FOS for Scaffold 2

[andriver]

I work for a large construction company, and we outsource our scaffolding to a very large scaffolding company. Per my company policy I need to review all 3rd-party engineered plans.

Recently I reviewed a plan to use 6x6 timber to span 17.5' (16in OC). I found an error in that they were using repetitive member factors and size factors that were only applicable for 2"-4" thick members not the 6x6 members being used. Consequently the FOS is 1.85 and not 4.0.

In talking with the company and our superintendents, this is a very big deal because the material is on site and they were going to begin assembling it this week, the area is a very large area so that this will impact quite a bit of material.

The EOR states that he does not need to reach a 4.0 FOS because this is an engineered system, and as EOR he can state what the capacity is. The 4.0 is more for people who don't know what they are doing.

Either way I am just a junior engineer grunt, it will not be up to me whether we accept this design. I wanted to know the consensus on his statement that engineered scaffolding does not require 4.0 FOS.

 

[thaidavid40]

If there is an exception to the OSHA rule governing the factor of safety, then I have never heard of that. I would ask that engineer to point you to where in a governing code that exception is stated. My experience has always been the CAPACITY equals or exceeds DL + 4*LL.
Dave

Thaidavid

 

[Lomarandil]

It depends what the meaning of the word "is" is. (or in this case, Factor of Safety).

As David points out, OSHA requires "each scaffold and scaffold component shall be capable of supporting, without failure, its own weight and at least 4 times the maximum intended load applied or transmitted to it." This is typically interpreted as a limit against nominal or ultimate capacity.

So if your designer is working in ASD (which many construction engineers do), it's possible that his FOS of 1.85 still complies with the OSHA requirement. Though it sounds pretty tight.

 

[andriver]

Thanks all, you bring up very good points. To clarify:

I am using 4.0(DL+LL), not just 4.0*DL. Originally this is how I interpreted OSHA, but my reviewer is on the side that says its 4.0 of the dead and live load. I can see the logic here, as the dead load (plywood on top of the timber in my case) is part of the intended load.

The FOS 1.85 is already stripping out the ASD FOS (which they claim is 1.66 per NDS 1997 commentary). My reviewer has a popular wood design textbook that claims this is actually 2.5 so this actually gets me closer than 1.85.

So going to ultimate strength, we do not reach the 4.0. This is the largest scaffold company in the US, their original calcs show a FOS of 4.0, it was the mistakes I found that brought this value down. I have checked many other of their drawings where they always reach a 4.0 FOS.

This is a big deal because its a huge area, we are talking 20,000ft^2 or more that will be affected, material is already here. This is why they are fighting it, especially because they are responsible per the bid. The Director of Engineering was who called me and surprised me, and him being a veteran I felt I was being muscled. I asked him to provide some literature to back up his claims. His claims being that, his company has stamped and sealed the drawing, the capacity is what they say it is. I have not received anything yet.

 

[CANPRO]

Often the 4 to 1 factor of safety for scaffolding and shoring is based on tests where the member in question is tested to failure. When using calculated values, you're going to come up with a number much lower than the ultimate based on testing (in my experience).

It sounds like the EOR is using calculated values based on code references (in Canada, CSA 086 for timber). If thats the case, I agree with him that 4 to 1 isn't required on the applied loads. If he is using calculated resistances, it should be compared to factored loads and not the applied loads. In formwork design, you can do either. For scaffolding, I've always used applied loads and allowable resistances (based on testing with 4 to 1, or values published by the manufacturer with a 4 to 1). If I had to incorporate timber into a scaffold, I would likely use material/load factors.

 

[CANPRO]

Just saw your most recent post after I replied - Stick to your guns and be respectful. Big companies and experienced Engineers make mistakes too. Either they'll admit their mistake if there is one or you'll learn something new and everyone moves on happy.

 

[andriver]

Thanks CANPRO,

I think your comment about being respectful is important. I spent some good time wording my original email correctly. I took out words like "Your Calculations" "Your Values" and switched it to, "Our values aren't matching" and I followed it with a statement "I am more of a steel guy so I may be missing something" etc.

I take little to no pride catching their errors, it's actually creating an issue where I wish there was none. I am deliberately wording all of my responses to indicate "we are in this together".

I would like to add that they have already admitted to their mistake, and they agree they only have a 1.85 FOS. There solution though is just ignore this time and going forward we will not do this again. Let this one fly, based on it being an engineered system.... Thing is, this is like the largest area we will be scaffolding, its huge!

 

[dhengr]

Andriver:
These kinds of construction problems are always very prickly and should ultimately be handled at the upper levels of the companies. I would do a good job of laying out what you have found, with substantiating calcs. and code refs., etc., along with some of your own summaries of some of the various telecons or e-mails with the people at the scaffolding co. Take this all to your boss and lay it out for him and ask him what he wants to do, or what he wants you to do. The engineering dept. at the scaffolding co. has stamped their plans and they are responsible for their design and its adequacy. You would usually not be responsible for their design unless you saw a gross error, on site, and did nothing, but know that your company has started such a commotion, by insisting on cross-checking every detail, they could end up involved if anything does go wrong, and they didn’t act properly. And, this is something that neither you, nor I (or the others here on E-Tips, who have given you good input), are in a position to make the final decision on, for your company. This is something that has to be worked out btwn. the two companies, as to who is responsible for what and who is offering what assurances, etc., at a much higher level than your’s.

OSHA does not usually cite companies for not exactly meeting their FoS’s, although I’m sure they could. There is merit to reduced FoS when the system is properly engineered and there are also instances when testing is used to prove a system is adequate. Exactly when each of these apply is kinda hard to determine as you read the OSHA stds. Then too, careful, well thought out and quality construction are important so as not to abuse the scaffolding. Their FoS’s are so high because there is so much poor thinking, engineering and workmanship going on within the construction industry. They cite people/companies when things go wrong and workers get hurt or worse. This is as much a legal and insurance issue for your company, as it is a sharpen your pencil engineering problem, at this stage of the game. Take it to your boss.

 

[SlideRuleEra]

Since this is temporary scaffolding, I'm surprised the scaffold designer is not taking advantage of a suitable load duration factor.

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[kgengr]

We provide structural calculations for scaffolding, shoring, and pedestrian protection in my office (as a very tiny portion of our work). For scaffolding (holds workers, materials) we use FOS 4.0. For pedestrian protection we use FOS of our own determination - needs to meet code (IBC/ASCE7 + supporting material code) requirements, but no other imposed FOS. For shoring (holds up structure, or formwork) we use 2.0 - 4.0.

 

[dicksewerrat]

The guy that makes the decision never dies on the job. I'd consider calling OSHA for visit. they used to have a program of visiting without a fine. But I wouldn't go up on the scaffold with 1.85. It will wobble. I s like walking on a floor that bends between floor joists.

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[andriver]

SlideRuleEra - We are already taking advantage of a 1.25 "Seven Day" Load Duration Factor which is being modest.

dicksewerrat - Deflection is not controlling, OSHA requires l/60 but we are actually better than l/180.

dhengr - This is what I did, I never substantiated his claims that engineered systems do not require a 4.0 FOS, nor has he provided any literature to support his claim.

As the client in this situation, I have asked politely that they modify their design to satisfy the OSHA requirements with my boss and field supervisions support. I was never worried about this because I knew we would do the right thing, I was merely gauging the industry and whether or not this was a typical approach.

 

[SlideRuleEra]

andriver - Thanks for the clarification. There is one point about this whole issue that has troubled me since your first post. A span of 17.5' for a 6 x 6 is very long (Span to Depth ratio of 35:1, even higher if the 6 x 6's are dressed, instead or rough cut). With spacing of 16" OC, I realize that the tributary area for each 6 x 6 is only 23.3 sq. ft., but it still bothers me. If you can tell us, what are the assumed dead and live loads? Are the 6 x 6 rough or dressed?

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[andriver]

The loads are 2.5 psf dead from plywood and 25 psf live load. I am not sure if it's rough or dressed but I have just assumed dressed during my peer review. It is Southern Pine No. 2 grade.

 

[SlideRuleEra]

andriver - I've taken a quick look at the stresses for the assumed load using dressed 6 x 6 properties. S = 30.3 in³
For DL, in addition to the 2.5 psf for plywood, I have added the self-weight of 6 x 6 (38 pcf). This gives at total DL of 8.5 psf. LL = 25 psf.

Using a total load of 33.5 psf, on 23.3 sf (tributary area), 17.5' span, I get a bending stress of 677 psi.

Southern Pine #2, Allowable F_b = 850 psi. Using C_D = 1.25, pushes that up to 1063 psi.
If the plywood is fastened (nailed) to 6 x 6, I would take credit for C_r = 1.15. If so, then Allowable F_b goes to 1220 psi. The 1.15 factor is very conservative, and has be repeatedly been shown to be valid by many tests over the years. If ET members don't agree, I understand.

Any way I look at this, 677 psi for the assumed load does not compare well with either 1063 psi or 1220 psi allowable when the 4 X LL requirement is considered. Following dhengr's advice, as you are doing, is certainly the right thing to do.

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[SlideRuleEra]

Took at look at the ultimate bending strength of Southern Pine #2, 6 x 6. From 1997 tests by the Forest Products Lab, the "equivalent allowable Modulus of Rupture" is 1780 psi - See Table 5 at this link: Properties and Grading of Southern Pine Timbers

For the 17.5' span, 6 x 6's in the scaffold I get a theoretical bending stress of 2200 psi for DL + 4*LL (109 psf). Comparing 2200 psi with 1780 psi, tells me the scaffold will most likely collapse at the required loading. Also, as the MOR is approached, deflection would be ridiculously large. Finally, the 6 x 6 for the FPL tests were 10' long, I would suspect that Southern Pine #2, 18' long, 6 x 6 would likely have more flaws (knots, splits, etc.)

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[SlideRuleEra]

andriver - A proposed solution for your consideration:

Change From:
#2, Southern Pine, 6 x 6 @ 16" on center to
#2, Southern Pine, 4 x 6 @ 10" on center. The consequences are as follows:

1. Only a slight increase in the number of board feet needed.

2. Allowable F_b increases from 850 psi to 1000 psi.

3. Repetitive member factor allowed by NDS.

4. Modulus of Elasticity increases from 1.2 x10⁶ to 1.4 x 10⁶.

This should go a long way toward settling the issue.

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[andriver]

Thank you SlideRuleEra,

Your numbers are comparable to mine, we are able to get a little more out of the timber by removing the FOS inherent in timber design. Per NDS Commentary 1997 there is a 1.67 FOS, per Breyer, D. (2006). Design of wood structures--ASD (6th ed.). New York: McGraw-Hill the FOS is 2.5.

I had proposed a solution of the 6x6 Southern Pine NO. 2 to be 12" O.C. or 4x6 Southern Pine NO.1 at the 16" O.C. The company is resisting the need to change their design, although they now agree that the 4.0 FOS is mandatory. Their solution? Drop the allowable Live Load.

We are having a meeting about this shortly but the consensus is, we paid for 25 psf live load, provide us with 25 psf live load. Also, the bureaucratic nightmare that would occur to approve below 25 psf working load in my company would be substantial.

Essentially they are asking us to sweep this under the rug, and that's not going to happen.

I would like to say you brought up a good point about the 18' span potentially having more flaws. I had always though of it by the face, ie the 6x6 will have more flaws than a 2x4. Your comment made me realize this is the case for the longer spans as well. I may be able to dodge several flaws by cutting this wood down this line, but the further I have to go, the less likely I am to not encounter more flaws. Thank you for that input.

 

[Lomarandil]

Agreed, limiting access platforms to anything less than 25psf is a head scratching exercise for most construction activities. (Or more realistically, the discrepancies just get ignored -- If I had a nickel for every time a foreman told me "That's what the FOS is for!")

I'm glad to hear that your company is standing their ground on this.

 

[Archie264]

Andriver, I believe you said the wood's already purchased and on site. Could you just purchase some more of the same size and cut the spacing to 12" oc? Or 10" oc? Or whatever's needed, taking into account everything mentioned including the deflection issues SlideRuleEra pointed out? It might be the simplest and fastest solution and, after considering all the meetings and correspondence other solutions would involve it might also be the cheapest.

SlideRuleEra, I enjoyed watching you dissect the issue in simple terms. It's a good reminder of the clarity that can be had by straight-forward hand calculation vs. losing the forest in the trees-worth of computer output that can happen.