Contractor calls about construction loading 6

[AggieYank]

"We just want to verify we can use a scissor lift on your slab"

How do you usually handle it when contractors call to make sure they can use a lift / forklift / other machine on your slab, either on grade or suspended?

We have basically said yes in an extremely roundabout, no liability way, assuming it seems reasonable, but I'm curious as to how others handle the calls.

 

[JAE]

I've had this happen before. My first reaction is to state that the floors were not designed with wheel loads in mind, but that if they provide me with spec sheets on the lift, including wheel load and wheel footprint data, I could check it...for a fee. Usually the fee is simply a couple of hours of work, but I insist that if I'm giving them an answer, and being held liable, I want to get paid for it.

In most floors, the live load value is at least 40 psf if not higher, and with a parking garage live load requirement of 50 psf, this usually doesn't pose a concern relative to overall capacity. But does present a wheel load issue that should be check in terms of slab capacity and punching shear.

 

[AggieYank]

Thanks JAE. I always check the slab shear capacity, including punching shear, before giving them the roundabout answer.

This brings up another question though. When designing floor slabs, why aren't they designed for the point wheel loads from scissor lifts? I don't think I've seen a commercial construction job that didn't use a scissor lift on each floor.

 

[whyun]

Means and methods are contractor's responsibility and they should retain an engineer to check this sort of thing. JAE is right about your right to a fee if you end up with the task. Personally I'd charge more than a few hours as it takes longer to obtain all the information you need than to actually do the work.

When checking construction loads such as scissor lifts, the point loads are usually not in addition to the full design live loads. I check the worst case point load in the midspan of each element without the full uniform live load (use 10 psf? or whatever you feel comfortable with)

 

[swivel63]

how far below is it shored?

 

[swivel63]

or is it even shored?

 

[JAE]

whyun, I knew someone would call me on my "couple of hours".

You are right that there's more time involved usually. Also, we as engineers should get paid for the value of the services we provide, not the time it takes to perform them.

 

[JStephen]

It seems to me, that the prudent thing to do, if you know a wheel load will be applied, is to specify maximum wheel load in the design.

There seems to be an assumption that the contractor that calls you is shirking his duty and the ones that don't are hiring their own consultants to re-engineer your building for that specific load. You have to consider the distinct possibility that the one calling you is the conscientious one and that no wheel-load design gets done in the other cases. Of course, if they've built enough slabs of similar thickness with similar equipment on top, they probably have a fair idea that it'll work without actually designing anything.

 

[jike]

I don't have my IBC code handy but I thought there is a minimum concentrated load specified for commercial buildings. Isn't it 1500#?

The contractor has the option of using the EOR or someone else to calculate those capacities but should expect to have to pay for that additional service.

 

[SlideRuleEra]

I agree that JAE's proposal is an excellent way to get the answer, but some Owners may have a problem with "their" Engineer being paid by both the Owner and the Contractor. Makes the Engineer appear to have a conflict of interest. I realize that this is really a "technicality", but there could be contract language that requires the Contractor to retain his own, third party Engineer to determine the answer.

http://www.SlideRuleEra.net

 

[whyun]

Scissor lifts are typically small and hold around 3 or 4 people max with little space to move around. They are common in construction sites even on concrete filled decks.

Fork lifts to transport construction material may impose more critical loading.

It is up to the contractor to decide whether they want to pay a nominal design fee or take a risk.

Per 2000 IBC, concentrated design live load for office floors is 2000#. Heavy manufacturing is 3000# and vehicular driveways subject to trucking is 8000#.

 

[Qshake]

Any construction related loading is the contractor's responsibility unless otherwise agreed upon between the owner and engineer representing his/her interest and is part of the scope.

We can't dictate means and methods and we shouldn't penalize our client for subjective construction loading (see Whyun's post regarding 2000# or 3000# etc) when in the final structure no such loading exists.

Regards,
Qshake

Eng-Tips Forums:Real Solutions for Real Problems Really Quick.

 

[COEngineeer]

What are you building?

 

[AggieYank]

jike, good point on the minimum design concentrated load requirement. Per IBC 2003, Table 1607.1 there is a minimum concentrated live load of:

Hospitals: 1000 lb
Libraries: 1000 lb
Light Manufacturing: 2000 lb
Heavy Manufacturing: 3000 lb
Offices: 2000 lb
Schools: 1000 lb
Retail: 1000 lb

Section 1607.4 says to assume the concentration occurs over a uniformly distributed area of 2.5 feet by 2.5 feet. There is a footnote under Table 1607.1 dealing with point loads in parking garages (distributed area of 4.5 by 4.5 inches).

A common slab we use is: 3 feet on center steel joists, 3-4" concrete on 1" metal deck (non-composite). This can handle a point load of 2000 lb live load distributed over 2.5 by 2.5 feet.

As for a scissor lift, I wouldn't assume it has a distributed area of 2.5 feet by 2.5 feet under each wheel. Also, the point load from one end of a scissor lift can easily exceed 1000 lb. Instead, I'd assume:
-it has a wheel bearing area of 3" by 1" (punching shear)
-the distributed area supporting each wheel is half the distance between wheels, or 18" width strip of slab, whichever is less when checking moment and shear in the slab.

A good guess when designing the slab is that a scissor lift will weigh between 2000-4000 lb. This slab can also handle the factored point load of (1/4 * 4000) distributed over an area of 18".

I think my future plan will be to tell the contractor that it's a means and methods, and that he can hire us or another engineer to check whether he can use a scissor lift on our slab. However, I'll also be able to sleep at night when I dont' get any call on it, and I'm sure the contractor hasn't thought to check before he lifts the scissor lift up there, which is almost always.

Thanks for all the helpful responses.

 

[socks15]

AggieYank
I've got a couple of articles on slab design that take into account wheel loads for fork lifts and such.

"Slab Thickness Design for Industrial Concrete Floors on Grade" by Robert G. Packard. It was in PCA's Concrete Information

"Concrete Floor Slabs ON Grade Subjected To Heavy Loads" Departments Of The Army And The Air Force Technical Manual
Army TM 5-809-12
Air Force AFM 88-3, Chap. 15
The copy I have is from August 1987. I'm sure there's a newer version out there.

Good luck.

 

[Loui1]

I usually do my best to answer this question with my liability in mind because #1 He will not likely hire an engineer to calc it if you deny him, and if something fails, you're going to spend more unrecoupable fee in litigation than the three minutes it takes to give an answer. #2 You are the most familiar with the system you designed. #3 It keeps relations good if you choose to do so.

Get the publication by Nucor "Designing with Vulcraft" 2nd ed. They have an example calc of concentrated loads for suspended slabs. And there is a plethora of design aids out there for wheels on slabs on grade. Go conservative on your assumptions and "suggestions" (never "recommendations") so if he overloads it and something does happen then there's no chance you're going to court.

 

[DaveVikingPE]

JAE said:

"Also, we as engineers should get paid for the value of the services we provide, not the time it takes to perform them."

That paradigm shift nearly knocked over my building.

 

[JStephen]

Yeah, I wondered about that, too.

If it's not properly designed, it'll fall over and cost millions. Therefore your work is worth millions. Good luck charging the customer for that.

I've seen previous comments on basing charging on a percentage of project cost due to liability, rather than just an hourly basis, and assumed that was what was intended.

 

[SperlingPE]

I am all for helping the contractor out.
However, giving away services (answering construction related, means and methods questions) only further erodes the value that an engineer brings to a project.
Typically your contract is with an owner or contractor. Your scope of work should be clear as to who is responsible for construction engineering. If it is not in the contract, additional fee should forth coming. The other thing to consider is your insurance carrier. Does it cover means and methods?

 

[cvg]

note that many (if not all) insurance carriers will not compensate for loss if you do work that is outside of a valid contract/scope of work. Free advice can then become very expensive...