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Flat-plate structure - sway analysis

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I performed lateral (wind load) analysis for a 24 story
shear walled flat plate structure as per ACI 318-99
clause 10.11.1. All the floors are post-tensioned slabs.
Plate bending elements were used to model the floor slabs
and walls in the 3D computer model. I used a reduced
floor slab stiffness of 0.25 Ig in my computer model.
For prestressed concrete slabs the code suggests to
take a higher value. But I used 0.25 Ig conservatively
for all my post tensioned slabs.

Many storys in the longitudinal direction were found
to be in sway condition (ACI 318-98 clause 10.11.4.2  )
and the maximum Q = 0.095. Shear walls are found to be
attracting 98 % of the lateral wind. The column moments
due to design wind load combinations are not significant
from design point because most of the wind load goes to
the walls. Architect was not inclined to increase the
wall length to make the building non-sway.

Lateral displacements determined using the service load
combinations (ACI R10.11.1) are less than building height/400
limit, which is common for most structures of this type.

At this stage I decided to design the building for the
sway effects. As per ACI 10.13, analysis was performed to
consider the effect of second-order displacements
for the two design wind load combinations. The second order
moments for 50% of the columns are found to be less than
the first order moments. This is because of the
redistribution of the forces within the structure where
in the stiff walls attracted even more lateral shear.

My questions are ...

1. My approach to the problem - is it correct ?

2. What stiffness reduction to be adopted for PT slabs ?
Where can I get the supporting information (published)
to convince the design reviewer ?

3. ACI 10.11.1 procedure  - is it applicable for shear-walled
flat-plate buildings when 98 % lateral load goes to the walls ?

4. For some columns the second-order moments are higher
than first-order moments ( both are less than 20% of the
moment capacity of the columns ). My reviewer feels that the
PT floor slab need to be checked for the second-order
moments because it is the floor that it transfering the
moment to the columns. To satisfy his querry I had included
the additional moment in the punching shear checks. Any
comments on this ??

Can some one please guide me.

Thank you,

Dr. M.V.K.V. Prasad
Senior Structural Engineer,
TRC Worldwide Enginnering
# 1052, 7th main, III block,
Koramangala,
Bangalore 560 034 INDIA




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