Designing Steel Beams for Long Spans in Residential Applications... 1

[KevinChez]

An architect would like a clear span of 50 feet for a one-story garage on a residential property. There may be a green roof and then snow loading (no gathering space)

Deflection will control my design but I don't want to use L/360 (1.67"). Thinking through what is practical criteria for this span. I will probably design for 1" total defection. They are prepared for a W24 or W36. But I do want to be reasonable.

I did not find any literature in my quick google search. Wondering if anyone has any tips.

Thank you.

 

[TheDW]

I'd punt it to the client.

Give them a few beam sizes with their design deflections, for example...what code minimum would be, what you'd recommend (1" sounds like a good starting point), a few that are more stiff...and let them decide what they can stomach.

 

[JAE]

For that long of a beam, I'd consider some initial camber, maybe 90% of the dead load, especially if there's a long term dead weight of a green roof involved.

Then all you are looking at is L/360 for the snow and perhaps 10% of the dead load.

 

[KevinChez]

Thx...JAE do you ever consider natural camber from the milling process? I read 1/8" per foot of length divided by 10. That would give me 5/8" to work with at the start.

I will probably do as you say DayWalker and give a few options. Thx.

 

[phamENG]

Kevin - I wouldn't count on natural camber. Natural camber is a maximum, so it could be any value in between 0 and .125L/10.

 

[KootK]

A couple of exotic solutions that probably would not meet the "reasonable" litmus test on a residential project.

1) Post tension just above the bottom flange so that you can sort of "tune" your camber.

2) If there's a green roof, I suspect that there's also a parapet. That might afford an opportunity to replace the beam with a simple HSS truss utilizing the depth between the top of garage door and the top of the parapet.

Sometimes there is marketing / perception value in having a couple of exotic, if improbable, solutions to table with your client. A useful stance for our kind to take is usually something to the tune of "We can engineer a solution to anything; it's only a matter of how much that solution will cost". How creative you want to appear will somewhat depend on whether you're more interested in impressing an architect or a contractor.

 

[SlideRuleEra]

They are prepared for a W24 or W36. But I do want to be reasonable.

To limit deflection, even a heavy W24 will be challenged (Span/Depth Ratio of 25).

The lightest W36 will probably be more than needed (Span/Depth Ratio of 17)

Don't arbitrarily limit the decision to those two depths, the "optimum answer" will likely by a W27, W30, or W33.

http://www.SlideRuleEra.net

 

[jayrod12]

do you ever consider natural camber from the milling process?

I would say no as that is the maximum allowed from the milling. You aren't guaranteed to get said camber from the mill.

If green roof, be sure that your drainage details are tight, and the drains are naturally located at the low point (think at the midspan of this clearspan beam). If they want to do perimeter drainage, be sure that they have more than the code minimum slope as then need to contend with the expected deflection and still ensure appropriate slopes.

Assuming this thing is maybe 24 or 30 feet wide. Minimum slopes would require 1.5" to 2" of slope. That's barely enough to cover the deflection. Combine that with the joist/truss deflection and you've got a recipe for minor ponding and drainage issues.

 

[r13]

For high sustained load, long term deflection need to be considered.

 

[JAE]

retired13
For steel beams long term deflection = short term deflection.

 

[r13]

JAE,

Thanks. I shall rephrase to "deflection for high sustained load should be considered".