Steel Beam - Bending Safety Factor

[Hammer335i]

I am designing an agricultural greenhouse type structure that will never be inhabited or be used for shelter by anyone (fenced off and presents minimal risk of danger to anyone). AISC defines the safety factor against yield in beams in bending as 1.67.

Since this structure has an estimated life of 20 years and really presents no problems, does anyone see an issue with reducing this factor to say 1.25? If the beam was over loaded & deformed, the owner would not care - possibly would just replace it.

What do the experts think!?!?

Thanks,

Paul

 

[UcfSE]

I would stick with the AISC. It's not worth the risk imo.

 

[TLHS]

This is not a good idea. If you're trying to take into account the fact that it's a low importance building, there are likely clauses in your building code that allow you to reduce environmental loading based on an importance factor. This would actually be within code rather than what you're proposing. Twenty years is a long time.

Also note that the failure isn't always yield. Even in allowable stress, the formulas are set up in such a way that you'll hopefully get your factor of safety over the governing failure mechanism. If your failure mechanism is plastic hinging where there's a redundant stable system you might get out okay even though it's still a failure. If your failure mechanism is something else like section yield, lateral torsional buckling, your connections, or even hinging without any redundancy then your whole thing breaks.

 

[msquared48]

He could always sell it to a new owner. Then what?

Your importance factors are figured into your wind and seismic values for the lateral resisting system. Never heard of one for the vertical except when combined with vertical seismic forces...

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[Hammer335i]

I am using ASCE 7-10 for wind and snow loading, etc and it is true that that there are reductions in the loads due to risk/importance. There are no building codes governing this installation and the client is looking to reduce the steel and this is one way I can see a possibility.

It is the standard factor of safety and I get why, but since this is not a typical building, but rather a small structure, I'd like to get more than 60% of the capacity of the beam.

If this is really a horrible idea, does anyone have any other suggestions on how I can be less conservative in my design?

Thanks!

 

[BAretired]

Stick with the code, Hammer. Temporary buildings have a habit of becoming permanent buildings.

BA

 

[JStephen]

If this is not your normal type of work, having somebody more familiar with it go through the design might be beneficial. You have different options for figuring strength, for figuring wind loads, etc, and prior experience can help with optimizing a design.

If you look at labor and coating costs, then just tweaking structural weight slightly may have a pretty negligible effect on constructed cost.

One advantage to using a code is it gives you some confidence in the results. You know it SHOULD work, since all the other structures built with that code stood up okay. If you intentionally undercut the code, you lose that confidence.

You mention this would not normally be inhabited, but if it's a greenhouse of sorts, it should have workers in it periodically, and it needs to be safe for them to work in. You'd have an awfully tough job justifying that reduction in safety factor if someone got killed in it. Reminds me of the Dallas Cowboys practice structure that collapsed.

 

[dhengr]

Hammer:
It is a fact, that in many locations agricultural buildings are not tightly controlled by local building officials or building codes. The logic is that a few dead cows, pigs or hay bales isn’t the same as a bunch of dead people. But, that won’t prevent you from being the goat if anything goes wrong, and the guy ends up with a bunch of dead tomatoes if the roof comes down. Don’t be less conservative in your design, be more thorough and thoughtful in your design. I wouldn’t argue with you about the difference between 1.67 and 1.5 in a few very carefully considered locations, but not at an early failure mechanism. And certainly, you must have some agreement with the owner that you will not be hung out to dry if something goes wrong in saving him a buck up front. Is the structure shaped such that it sheds snow, and will the owner clean up the sloughed off snow around the bldg., so that the bottom 6' of the side walls don’t become retaining walls, with an unbalanced loading. What can you do in your design to make this structure very easy to fabricate, erect and maintain? Remember, an hour saved there will buy you a hundred pounds of steel. Simple, repetitive details to fab and assemble and maintain may be worth more than a few pounds of steel saved, up front. How is the structure going to be enclosed? Clean and simple and easily repaired, I would hope. Remember the humid interior conditions and the toll that can take on steel, therefore a very durable paint system, maybe a material other than steel.

 

[Hammer335i]

Thanks for all your input! I suppose I will not move forward with that design modification.

I actually work for a company that does little structures like this all the time and they are also pressuring to lighten the system as well (in this case the client is pushing harder than my company). We build thousands of these small structures so we are not looking at saving a few hundred pounds of steel, rather a few million pounds of steel. The design is about a cheap and quick as can be to install - all that is left to do is lighten the steel (aluminum is cost prohibitive these days). I can tweak hardware, but literally, this one beam makes up about 40% of the weight and cost of the structure and that is compounded in galvanizing costs, shipping costs, labor costs, etc.

I suppose the best way to reduce loads is to do wind tunnel testing.

Thanks all!

 

[cliff234]

Wind tunnel testing will not be a solution for your situation. Two reasons. First, wind tunnel tests will often increase loads. Second, wind tunnel tests are specific to the locations where structures are located. Since you are designing structures that could be located anywhere, you cannot (to my knowledge) do a generic wind tunnel test.

 

[BAretired]

If one beam makes up 40% of the weight and cost of the structure, you may wish to consider replacing it with a lighter member, particularly if it is repeated in many similar structures. A truss or castellated beam would undoubtedly save weight, but the fabrication costs may rule it out economically.

BA

 

[JStephen]

Possibly use higher-strength steel if yield strength is a governing issue, or have a finite-element analysis performed to evaluate buckling if that is the limiting criteria?

 

[Hammer335i]

BA - yes, I am trying to reduce the member.

JS - higher strength steel is a option.

Just to give an idea, each structure is about 30' long by 15' wide, monosloped open to the air. The beam in question is a 4" round tube that I would like to reduce the wall thickness of in order to reduce the weight. Each foundation costs about $350, so the idea is to reduce foundations first, then thin out the pipe. No chance I turn it into a truss economically. I am looking at going to a thin roll-formed shape.

Thanks all.

 

[rb1957]

is round tube the most efficient section ? maybe square or rectangular are also cheap and available ? I-beam ??

locally reinforce the critical section would be cheaper than high strength steel, no?

Quando Omni Flunkus Moritati

 

[dik]

likely rectangular HSS is the most cost effective...

Dik

 

[BAretired]

I don't know why a hollow section would be the most cost effective for a member in bending. Maybe a sketch of the roof framing plan would suggest an alternative idea.

BA

 

[fancypants]

A tube of some type would be more bird proof it that is going to be a problem.

 

[steellion]

Have you considered roof live load reductions as permitted by the code? What is your deflection criteria? It's hard for me to believe that a 4" thin-walled tube will meet deflection requirements for this roof. If it is a glass roof, excessive deflection could cause failure of the glass.

Based on the info provided, it's possible an I-shaped member will increase your bending capacity, reduce deflection, and perhaps be easier and cheaper to connect.

 

[spats]

I'm surprised nobody has mentioned that the the building code is a MINIMUM standard. The statement "there are no building codes governing this installation" is not correct, unless there is no juristiction. Building codes apply to greenhouses as well as habitable buildings. ASCE7, which is included by reference in most US codes is entitled "Minimum Design Loads for Buildings AND OTHER STRUCTURES"... seems like that says it all, unless you're not concerned about your liability.