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Reason for considering compression reinforcement for long term Deflection

 
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Mayur Mistry
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Joined: 17 Sep 2016
Posts: 7

PostPosted: Sat Aug 05, 2017 11:57 am    Post subject: Reason for considering compression reinforcement for long term Deflection Reply with quote

Dear SEFIans , I want to know reasons for considering compression reinforcement for calculating long term Deflection .
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arunangshu
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Joined: 18 Sep 2009
Posts: 36
Location: New Delhi

PostPosted: Mon Aug 07, 2017 11:53 am    Post subject: Reply with quote

Mayur,

Compression reinforcement changes the stress-strain relationship of concrete section (by changing the transformed section) resulting into changes in cracked section property.
See Table 10-2 of PCA notes on ACI 318.

Apart from that, compression steel reduces creep and shrinkage deformation. Excerpt from ACI 209 is given below:

Compression steel in reinforced members and non-tensioned steel in prestressed members tend to offset the movement of the neutral axis caused by creep. The net movement of the neutral axis is the resultant of two movements. A movement towards the tensile reinforcement (increasing the concrete compression zone, which results in a reduction in the moment arm). This movement is caused by the effect of creep plus a reduction in the compression zone due to the progressive cracking in the tensile zone.
The second movement is produced by the increase in steel strains due to the reduction of the internal moment arm (plus the small effect, if any, of repeated live load cycles). As cracking progresses, steel strains increase further and reduce the moment arm.

Sum of above contributes to total long-term deflection and/or instantaneous deflection under live load.

Regards
Arun
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