We are currently working on a RC garage model (11” slab thickness, max span ≈34’). We are noticing that our load history deflections are much larger than we are expecting.
When we run load history deflections we get a Final Instantaneous Deflection of 3.257” in our worst span.
We created a load combination based off of ACI 318-9.5.2.5 to compare this to. The load combination we created essential takes sustained loads (DL + 10%LL + 50%ParkingLL), multiplies them by the long-term multiplier from ACI, and then adds in Service loads (DL+LL). Using 2.0 as a conservative value for the long-term multiplier, our resulting load combination is as follows:
Using this formulation, we get a maximum deflection of 1.044" from the 'Max Deflection' plot under the load combination.
We are aware of some of the reasons stated in Section 65 of the Concept Manual as to why some deflection results are lower than those calculated in Load History deflections, but we are having a difficult time making sense of the difference being 3x what ACI predicts.
Are there any other reasons that could explain the large discrepancy?
I have attached the model in question for reference.
Thanks
When the Load history deflection predicts significantly larger deflection than a standard combinations times the creep factor, it's usually a matter of cracking. I use the Service Design - ECR plots to gauge the amount of predicted cracking, and in your model at many of the supports, the cracking results in approximately double curvature, over 50% cracked.
The other thing to watch out for are sections with no equilibrium. In your Load History Calc log there are dozens of these warnings. Here is the first:
Calculating Instantaneous Cross Section Load History Curvatures for Load Step Maximum Short Term Load Iteration 3 Cross section 437 has no equilibrium at (-251,9.199) (-257.6,9.199). Setting element stiffness to be very small.
Calculating Instantaneous Cross Section Load History Curvatures for Load Step Maximum Short Term Load Iteration 3
Cross section 437 has no equilibrium at (-251,9.199) (-257.6,9.199). Setting element stiffness to be very small.
So with all these fully cracked sections it's no surprise that the final load history results are so large. To improve the behavior, you might have to add some more user reinforcement, assuming there are no problems with the loads, load history steps or strip layout. As a test I increase the #5 @ 12 bottom bars to #6 at 12 and the max long term deflection dropped slightly to 3.0".
Answer Verified By: PPesek
Taking your suggestions from above, we tried to remove those two issues.
If we set our slab thickness to 18" in that same model, and rerun, we no longer get ECR spikes at the columns and equilibrium errors are reduced to just a couple that are not located near the maximum deflection point.
With those two errors/issues worked out, we were left with the following deflection values:
ACI 9.5 Method: 0.4153"
Load History Final Instantaneous: 1.17"
Any other thoughts on what might be causing the large discrepancy?
Garage Model w 18in slab.zip
Your Creep factor is 3.35, but your Dead load factor is only 3, reducing the creep factor to 3 would align better with your custom combo.
It's also informative to look at the second step "Sustained Load" vs. the "Final Instantaneous Load" step since your custom combo only represents half the Parking load sustained.
When I even the playing field in those ways, the load history sustained result is 1.0", still significantly larger that your combo.
We can deduce that the effects of shrinkage are significant. As a test I reduced the shrinkage restraint percentage from 10 to 1% and then the sustained deflections are dramatically reduced to 0.36". For more on the use of shrinkage restraint in load history calculations, check out this article:
communities.bentley.com/.../6191.aspx
Thanks for the help. We'll be looking more closely at some of these factors in the load history deflections.