In all the codes in one way or the other the following parameters are involved. • elastic seismic coefficient (base shear); • design force reduction; • lateral force distribution; • overturning moments.
Comparison of codes
In all the codes in one way or the other the following parameters are involved.
• elastic seismic coefficient (base shear);
• design force reduction;
• lateral force distribution;
• overturning moments.
1 Base shear
NEHRP provisions, the source document of the IBC, gives the period formula identical to the Rayleigh method in which static deflection due to a set of lateral forces Fi is ui assumed as shape function. The period formula given by the NBCC has the same base except that the lateral forces used to determine the static deflections are equal to the lumped weight at the floors. For a linear elastic system we have seen that elastic seismic coefficient Ce is related to pseudo-acceleration spectrum for an SDOF system
By taking R = Q′ = q′ = 1 Ce in the building code corresponds to A/g and these two values given by Eq. 19.65 and 19.66 are not identical.
2 Design force reduction
Most codes specify that the design base shear to be smaller than the elastic base shear (determined from using elastic seismic coefficient Ce). For most of the codes discussed the reduction factors are R, Q′ and q′ where R = Q′ = q′ = 4 factors independent of T1 in IBC and NBCC and depends on period in MFDC and EC. The actual strength of the building exceeds design strength, especially for short period systems. The over-strength of a building is usually not recognized explicitly in building codes.
3 Lateral force distribution
According to structural dynamics force at any floor level i
If φjn is proportional to hj then NBCC agrees with structural dynamics formula. Linear mode shape is a reasonable shape for a building in between ρ = 0 and ∞ (ρ = beam to column stiffness). In IBC the height-wise distribution of lateral force based on the assumption that lateral displacement is proportional to the heights when T1 ≤ 0.5 s and to h 2j when T1 ≥ 2.5 s and to an intermediate power of the height for intermediate values of T1. This is intended to recognize the changing fundamental mode and increasing higher mode contributions to response with increasing T1. The Indian code uses
but stipulates dynamic analysis for regular buildings h > 40 m in Zones IV and V and h > 90 m in Zones II and III. In such cases
and storey shears for all modes must be done using SRSS and CQC rules.
4 Overturning moments
Certain building codes such as NBC, IBC, and MFDC allow reduction of overturning moment relative to the values computed by lateral force Fj by statics because the response contribution of higher modes are more significant for storey shear than the overturning moments. The reduction factor in IBC at the base of the building varies between 1 (no reduction) and 0.8, depending on the number of stories. The EC-8 permits no reduction of overturning moments relative to the values computed from lateral forces by statics.
In the following, we will calculate shear in various buildings based on IS1893 2002, Part 1. The method used in the other codes will be similar except for some variations. The reader can adapt the following problems according to their country’s seismic code or international building code.