Cracked Section Properties for Wind/Seismic Serviceability Analysis

[FootNMouth]

The policy is my office is that we used cracked section properties per ACI for wind strength analysis, however we use the gross section properties for serviceability analysis. My question is why can this be assumed. Let say for example that you have a design wind storm in the first year of service. At this point this building is likely completely cracked but may still remain structurally adequate. Are we not then saying that for the remaining life of the structure the building will experience drifts much greater than H/400? My guess is that this is assumed based on the probability that it will occur within the service life of the building.

 

[JoshPlumSE]

Remember that the code assumes the structure merely avoids collapse during a design event. It may have to be repaired, reinforced, retrofitted, torn down, whatever. The goal is merely life-safety.

So, in your example the building experiences it's design event in the first year. If it didn't fall down, then it worked as planned. Anything that happens after that is a bonus.

The better question is what happens if it gets a wind event that is 50% of the design event every year for 10 years. Is it sufficiently cracked at the end of that time that the next wind event (let's say at 70% of design event force levels) causes deflections significantly greater than H/400.

If the answer is no, then your analysis using gross properties is justified. If not, then maybe using a reduced stiffness in your analysis would be a better idea.

 

[bookowski]

Lots of opinions on this topic and there does not seem to be a standard in the industry. Most common thing that I see is running the service load combos and checking for either net tension > 0 or net tension > fr and then cracking those regions and iterating. Check out 8.8.1 in ACI318-08, this gives some guidance for modeling under service loads.

 

[wannabeSE]

ACI 318 (08 and 11) section 8.8 provides some guidance for member stiffness when considering lateral deflection.

 

[jvvse]

Isn't wind design made to be elastic and therefore not supposed to have any cracking? Or are you talking hurricane level wind?

Seismic governs where I come from, so was just curious about all this wind serviceability talk.

 

[JAE]

If the wind forces create effects in the members that create stresses higher than the cracking stress then the member is cracked.

It depends on the structural arrangement and magnitude of the moments and axial loads relative to the member sizes.