For scheme design, you might have increased that wall thickness from a nice looking 400 mm for a 22 storey building to an awkward size of 800 mm thick and yet instead of the demand decreasing and solving the problem once and for all, the failing walls just seem to creep up to the next floor level.
That’s what happened to one of my projects recently. With everything else checked and assured sensible, the thicker wall sucked up more forces than the previous thinner section did and it affected portions of the wall on the succeeding floors.
Aside from that, the link beams are all over the place, stretched well beyond their limits. Add to that our agitation to the thought that no more walls are allowed by the architects and we need to finish it soon because of the deadline.
If you’re relying on shearwalls to stabilize your structure from lateral loads and you’re basically out of options to decrease the load or alter the current layout and you have to make it work, (“piniga ang design” in my native idiom) there is a way.
Crack that. No, forget Souljaboy, crack the walls I mean.
The idea is to crack the walls that have stresses exceeding the modulus of rupture (see above equation from ACI 318M-11). Since the cracked walls will have reduced modifiers, the excess force that it can no longer carry will be passed to other walls that still have reserve strength (that is, it did not yet reach it’s theoretical cracking limit).
I also checked the walls adjacent to the problematic ones that are supposed to be taking care of the push-pull action. They’re underutilized. We cant find a way to tie the underutilized walls and the problematic ones to hopefully engage them together in a composite action. “Crack the problematic ones,” said my design manager, “para maipasa sa mga tamad” (so that the slack walls will work.) We’ve had a good laugh after that.
Thankfully it did work! The slackers finally took part. Before cracking the walls, I was getting a 3.98% (maximum allowed by the ACI 318 is 4.0%) reinforcement for the problematic wall but now it was down to 2.61%. at least there’s a bit allowance for contingencies.
Now, how did I crack the walls in ETABS? Follow this procedure:
- First is to create an envelope for ultimate load combinations (you wouldn’t want to investigate all the combinations one by one!)
- Make sure to select the “Max” option before viewing the f22 contour. A positive f22 contour denotes walls in tension in units of kN/m.
- Next is to get the equivalent cracking load per wall thickness. That is fr * thickness which is 0.62*(fc’)^0.5*thickness with a resulting unit of N/mm or kN/m. All walls exceeding this value need to be cracked (you need to do this visually and manually by checking each wall elevation) . Crack only the panels that exceed the modulus of rupture.
- In cracking, apply a property modifier of f11 = f22 = m11 = m22 = m12 = 0.35 which is pursuant to section 10.10.4.1 of ACI 318M-11.
- After cracking, run analysis, make sure static and dynamic earthquakes are balanced, and design the walls. There will be a redistribution of forces until everything is in equilibrium.
Hopefully all walls will now work. This is a tedious solution and we only did it once and we never did another round of iteration after that. And judging the results which we found sensible, cracking the walls is definitely one of the solutions to go to if everything else (from reducing the load to altering the framing) is not possible or simply not allowed.