modifiers for wall

[chekre]

hi all,

regarding the modifiers to apply to walls, I assign 0.7 to f22 for walls and then check the stresses using P/A + Mv/I to see if there is any tension area. if the tension stresses are higher than the modulus of rupture(generally in the upper stories where axial force is less) , 0.35 is assigned for f22 and drift(if any) is checked.

Any comments ?

 

[slickdeals]

You will start with Ig = 1.0 for the walls, as allowed by ACI (using a 1.43 multiplier on 0.7 Ig). If walls are cracked, apply 0.5 Ig and continue.

There is a good discussion in this paper

 

[chekre]

i generally start by 0.7. when checking serviceability, i multiply all the modifiers by 1.43. i think even when the walls are uncracked, u cant use the 1.0 modifiers since ACI puts an upper limit of 0.875 for compression members (section 10.10.4.1).

 

[chekre]

I was reading the paper. I dont understand the fact that the walls at lower level are cracked while the upper are uncracked. Generally, it is the inverse. At upper stories, the axial load is still low compared to the below. P/A + My/I will show tension stresses at upper while the P effect at lower is more pronounced.
Any comments ?

 

[johnbridge231]

Very useful document, I have to admit.. Thanks for sharing.

Analysis and Design of arbitrary cross sections
Reinforcement design to all major codes
Moment Curvature analysis

http://www.engissol.com/cross-section-analysis-design.html

 

[PicoStruc]

For seismic purpose, I start with 0.7 for F11 and F22 for seismic analysis, as prescribed by Concrete code A23.3-04 in Canada.
For wind, it is iterative depending of the cracking state as described in previous posts.

 

[chekre]

0.7 for f22 is enough i think. modifiers for f11 should be applied only to spandrels.
when columns ask for much more reinf't at the higher story and this ratio decreases directly to the minimum the story below, do u add reinf't just at that story ?