Talk me down, please 6

[atrizzy]

So I'm tasked with assessing if a crew of workers can enter a building safely. It's an old industrial PEMB type building (moment frames in one direction, bracing in the other, wind resisting gable ends) and in pretty bad shape, but has stood in place in its current state for probably 25 years.

The crews job will consist of a walkthrough, testing for hazardous materials, and planning for the eventual demolition of the structure. This will probably take about one month.

The building has problems, namely:

-A water ingress issue that is affecting some areas of the slab on grade, and will likely one day cause perimeter foundation damage
-To make room for equipment, someone, years ago removed ALL of the cross bracing. ALL of it. They even removed one bay of roof bracing for some reason.
-Several areas with damaged wind girts, major corrosion, but not stuff likely to cause a stability issue (If this was all that was wrong I'd have no concerns)
-A lot of hazards, post-hoc platform 'designs', dangerous stairs... but all with a load path and very unlikely to be of structural concern.

So of course my brain says, no bracing, no way. A critical component for stability is missing, and entry is at one's own risk.

But I've just spent days in this building and I'm not particularly concerned... so my heart says... well, maybe we can specify that they can't enter under any snow load or when wind speed forecast exceeds, say... 10 kph? Just the first thought to come to mind...

What say you? Is there a way to qualify an answer to let a few scientists and demolition planners enter?

 

[3DDave]

Assume there is asbestos and plan accordingly. What else is supposed to be there? Most everything else is in the ground water or has evaporated. Probably the greatest risk is from diseases in bird or rodent droppings.

 

[canwesteng]

I'd take a step back and ask how much work is it really going to be to add some temp bracing to the structure? In your shoes I'd required it before demo anyway, so you're just bringing it forward.

 

[atrizzy]

Thanks Dave, except I've been asked to certify the building as safe to enter from a structural perspective...

 

[JAE]

Probably OK. Who knows?

 

[atrizzy]

I'd take a step back and ask how much work is it really going to be to add some temp bracing to the structure? In your shoes I'd required it before demo anyway, so you're just bringing it forward.

That's a good idea.

 

[Just Some Nerd]

maybe we can specify that they can't enter under any snow load or when wind speed forecast exceeds, say... 10 kph?

I'd be shitting my pants if I was relying on the accuracy of the weather forecast for safety, to be honest. I'd go for temporary bracing throughout

----------------------------------------------------------------------

Why yes, I do in fact have no idea what I'm talking about

 

[bones206]

Yea, I agree with canwest. If you could do so in consultation with the demo contractor, even better. They are going to have a specific plan of attack.

If nothing else, adding some stability bracing provides a little peace of mind for everyone involved.

 

[MintJulep]

I'm tasked with assessing if a crew of workers can enter a building safely

Sounds like you've done that. Stop and issue your findings. Why are you volunteering additional scope?

"The building is not safe to enter in its current condition because all of the structural cross bracing, and some of the roof bracing has been removed. Any number of external factors, that cannot be predicted nor controlled might cause collapse."

There, task complete.

 

[Tomfh]

In some parts of the world they don’t bother with bracing and just rely on the metal cladding and girt system. Is there cladding fixed to girts/purlins throughout? In reality this is how most metal clad frames work.

Another thing I have relied upon on occasions is nudging buildings with a long reach excavator in a few strategic locations. You quickly get a sense whether the building is sturdy enough for people to approach or enter.

 

[atrizzy]

In some parts of the world they don’t bother with bracing and just rely on the metal cladding and girt system. Is there cladding fixed to girts/purlins throughout? In reality this is how most metal clad frames work.

Another thing I have relied upon occasion is nudged buildings with long reach excavator in a few strategic locations. You quickly get a sense whether the building is sturdy enough for people to approach or enter.

I have no doubt that this is how it's working, but it was certainly not meant to by design.

I can see the ends of the bracing that was cut. In one location they left half of it lying against the equipment they installed. In another, you can see from when the building was built a note written on the frame "sway bracing this bay". All missing brace bays also have corresponding roof bracing... it's fairly obvious in this case.

 

[3DDave]

If it is too dangerous to walk through and get samples it is too dangerous to make structural modifications.

Of course all of us are at a total disadvantage as no photos or other indications of the quality of the structure have been shown here.

 

[LittleInch]

Nowadays I try veery hard not to use the word "Safe" or "safely". This is a highly variable term and what people really mean, even if they don't say it is "Is the risk sufficiently mitigated and low enough for me to accept that risk in conjunction with the benefit I get from the activity (in this case doing work and getting paid).

Don't know if its common in Structural Engineering, but in the Petrochem world we often start with a HAZID to assess the probability vs consequence pre and post mitigation, usually a 1 to 5 matrix.

SO this identifies the key risks and what you can do to reduce it.

It is qualitative so usually needs more than one persons view, but demonstrates the effectiveness or otherwise and gives people an idea of the risks.

PEMBS as most people know are designed with wafer thin margins against collapse and this building has removed some of the features which prevent this.

SO a difficult one to be "certain" on anything. Maybe build a scaffold cage or have a few shelters to run into if the building starts to fall which can withstand a collapse?

Or if they do need to make it stronger before they pull it down then that certainly adds to it, but how often does a building fall down when people start messing about with the structure??

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Tomfh]

We do 5x5 risk matrixs in structural assessment work in Australia. Clients (particularly bigger companies) typically want that sort of traffic light rating system during audits so they can make their decisions.

It’s a bit wishy washy, you can game them a bit to get the answer you want, but even so I agree it’s far better than the safe/unsafe binary, which doesn’t tell you much. It’s a question of risks and benefits.

 

[MintJulep]

Do not confuse a long period of good luck with safety.

Do not compound that confusion with confirmation bias --> "I worked in the building for a week and it didn't fall on me."

You made a mistake by continuing to work inside after you saw that all the bracing had been removed.

Formal risk assessments are intended to avoid those mistakes.

Yes, the probability is low --> "It's been like this for 25 years."

Regardless, the consequence of "building collapses on work crew" are catastrophic. --> Risk - unacceptable.

The demo crew can use remotely operated wheeled and tracked things to get their samples, and flying drones for a closer look. HD video, maybe LIDAR.

No need to go inside.

 

[Tomfh]

Regardless, the consequence of "building collapses on work crew" are catastrophic. --> Risk - unacceptable.

Most risk assessment matrices aren’t calibrated like that. It defeats the purpose of them if you’re just going to label any risk that results in fatalities as unacceptable.
Any building carries some likelihood of collapsing, potentially killing those inside. In terms of assessing the risk you have to assess the likelihood. You don’t just say people might die therefore a risk is unacceptable.

 

[LittleInch]

I agree with tomfh,

All risk is a multiplication of probability times consequence. Hence consequence here for many of the risks could be 5 - Collapse of building, loss of life, big costs etc. The issue is really the probability.

If you judge it as 1 - Very unlikely, then you have a risk of 5 - Acceptable
If you judge it as 4 - Likely - then you have a risk of 20 - unacceptable unless you do something about it to reduce the probability - e.g. no access when wind > 15 kts, snow, heavy rain etc

They all vary a bit but this is typical

Also maybe limit the consequence by having no more than say 4 people in the building at any one time and only one activity at a time.

Strengthening the building will reduce probability, but the act of strengthening the building also needs to be assessed for risk.

Is there anything you can do the exterior of the building to improve its stability / strength?

Yes its a bit crude, but helps to identify the risks and then work it through. It will also inform the people with skin in the game - i.e. the companies and workers who will be inside / next to the building.



Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[JStephen]

Could you add external bracing? Guy wires, maybe?
Discussing the problem with a demolition contractor might be informative, surely this is not the first building like this.

 

[atrizzy]

I think external bracing is probably the way to go. One side is inaccessible so it'll have to be rigid, but a small price to pay, and one the demo contractor will be thankful to hear of in advance anyway.

 

[Rabbit12]

I agree with the external brace option. Shouldn't be too hard to install something like that. I'm thinking some concrete dead man w/ cables and then maybe limit the environmental conditions that the building can be occupied, ie wind speed no more than 20 MPH. Should not be that hard to monitor wind load.