Most building codes require the following elements to be designed for VERTICAL seismic excitation:
1. Cantilever more than 5m span
2. Beams more than 20 m span
3. Pre stressed concrete
I can understand why the first two elements shall be designed for vertical excitation. What I want to understand is why pre stressed concrete is supposed to be designed for VERTICAL seismic excitation?
Please help me.
You may well understand this concept if you know the method proposed by T Y Lin i.e. load balancing concept. When acceleration is up it is safe as this will lead to further reduction of load balancing. However when this is reverse the load increase and same has not been compensated and therefore this must be considered in addition to the imposed load.
This is mainly because pre-stressed floor components generally have long spans.
Firstly as Hamdy Abou-Elfath said sections are of large lengths, Secondly if you might have observed a pre-stressed section, especially beam or slab they get convexed upward after pre-stressing is provided in the sections, if there occurs more upward force due to vertical excitation of the building there can occur formation of cracks in the slab/beams. These upward accelerations are usually unable to be transferred to the Columns, as they are compression members failing in the case of tensions. Some time back I had discussed the character of a pre-stressed slab with a Structural Engineer working in it from a decade, he stated that they usually prefer the slab as a combination of various cantilever slabs.
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