Cracked Section Properties for Walls

[EBF]

I am using ETABS to model a concrete shear wall building. Section 1910.11.1 of the UBC provides modified properties to be determine lateral story drift. How do I determine if I should use the value for a cracked or uncracked section? Thanks!

 

[WillisV]

Determine if the tensile stresses in the shearwall are greater than the modulus of rupture (7.5 *sqrt f'c) of the concrete (these would be the S22 stresses if the local x axis is in line with the global x axis). If so then the section is cracked. A very easy way to see this is to set the color gradient for the stress plot in etabs to show everything less than the modulus in shades of gray and everything larger than the modulus to bright red.

 

[UcfSE]

Verify what the modulus of rupture is per the UBC. I believe the UBC uses a different value than the value from the ACI that WillisV points out.

 

[jen4950]

This is not a clear cut thing. My colleagues in California AND New York completely blew a gasket when they figured out I actually used cracked sections in my seismic design of our big project in Las Vegas.

Supposedly everyone in a seismic zone ignores cracked sections and uses 0.7Ig. This includes our peer reviewer (Saiful Bouquet- the "AUTHORITY" on seismic design despite us being the EOR on the most recent tallest buildings in the world.. work including 101 Taipei, Petronas Towers, Burj Dubai, Soldier Field, 1180 Peachtree (great website btw), 181 West Madison, Shanghai Plaza 66 etc..)

 

[jen4950]

Using a color gradient on each shearwall face is what we do. Set it to your limit- say 2 root f'c and see what's cracked. Then tweak it depending on how intense your model is.

Most buildings are really straight-forward.. And ETABS 9 has made some really great improvements.

 

[WillisV]

Well MY peer reviewer "Shaky Dan" agrees with me as I have designed some of the most important structures around here including the local Walmart, the fire station, and even the new Red Lobster facade renovation...

 

[Murali27]

I agree with jen. Assumptions on cracked sections are not clear in the analysis. The values mentioned in the code are not manadatory, just an intial estimate only.

If we use 0.7Ig for wall design, which is really conservative (attracts more force). In the same analysis, if we assign 0.5Ig for coupling beams/frame members, then it is fine because it is going to effect distribution of forces.

Cracked sections should be arrived for serviceability and limit state in all the structural members(horizontal/vertical), which is really tedious.

Any more thoughts...

 

[dawn836]

well jen4 i agree with you in using stiffness reduction factor for walls to account for cracked section in analysis of tall Buildings and the other way to account for cracking of concrete sections would be material NON-LINEAR ANALYSIS,but i would to share a comment with you.

suppose i have shear wall section formed of 4 shell elements and stress in one or more shell element forming the shear wall section exceeded cracking limit then the four shell elements must be assigned reduction factor for tension cracked section.

In Most analysis cases when modification factors are used to allow for cracking this lead to increase stress in walls and columns sections than analysis done by using uncracked section.

What are the great improvments ETABS 9 has?

 

[jen4950]

Well- my current reference is a building that defies most logic. I have a building that is 600 feet tall (900 feet wide), with a bit over 120 individual and unique shear-wall segments. Six wall segments have the same local axis orientation in the whole building.

ALL uniformity in geometry was neglected in SD phase.

IMHO today's software does not have the capability of assigning cracked section properties.. period. Hence the value of the structural engineer and my position (i.e. not quite ready to get fired..) in determining the approximate behavior of the system.

For instance, with all of the proper algorithms, I got an estimate from one of the FEA software teams.. a perfect, fully automated analysis and design of the said project would take until 2015 with out current capabilities- and current design locked in.

Headline- the project WILL be finished fall 2009. (hell or high-water..)

 

[jen4950]

First pass- Semi-rigid diaphrams are worth every single penny.

 

[Murali27]

Dear Jen

I need a quick answer. Under ultimate state, though wall is not cracked, 0.7Ig need to be used in the analysis. How do we modify the stifness in ETABS. Are they f11 & f22.
Please let me know

We are currently using UBC code as a reference for analysis purpose. Stiffness modifiers as stated in UBC 1997 are for ultimate. It doesn't clearly indicate what to be used for wind tunnel study and vibration studies. But ACI has got the right direction. But we need to satisfy the UBC requirements.

Do i get any reference or cross reference from UBC to ACI with respect to stiffness modifiers?

Thanks
Murali G