this thesis, an attempt bias been made to study the structural economics of Ordinary Braced Frames (OBF) with respect to Ordinary moment resisting frames (OMRF) and Special moment resisting frames (SMRF).' The meaning of `structural economics' is restricted to comparison, of the structural cost of the material, i.e., steel and concrete required in the beams, columns and bracings of the considered buildings. The material required for foundation, slab, labour charges, etc, are not included. , In the present study, a regular RC building is analyzed and designed for zone IV and medium soil strata as per IS 1893 (part 1): 2002 both as 'OMRF (Case 1)' and' as a SMRF (Case 2). Theh-the building as designed as OBF with ~ kCase 3). The sizes of'beams are kept same but sizes of column are 'different for all three cases.°Cost of steel reinforcement and cost of concrete for all the three cases is then calculated, to study the effect, of bracing on-the•structural economics in earthquake resistant design of RC buildings. It was found that the structural cost of OMRF (I = 1) and SMRF '(I' _ ' 1) 'is' increased by 48.16% and 17.46 % respectively, in comparison to the structural cost of OBF (I = '1). To study the relative efficiency of OMRF, SMRF and OBF, the seismic forces where" increased-by the increasing the Importance Factor to I = 1.5. Again, the' analysis and design of the building is carried out as OMRF (Case 4), SMRF (Case 5) and OBF (Case 6):' 1'' It was -found that the, the structural cost of OMRF (I = 1.5) increased by 17.81 % in comparison to OMRF '' (I = 1). Similarly for S'MRF (I = 1.5) increased" by 1803' % - in: comparison to SMRF (I = 1) and for OBF (I = 1.5) increased by 6.19 % in comparison to OBF(I=1). Buildings are modeled and analysed using SAP 2000 software while their design conforms to IS 456, IS 13920 and IS 1893 (Part 1):2000.