Two-way Slab: Balance 50% - 90% Dead Load in Each Direction?

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Two-way Slab: Balance 50% - 90% Dead Load in Each Direction?

Postby ericqianwang on Thu Jan 27, 2011 9:06 pm

To design a two-way PT slab, we usually design the tendons strip-by-strip in Adapt PT first and then export the tendons into Adapt Builder for analysis.

As suggested by Bijan, we should have the PT tendons balance 50% - 90% of the slab dead load in each direction. But the question is: For a two-way PT slab, if we have the banded tendons balance say 90% of the dead load in one direction and have the distributed tendons balance another 90% in the other direction, the slab may be overbalanced to 180% that will cause cracking at the slab top.

Does the above concern make sense? How much balance % shall I specify in Adapt PT when designing banded tendons and distributed tendons respectively? 45%?

-Eric Q. Wang, PE, MSCE
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Re: Two-way Slab: Balance 50% - 90% Dead Load in Each Direction?

Postby Dr_Bijan_Aalami on Mon Feb 07, 2011 4:46 am

Column supported floor slabs, in particular where the supports are an array of orthogonal columns are classified as two-way systems. There are two central reasons for the structural system of such floors to be treated as two-way.

First, the common practice is to place the slab reinforcement along the line of columns, as opposed in the diagonal directions. This necessitates that in each of the two support lines the selected design strips to be designed for the entire load on the slab (100 % of load). This is not a conservative assumption. It is the minimum requirement for safety of the structure. I hope to develop a Technical Note with greater details within the next week or two and post it on the web site.

Second, Since bending moments and stresses in column supported slabs significantly vary along the line of supports, a number of major building codes, such as International Building Code (IBC, ACI) define a specific set of allowable stresses for two-way systems. The allowable values for two-way systems are intended to address the large rise of stress over the columns.
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Re: Two-way Slab: Balance 50% - 90% Dead Load in Each Direction?

Postby sunrise1973 on Wed Apr 06, 2011 12:13 pm

[quote="Dr_Bijan_Aalami"] I hope to develop a Technical Note with greater details within the next week or two and post it on the web site. quote]

Dr_Bijan_Aalami,would you please tell me how to get this TN,thanks
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