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When Water Management Becomes a Structural Issue

Patrick OSullivan | Posted in General Questions on

It’s obviously very early to be drawing conclusions about the recent Florida apartment building collapse, but the initial observations lifted from a 2018 review by a structural engineering firm are sobering. Some of them are mentioned in the article here: https://www.npr.org/sections/live-updates-miami-area-condo-collapse/2021/06/26/1010542570/florida-condo-structural-damage-engineers-report-surfside-miami

I want to be very clear here on primary vs. secondary sources, but the secondary sources (i.e. the above referenced news article and others) report that the engineering firm commented on damage to concrete due to failed waterproofing details of a pool level because of improper slope and drainage.

The spalling due to water damage is interesting as we often discuss here about concrete’s ability to be wet and suffer no ill effects. Spalling would typically be associated with colder climates where water in the concrete could freeze, but that’s obviously not the case in Florida. It will be interesting to see further analysis of these underlying issues. Thinking off the top of my head–one wonders if the spalling could have been symptomatic of corrosion of rebar brought on by the water issues.

Here, we deal largely in single family, wood framed structures, which of course are incredibly important to the handful of people that occupy them. We’ll see how this particular incident plays out, but I can’t help but think that it’s a good reminder of, despite our frequent quibbling over details, sometimes details really do matter.

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Replies

  1. Walter Ahlgrim | | #1

    From what I can tell we are all still guessing at the root causes of the collapse.

    I do not think anything in the engineers report suggested in any way that the building was in danger of collapse.

    Most of the recommended repairs almost sound cosmetic.

    Walta

  2. John Clark | | #2

    The salt in the ocean air that is the issue. It reacts with the concrete to cause internal cracking and weakens/corrodes the rebar.

    Regrettably it appears that the HOA and the homeowners were a couple of months behind with regards to approving and starting repairs. I imagine it's entirely possible that once some concrete was opened up that the building could've been condemned until repairs were completed.

  3. Expert Member
    Deleted | | #3

    Deleted

  4. Jon R | | #4

    Some info about bad things that can happen to concrete, many of them because it is wet.

    But maybe it was a sinkhole - which is about water moving in soil.

  5. Expert Member
    DCContrarian | | #5

    My understanding is that water acting on the soil under the foundation is a much bigger risk than on the concrete. The reason buildings have perimeter drains is to protect the soil.

    1. Jon R | | #6

      On one hand, a perimeter drain can reduce soil saturation, preventing weak soil. On the other hand, such a drain might carry away soil, leaving weak voids. Better to divert the water at the surface.

    2. Expert Member
      Zephyr7 | | #7

      Correct, which is why code always says something along the lines of "must rest on UNDISTURBED soil". This is also why I tell property owners in my consulting work not to allow directional boring under their foundation wall -- the bentonite slurry can sometimes erore more than you'd think, so it's best to acutlly bore THROUGH the wall, not under it.

      I'm not a structural engineer (I'm an electrical engineer), but I have been on teems evaluating buildings for special purposes (data centers, usually, and other "interesting" special occupancies). We spend a lot of time looking at structural members and their connections, since we put in VERY heavy things (big generators, big UPSes, etc.), so we have to be sure everything can support the load. One of our structural engineers did find a building unsuitable for a brodcast antenna tower on the roof once, due to concentrated loading on the foundation. The loads can be a lot higher than you'd think when wind (and ice loading here in the north) are factored in.

      I've seen salt spray get into a concrete column and eat it's way down the rerod inside before. It's entirely possible there was a lot of hidden damage. When my team does building inspections prior to a project, we're usually not allowed to actually cut into anything -- we can only look at what we can see, and what we can access by moving ceiling tiles or opening access hatches. The building owners don't usually let us cut or drill anything. We solve this on our end with contract language stating we're not responsible for anything we can't reasonably access at walkthrough. My guess is the reason the engineer's report on that condo complex estimated that the number would go up after the work started is because they probably expected to find other hidden damage that they couldn't see during their initial inspection. That is entirely reasonable on the part of the consulting firm.

      I've seen a lot of people trying to blame the condo association. I think that's premature at this point. If there was a lot of hidden damage, no one would have known about it -- yet. And it doesn't necassarily mean anyone was penny pinching or being negligent, either. Sometimes there can be hidden defects from the time the property was built. A bad concrete mix, poor weather conditions during construction, just the exact right combination of events in terms of weather damage, etc. For a good example of something like this, look up the structural repairs done to the Citigroup Center building in New York. I first found out about that during a class in college, since the professor knew another professor whose student found the problem during a class analyzing that particular building.

      We all need to wait for REAL findings (not news media speculation) here. It's too early to assign any blame. If the condo people acted to bid out the work as soon as they found out about the problems, it really wasn't their fault.

      All that fun stuff said, YES, water can be a structural issue! Water can erode foundations, water can rot wood, water can rot steel (by rusting), water can saturate materials and cause them to weaken, water can cause firm soil to act like a fluid (quick sand is the best known example). Water can accumulate and build up pressure, causing failure from weight alone (just like snow buildup can cause a roof to collapse). Look how quickly heavy ran can cut a "stream" into the shoulder of a road, for example (which just happened on my own road over the past few days). It's possible to design for these kinds of things, but any desing will have limits as to how much it can handle.

      Bill

    3. John Clark | | #9

      I guess it's possible with with the building being so close to the ocean I would think that the foundations are tied to the limestone bedrock below.

      In any case apparently the footprint of the foundation is much larger than the building itself. All of the landscaping and such is sitting on top of concrete which had waterproofing applied. Apparently the sister building had the waterproof barrier re-applied at considerable expense.

  6. plumb_bob | | #8

    I was recently at a SFD where the basement slab and 1 exterior wall were completely undermined and the soil was gone. The problem ended up being a leaking sump pump pipe that was washing the soil back into the sump to be ejected into the storm system.

    My immediate thought when I saw the towers collapse is soil failure of some sort. This will be a case study that future generations will learn from. Things like this cause codes to change.

    1. Expert Member
      Zephyr7 | | #10

      I'm not so sure a code change would have helped if it was an erosion issue. Sometimes errosion is caused by development of neighboring lots. My previous house had an issue like that -- there used to be a stream in the woods, but when the lots behind me got developed, the stream "dried up". You can, to this day, still see the depression where the stream is. If you dig down about 2-3 feet or so, you find the stream -- and it's MOVING too, just underground now instead of on the surface. I have heard that the houses behind my old house have a lot of water issues in their basements.

      It's entirely possible that the building was fine when built, then as neighboring properties got developed or modified, something may have changed that impacted the building that collapsed. I would not be surprised to find erosion as at least a contributor to the structural failure here.

      Bill

  7. Jon R | | #11

    > pool level because of improper slope and drainage

    A good reminder that small amounts of salt in an area where water is forced to evaporate will eventually be a very concentrated salt solution. This can cause rebar corrosion which then causes spalling/cracking, causing much more rebar corrosion.

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