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?

 

[AZPete]

Install strain gauges in select locations and monitor for changes. Short of instant collapse, you at least have a plausible sentry system for issuing an evacuation alert.

 

[geesaman.d]

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

Indeed - many machines and structures cruise along its usable life with few obvious changes for most of it. Then when it really finally breaks down, it breaks down quite quickly. So while the probability is/has been low, as time passes that probability is increasing. It's a true logical fallacy.

 

[GeorgeTheCivilEngineer]

"The structural engineer said it was safe, get on in there..."


Report findings of fact, the ramifications of those facts and options to mitigate. The decision to enter or not, or to apply mitigations, shouldn't rest with you.

 

[LittleInch]

Getting back to the probability thing, is there a database or ready made statistic anywhere where you could say something like,

Based on 1000 structures ( or whatever number you take),

A new structure designed to IBC and built to plan made of concrete and steel - chance of one collapsing in the next 12 months is ??0.1, 0.01?
New PEMB - same thing - 2.5??
This PEMB in its current condition - 100?, 200?

If its windy multiply by 2, Snow 5, earthquake 10

Always helps to put it into context and let people assess the risk themselves. Everyone has a different "risk appetite". Like most things it tends to peak at about age 25, then fall gradually over time.....

Or just illustrate your e-mail presentation whatever with this.... I know this isn't the exact phrase which is "You need to ask yourself, Do I feel lucky? Well, Do ya?"

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

 

[atrizzy]

LittleInch, rest assured I'm thankful for all of these responses.

 

[Heaviside1925]

OP,
I am leaning toward MintJulep's advice on this.

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

I think this is not an opinion you can make. Is the building safe for occupancy? No. Are there risks inherent to entering? Yes, and these are the risks based on my walk through. At that point it is up to the company's and the crew's competency and acceptable level of risk to determine if they want their crews entering. Are there mitigation strategies from a structural standpoint to lower the risk? Yes, and if you are asked, you can offer them up.

I am pulling these thoughts from some past experience with urban search and rescue. We already knew the building had been compromised and we would have a structural provide commentary on how compromised the building was. Based on that commentary, we could make a discission based on risk and life safety considerations on whether or not a search could be conducted "as is" or whether certain remediations needed to take place first to make it "safer". In the end, that decision was based solely on our team and preexisting standards that we as the rescuers were required to make on a risk vs. reward basis. In industry this is in the form of the Risk Matrix that LittleInch provided. There was never an expectation of the structural to make the call on whether the building was "safe" to enter or not because we already knew it wasn't.

 

[TLHS]

Yeah, if you have space it's pretty easy to get some concrete blocks and guy wires or angles that will give you a limited lateral capacity to be able to justify short term limited entry with controls.

You could try to calc some diaphragm capacity, but that's going to be fairly out there with the types of siding and connections used, especially if the girts are degraded.

It's likely not worth it to try to get comfortable with, especially since it seems like the stakeholders are already aware that the building is sketchy.

If the bracing connections are still there you could also just reinstate some of it if you're comfortable with the cladding holding up in fair weather for a one or two day job.

Edit: I have done the probability thing for industrial structures. You basically back calculate the return period the thing can take and use that to inform risk. This is also what's done for reduced construction loadings. You still need to make a judgement on capacity though, and I wouldn't want to do that with a stability issue unless someone's paid me to work really hard to evaluate the strength behind the potential stability mechanism. You should only get complicated on this type of thing if you have a really big feasibility of cost issue with the simple brute force reliable options

 

[Strong]

Robots and Drones?

Walt

 

[human909]

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...

That is exactly what I would do. You know this building has stood up to significantly higher winds. Set conservatively low environmental conditions and exposure time for entry and investigation and the job is done. Charge appropriately. The risk you are taking on here is extremely low as you have already recognised.

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.

Your clients will eventually ask somebody else if you have an attitude like that. But that is fine if YOU don't want to take on that risk. But we do need engineers in our society that are capable of making such calls.

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

I disagree. He was in a position to make the risk assessment. He made it. That isn't a mistake, that is good engineering.


Even with NEW structures that are in the process of being construction there are often stages during construction where they are in a temporary state where high winds could cause collapse. Decisions are made based on regulation, code, judgment or engineering calculation on how to proceed safely. This is no different.

As structural engineers we dealt with likelihoods in all our work. We can never guaranteed 100% structural safety against all conceivable circumstances.

 

[human909]

An example of what I might write:

The structure in its present condition does not meet current (or historic) structural capacity requirements for a structure of its type. As such it is not structurally adequate.

Based on its assessed condition, the structure is likely to withstand a 1 in 5 year wind event (~39m/s gust strength). However given the uncertainties involved, the structure should be considered to be at elevated risk of failure during and wind events where gusts greater than 15m/s are expected. (eg winds greater that 20mph)

Any work required should ensure that it is undertaken in conditions where winds are are expected to be under 20mph.....

 

[Gforce1]

Install temporary bracing?

This could be formed from scaffold tube with gravock connectors clamped to the steelwork. It could possibly be installed from the outside. Design wise a reduced wind return period would be applicable.

I think you need to ensure the stability of the building. If you can't prove this install a protection system; i.e. temporary works.

I am involved extensively in demolition and measures such as this are common. You can't put people inside without a justification.

 

[LittleInch]

Getting back to the key question which is

"I've been asked to certify the building as safe to enter from a structural perspective..."

Now that suddenly makes it you and your insurers risk when asked to CERTIFY something to be as nebulous a word as "safe".

I wouldn't go there myself and would advise a different approach, namely list the structural defects you have been able to see and what that means in terms of structural integrity and risk of collapse.

Then you can advise things that could reduce that risk to the point where limited work could be undertaken without excessive risk to those doing it, plus a wind limit to be monitored before and during any internal inspections.

Just avoid the use of the word "Safe" or "safely". Means nothing.

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

 

[BFstr]

atrizzy,
May I ask what's the size of your building by the way?
And is it in high seismic or costal location?
Is it at middle of a populated town or out nowhere in a farm?

 

[BFstr]

Any pics you possibly can share?

 

[atrizzy]

BFstr,
I'd rather not share any pictures as to limit chance of identification.
The building is a slapped together old mining mill way out in the middle of nowhere. Minimal seismic, but decent snow loads.

 

[atrizzy]

I just want to take a moment to thank everybody who contributed to this thread. There is a LOT of food for thought here and this will surely impact our approach to the wording in the report and the information we provide. If anything interesting comes of this, I'll be sure to update the thread... hopefully nothing will... boring is best!

 

[LittleInch]

Well you've got a wide variety of responses here from "Just get on with it, you survived 5 days ok" to "board it up with Do Not Enter, Dangerous Structure signs"....

Let us know what you decide to close this out as its always good to see which bits of advice you took up and hopefully worked with the client.

If you get a big wind it might have fallen down already!

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