about base shear

1. Base shear should be more or less than what?
Let us try tounderstand structural dynamics using some static approaches.
Building is heavy -> High Base Shear
Building is stiff -> High Base Shear
Building located near fault -> High Base Shear
1. Building Weight
Imagine you have two buildings. One weights 50,000 kips while the other weights
20,000 kips. And let us say that the ground is excited at 0.4g of acceleration. Now in no
way the entire building will be shaked at this value of acceleration, but just for the sake
of simplifying we say that it is.
So building 1 will have a base shear of 50,000 * 0.4 = 20,000 kips
Building 2 will have a base shear of 20,000*0.4 = 8,000 kips
So there you have variation of building base shear with respect to weight of the
structure.
2. Building Stiffness
Oh, this one is the ultimate parameter tounderstand the building base shear. Now, each
structure has its own natural period of vibration. Some buildings have 0.5 seconds,
some have 1.5 seconds and some have 8 seconds of natural period. Higher the natural
period of structure means the more flexible the structure is. A flexible structure
generally experiences lower accelerations than a stiff building. This is called response of
a structure. (Response Spectrum Analysis).
Now because a flexible building is hard to excite, it will have lower base shear as
compared to a stiff building. For example if you compare a shear wall building and a
moment frame building for the same building height and weight, then the base shear of
moment frame building will be lower than that of a shear wall building.
3. Distance from Fault
This is kind of understood. Where would you experience more violent shaking? Near
fault or away from fault? The amount of ground excitation plays an important role in
determining the base shear. In a site specific response spectrum, the acceleration at
ground is the value of acceleration at zero time period in the chart.
The most important part in designing the building to resist earthquakes is not just
capacity of structure to resist force. But the ability of structure tobe ductile enough to
dissipate energy. The ductility of the structure lies in capacity based design. If a

2. structure is well detailed, then it has more chances of surviving earthquakes than a
poorly detailed one.