This was a real project that was in litigation for quite some time. Finally settled. Here is one of the issues that I would like to get comments on…
Project Information
High rise building, 20+ stories located along a body of water with a fetch distance of about 1 mile in one direction, less than a mile in the other direction. Area design wind speed is about 120 mph.
Reinforced concrete frame with post-tensioned slabs
Cold formed steel infilled exterior walls with numerous windows of various sizes.
Portland cement plaster (stucco) exterior cladding on gypsum sheathing
Here is the serviceability issue:
Cold formed steel studs were sized properly for the spacing at the stress levels from lateral wind loads and other loads .
The studs were checked for deflection to L/360 to comply with the requirements for stucco deflection.
At windows, studs were doubled to accommodate the opening.
Should the additional stiffness of the doubled studs (creates a “hard” spot in the wall) as compared to the single studs be considered in limiting the stucco cracking because of the differences in horizontal bending of the wall system or was it only necessary to check the vertical deflection of the individual single studs.
Your thoughts (I’ll give you the outcome after sufficient discussion)
@Ron I would not normally consider that as a problem. Depending on wind conditions (gustyness) and location of the openings on the surface (near corners) the l/360 might be light for a brittle surface. I would not use cement stucco on a highrise, the ‘plastic stuff’ has some flexibility.
Doubling the studs would likely solve more problems than it would create. Cracking in the vicinity might more likely be due to the discontinuity of the surface (aka ‘reentrant corners of windows and doors’) rather than the increased stiffness…
@Ron
I’ll bet most of the cracks in the stucco originated at the reentrant corners of the window and door openings. What did the stucco people do to prevent this, to reinforce the reentrant corners, with some diagonally placed mesh strips, or some such, to prevent this cracking? And, what do the drawings, details and specs. say should be done in this regard? The old std., first thought, has been to approx. replace what you’ve cut out, by adding studs (rebar, stressing strands, etc.) at/near the edge of the opening. The primary reason for the cold formed steel studs in this case is to support the exterior cladding, and it is assumed that the stud wall designer knew what the ext. finish would be. So, with a flexible stud wall and a brittle ext. finish material, one might spend a little more design time thinking about how the two interact. This type of problem might be one of the drawbacks of subbing out all of the specialties systems design. Everyone thinks about their own little niche, and the myriad of issues which fall in between each niche get lost in the confusion, and no one pays any attention to them. Then, you also have Arch’s. these days, insisting on all kinds of details, structural conditions and finish systems which are really difficult to make work in a reasonable and practical fashion, without stretching every aspect of the design process, and every one of those systems to the limits; and we wonder why we end up with these problems in the finished product. I’ll bet there is plenty of blame to go around here, irrespective of who have the deepest pockets. Without knowing exactly what you are looking at, what the actual conditions and cracking are, I might have missed this condition too, but I likely would have designed a slightly stiffer stud wall system in general, and caught hell for those $$ wasted. I’ve certainly seen this condition (reentrant corners) be a problem before on rigid exterior finishing materials. A detail I’ve recommended which always causes a fight with the Arch. is a caulking (caulk filled) reglet, at the opening jambs, running from floor to ceiling. Of course, those are the ugliest thing the Arch. has ever seen, until the cracking starts.
@Ron@dhengr Part of building design and construction is selecting envelope materials that are appropriate. A brittle material used in the envelope, for a highrise, or medium highrise would, to me, not be a suitable material. More $.02…
I’ve spent decades designing to solve problems… and, a lot of time spent on eliminating them, before they become problems.
I had an attached carport converted to a bedroom on my (former) home, a 1 story CBS house. The contractor used stucco over lath and wood studs to enclose the major openings.
Disaster; the stuff cracked almost immediately, and let in ants and rain.
We had another contractor replace the bottom of the windowed wall with solid concrete; much better. I guess CBS houses sway too much in the wind.
@dik…
The outcome was that I felt the design engineer should have considered the differential deflection between the “hard spots” and the open wall deflection. They disagreed and filed a complaint with our state board against me (1st complaint in 40 years of practice) when I stated in deposition that they did not follow an appropriate standard of care in evaluating deflection in two directions, not just one, and that even though the building code allows a load reduction for serviceability, that they had an obligation to evaluate the building, not just meet the minimum mandate of the code.
The state board evaluated the case (thousands of pages of documents on both sides of the issue) and decided there was no reason to move forward with their complaint against me and dropped the complaint.
My premise is now slowly being codified with serviceability more closely reaching the requirements of stress compliance. My position is that meeting minimum stress compliance does not necessarily mean the building will perform as expected and therefore serviceability needs more attention.
@dik…it did come up, but with cold formed steel wall infill, you have few choices for the exterior facade. Stucco would work fine for this if properly designed and constructed.
I did a 12 story building a few years back with appropriate breaks at each floor for drainage and it works fine.
@Ron Thanks Ron… I guess having gone down the road of using a brittle material, you have to design for it, and accommodate it using whatever stiffness and jointing is appropriate. I’m not sure what stiffness I would use… for steel studs with brick veneer I usually use L/720. For stuccco on a highrise??? Deflection and jointing would be a real concern.
@dik…you’re right. L/360 is the minimum for stucco by code in the US; however, that’s not tight enough for a high rise that moves more than a low rise building. I would look at a minimum of L/600, maybe even tighter.
@Ron What is it for masonry veneer? Stucco, should be higher than veneer. If it was code conforming, what’s the issue with the engineer that did the design, other than codes are a minimum standard.
For a concrete slab over a wood floor for a residential garage, I use l/1000 or better for the system, not just an individual piece of the system. WAy beyond code requirements here… some designs are inherently deflection controlled, I.e., serviceability controlled, whether for comfort, material preservation, or environment preservation.
@dik…masonry (brick) is L/600, but stucco is only L/360, I agree it should be tighter. The problem with this one is the engineer only consider that the wall would deflect in one direction…vertical. He therefore analyzed each stud for deflection in its long direction. Logical and correct; however, the wall doesn’t only deflect in one direction. It has a hyperbolic deflection profile and the doubled stiffness at the windows exacerbates the problem.
@Ron Yup… agreed. I find that mostly the cause of a problem is easy to identify, then it’s a matter of quantifying it. Sometimes, however, the cause can be elusive. Great topic.
Added… sorry that you were reported on this; I would not be at all happy about that.