Allow Absurd Load Combination to Control? 7

[SandwichEngine]

Ran across this yesterday. I've simplified the numbers on this example to make the question cleaner.

Existing building is a 25,000 square foot low rise manufacturing facility in Texas. Ground snow is 5 psf.

End user wants to repurpose the building which would require hanging additional piping from the roof that weighs 5 psf over what the building was designed for.

Checking the existing building shows that it's slightly overstressed in some areas but only for the load combination DL+0.45W+0.75Lr, IBC Equation 16-13.

This equation checks to see how the roof members would fare if there was 75% of the roof live load on a completed roof during a 10 year wind event. For this building that's 500,000 375,000 lbs of men or (2500) (1875) 200 lb men on the roof during a 10 year MRI wind event. This amount of men of a roof would never be more than about 20 or 30 during construction, not to mention years later.

The worst case unity check for any other load combination is 0.50.

I have no problem allowing the new larger loading from a safety perspective but how do you navigate that it would require you to disregard this portion of the code.

This has me thinking I should call BS on the code when it doesn't make sense, like you would with software.

Haven't decided what I'll do yet but would like to hear other perspectives, especially ones that disagree with my initial take.

 

[Ron247]

I have not looked at the code but is the 10-year wind = .45W or is the wind .45x(10-year) wind? The second is more likely to occur than the first. As far as square footage, it would be the smaller tribs to be concerned with. I have seen a stack of plywood on a roof with 6 guys sitting on it eating lunch. I have seen a 3' tall stack of 4'x12' drywall on a 40 psf floor live load wood dining room floor. I had a handrail at a 15' drop to stone that was wobbly that also had a view at the time of 2 women playing golf. The person looking to buy the property said he was not worried because he thought there would never be more than 2 or 3 people at the rail at one time. I knew he said he and his wife liked to entertain such as Super Bowl parties. I asked him if one of his buddies saw the 2 women get into a fight, would all of his guests run outside to watch. He decided the rail needed reinforcing. You can never predict what can go wrong.

Have you tried locating the future loads at specific locations rather than a general square footage. I have done that in the past and it worked better. Took more time though. I am not willing to risk a lot for someone else to benefit. As far as getting anything in writing, it will work if you have nothing to lose. If you have assets, you will still get sued. Winning a law suit generally costs you $16k.

 

[BridgeEngineer21]

Just thinking of another potential local overload scenario - say a rare (or not so rare, depending on your location) tornado or hurricane comes through the area, and a bunch of daredevils from the local college football team (all easily 200 lb+ guys) decide the roof of your building is the ideal spot to watch the storm come in. So now they're all crowding at the edge of the roof over the fascia beam, which is maybe even experiencing 20- or 50- year winds with the storm.

Is it likely, probably not, but the point of the code is to cover even unlikely (but still possible) events.

 

[BS2]

I have run into this scenario several times. In my case, the structure was a decent size - approximately 400'x400' with a very lightweight roof. Applying the roof live load reductions resulted in a 12 psf roof live load. This is approximately 1,920 kips of roof live load. The structure was quite capable of supporting this load locally but when the entire frame was analyzed, the columns and a couple of other members were somewhat overstressed by the DL+0.45W+0.75Lr load combination. I reduced the roof live load to approximately 10 psf and the code checks passed.

I used some engineering judgement to logic that there is no possible way they could ever place anywhere close to 1,920 kips on the roof so a slight reduction to 10 psf over the entire roof was acceptable. Of course, the 20 psf roof live load was applied locally over several bays at a time as this is a realistic load case.

 

[JoshPlumSE]

This amount of men of a roof would never be more than about 20 or 30 during construction, not to mention years later.

I don't look at Roof Live Load as people load so much as construction loads. You ever see a roof that is getting ready to be replaced. You've got the existing roof, the contractors, and a crap ton of roofing material stacked up over a relatively small area.

Now, this construction load doesn't occur over the whole roof. So, if you're assuming it does then that becomes the absurd load combination. But, in a localized area, I don't see anything absurd about it.

 

[CrabbyT]

You're not legally allowed to call BS on the code. The code is literally the law. I'm not even sure the AHJ would have the authority to allow you or the owner to ignore the default load combos. Plus, you would be taking on a ridiculous amount of liability and might potentially be accused of negligence if the roof were to collapse

Also, you don't need to apply the load to the entire roof for a single element to fail. What if someday they do a reroof and what if they stage all the roofing materials and waste along a single truss/beam?

 

[Rabbit12]

Josh Plum nailed it. It's not just people you need to worry about. It's tools, and materials as well.

A few years ago I got a call from an owner. They have a roofing contractor re-roofing a ballasted roof. This was a school and the gym was in the middle of the building and it's roof was higher than the surrounding roof. The contractor pushed all the ballast from the gym roof onto the lower roof. There was 3'-4' of rock all around the gym walls on the lower roof.

Don't underestimate the dumb shit a contractor will do. Will the whole roof see that load? Really unlikely but the chance of a single bay seeing that load is much much better.

 

[MotorCity]

The concern discussed in this thread is addressed by the concept of live load reduction in the code where you don't have 100% of the load, 100% of the time.

 

[hetgen]

I was tasked to review an interesting roof design a while back, the building owner was angry and the designer thought I'm crazy when I labeled the building " NOT CODE COMPLIANT "

The problem was that there are 100's similar structures built for the same client and are performing well.


Check this thread.
thread507-481518

I say stick to the code requirements unless you're in some part of the world where design codes are only seen as guidance, not as minimum requirements.

 

[Settingsun]

 I reduced the roof live load to approximately 10 psf and the code checks passed.

I used some engineering judgement to logic that there is no possible way they could ever place anywhere close to 1,920 kips [12 psf] on the roof so a slight reduction to 10 psf over the entire roof was acceptable.

I was reminded recently of five warehouses that collapsed in Australia in 2015. It was blamed on >100mm of hail which would be in/around that range of load, and obviously plausible to combine with storm winds. The rooves should have had an ultimate capacity >8.7 psf based on nominal design live load (no snow load required in the area), load factor, and capacity factor, so it tallies with the load getting up to 10psf-ish. It's not always men and materials.

 

[Lutfi]

@SandwichEngine, next time when you are in the field, take a look and see how contractors stack and store supplies and equipment on roofs. Yes, it is not all over the entire roof. However, it can occupy a large area. All local members must be designed for such live loads. Also, see 2015 International Existing Building Code provisions, I am sure similar language if not the same is in the 2018 edition, as it relates to gravity:

“[BS] 402.3 Existing structural elements carrying gravity load. Any existing gravity load-carrying structural element for which an addition and its related alterations cause an increase in design gravity load of more than 5 percent shall be strengthened, supplemented, replaced or otherwise altered as needed to carry the increased gravity load required by the International Building Code for new structures. Any existing gravity load-carrying structural element whose gravity load- carrying capacity is decreased shall be considered an altered element subject to the requirements of Section 403.3. Any existing element that will form part of the lateral load path for any part of the addition shall be considered an existing lateral load-carrying structural element subject to the requirements of Section 402.4.

[BS] 402.3.1 Design live load. Where the addition does not result in increased design live load, existing gravity load-carrying structural elements shall be permitted to be evaluated and designed for live loads approved prior to the addition. If the approved live load is less than that required by Section 1607 of the International Building Code, the area designed for the nonconforming live load shall be posted with placards of approved design indicating the approved live load. Where the addition does result in increased design live load, the live load required by Section 1607 of the International Building Code shall be used.”


Regards,
Lutfi

 

[Brad805]

I think it is important to keep a few points in mind here.
1. If you have read any of Sandwichengine's posts you will see he has been in the pre-eng business a long time. I am confident he is very well aware of how these roofs are erected.
2. It is not uncommon for these manufacturer's to have a division that erects the building, so many working for the manufacturer have a wealth of construction experience.
3. If this is a pre-engineered frame roof like I posted above the material stacking suggested is not a common practice. Why? The most common method I see to erect the roof of one of these buildings is shown below. The lifts used do not have a great deal of load capacity.
4. The roof purlin spacing here could be in the realm of 6' or more based on the loads suggested. I am not sure if many have tried to walk on 26g or thinner tin spanning that far, but even the craziest worker will not do that very often for fear of falling thru or damaging the tin.
5. The most cost effective roof for these involves a rolled batt roof that comes in large rolls about 4'-0" wide. It is rolled out over the wall girts and roof purlins from the spool and is not stacked on the roof.
6. Failure to erect these frames safely has resulted in a number of collapses. Google "Pre-engineered steel building erection failure" and you will see a number of cases.
7. The building being worked on is existing. Unless they are adding a ton of electrical, the most common additional load is hanging pipes from the frames. The cases I have looked at do not commonly add a lot of load to the purlins except when the roof is being improved or new RTU's are needed.

This is a case where you have two options:
1. Reinforce if necessary per code as I expect Sandwichengine has already discussed with the client.
2. Double or triple the engineering costs and have them evaluate the frames for the precise piping needed. The global load being considered could be conservative.

I applaud Sandwichengine for helping this customer. We have asked suppliers an endless number of times to assist, and very few do. Why? No $$ in it for them. I get it, but everything about these buildings makes them a challenge to check.

 

[JLNJ]

I too applaud SE for trying to "make it work".

Can you use the 5%/10% rule for the additional load? Sometimes non-conforming buildings will pass this provision.

I've dealt with a LOT of buildings with hanging piping - sometimes it works out that the load is overestimated (e.g. the mechanical guys just guessed and they weren't very good at it), sometimes I really sharpened my pencil (gas lines, compressed air lines? use the empty pipe DL only, not full of water), and sometimes the load is surprisingly more than I expected. Cable tray, pipe anchors, vertical runs of pipe, insulation, heavy fittings, the structure to hang the pipe, aggregated point loads, etc. sometimes get overlooked. Get as close as you can to the process and see what's really going on.