Page 175 - Dynamic Loading and Design of Structures

Page 151

The design base shear is

(4.27)

where W is the ‘seismic’ dead load, including the total dead load and applicable portions of
2
other loads (partition load of at least 0.5 kN/m , permanent equipment, etc.), and the factors
Ca, Cv, I and R, that define the design spectrum were discussed in the previous section. Two
lower bounds to V , are set in UBC
b
●For all seismic zones

(4.28a)

●For seismic Zone 4 (highest)

(4.28b)

Note that this is to account for near source effects, as discussed in Section 4.3.2. The
fundamental period T1 is calculated using the same procedures as in EC8. The distribution of
lateral forces along the height of the structure is given by

(4.29)

It is seen that this is the same as the simplified version of eqn (4.24), wherein the heights zi
replace the mode shape amplitudes jy, with the exception that part of the total base shear is
applied as a concentrated force at the top, F=0.07 T V , which need not exceed 25 per cent of
t
l
b
the total base shear and may be taken as zero for T ≤0.7 sec. The top force F is a simple way
t
1
of accounting for the effect of higher modes on the force pattern and is important for tall
buildings only.
The 0.05L accidental eccentricity discussed previously for EC8 is also specified in UBC,
but no provisions are included regarding the ‘dynamic’ eccentricity (e in EC8).
2
The following combinations involving the seismic loading E are specified in UBC (the
notation for loads has been changed here to facilitate comparison with EC8)

(4.30a)

(4.30b)