Unbraced Length of Steel Beam

[Lake06]

I am designing a garage with a 24’x24’ foot print. I would like to use a steel beam to support the 2nd floor with a live load of 40 psf. The steel beam will be located 12 feet from the end of the wall. 2x10 dimension lumber will rest on top of the steel beam spanning in the perpendicular direction. The steel beam will have a 2 x (match width of top flange) wood board bolted to the top of the steel beam. Their will be 2 bolts every 2 feet along the beam. Based on my calculations using LRFD a W12x22 beam will work for this application, however the unbraced length is too long. According to the calculations I need to support the top flange of the beam in two locations minimum. The 2x10 joist will be spaced 16 in on center and toe nailed into the wood attached to the top flange of the steel beam. My question is, does the 2x10 joist attached (as referenced above) to the steel beam provide an unbraced length of 16 in on center or do I need to use some other type of connection.

 

[BAretired]

It is standard practice to consider wood joists toe-nailed to a wood nailer on a steel beam to provide lateral bracing for the beam. This assumes a plywood or OSB diaphragm nailed to the joists.

BA

 

[JoshPlumSE]

Take a look at appendix 6 of the AISC 13th edition (or the just published 2010 spec). You will get some guidance on what should be or should not be considered a point of bracing.

I do not believe that what you describe would meet AISC's criteria. That being said, BA is correct. In the past, I would say that this would have been considered bracing.... But, then again so would a point of inflection. And, that is clearly verboten these days.

I tend to deal with this issue by being a bit more generous with my Cb calculations where I may have used a Cb of 1.0 in the past. Even so, how much are you really adding to the cost of the structure if you upsize this beam? If it's my stamp, then I probably increase the size of the beam slightly until I'm comfortable with the design.

 

[Lake06]

BAretired......Is the diaphragm you are referring to the same method as shown in the attached detail?

JoshPlum…..I will take a look at the AISC’s appendix when I return to the office on monday. If I design this beam based on current AISC requirements and assuming the wood joist do not provide lateral bracing then I would have to increase the beam size, as you suggested. If the beam becomes too big, due to the 24 foot span and 40psf LL, then I guess I could take a look at using two smaller steel beams with steel diaphragms attached between them. I would prefer to use one beam though, maybe with someway of providing additional support from the wood joist resting on top, any ideas?

http://files.engineering.com/getfil...-aef3-55bbe89d11cc&file=steel_beam_detail.pdf

 

[bookowski]

Why not add a more substantial connection to the joist in a few locations in place of the toenail? Assuming that you have a diaphragm on those joists I would guess that you will satisfy the stiffness requirements of AISC, the only hangup may be the capacity of that toenail (maybe), if that's the case you could use an angle clip in a few locations to provide a more substantial connection.

 

[hokie66]

Lake06,
There is no diaphragm shown on your attached detail.

 

[rscassar]

I would use the wood joist to provide restraint for lateral torsional buckling. I believe the requirement to be considered a lateral restraint is the ability to resist 2.5% of the flange force laterally. For a 2x10 floor joists which is fixed to the nailing plate with 2 skewed 3mm dia nails (assumption) which can transfer laterally 2kN shear, you would need to use 4 or 5 joists acting togerther to provide restraint to the beam. Or I would detail a connection details which has the ability to resist the required lateral force to be considered a restraint.

 

[JAE]

Appendix 6 of the AISC specificiation outlines the strength and stiffness required for a beam.

These values are indicated as singular values for "a brace". What AISC does not tell you is the strength or stiffness "per foot" of beam.

The inherent intent within this spec section is that, for a beam that works for a specific Lb, you need a brace of X stiffness and Y strength at that Lb spacing. The strength and stiffness rely on the characteristics of the beam chosen.

A larger steel beam will be able to resist the moments with a longer Lb so the brace values go up significantly. A smaller beam may be able to resist the same moment but with a small Lb and so smaller braces at the smaller Lb result.

A singular 2x wood joist with toenails may not be adequate by itself to provide the necessary X and Y values (maybe the stiffness I don't know).

So you would have to use some kind of iterative procedure (or trial and error) to determine the beam size and whether the 2x10's can work. I would normally utilize ALL the 2x10's within an Lb region as serving as the singular brace.

Normally, I would assume that the 2x10's would certainly be a brace and I would typically assume Lb = 16". But under the new Appendix 6, I'm not sure that is "true" anymore.

 

[BAretired]

Lake06,

As hokie stated, your attachment does not show a diaphragm. All I meant was that there should be a wood deck fastened to the joists to provide diaphragm action. Without deck, the joists do not provide lateral bracing to the beam.

The Canadian Code, S16-01, Article 9.2.7 states "When bracing is effected by a slab or deck and the means by which the calculated bracing forces are transmitted between the flange or chord and the slab or deck shall be adequate to resist a force in the plane of the slab or deck. This force, taken at least 0.05 times the maximum force in the flange or chord unless a lesser amount can be justified by analysis, shall be considered to be uniformly distributed along the length of the compression flange or chord."

I would regard the joists and the nails attaching them to the nailer plus the glue and screws from deck to joist as the means by which the bracing forces are transmitted between flange and deck.

I am not very familiar with the AISC code and would need to study it in more detail before commenting further. I do not quite understand the difference between a relative brace and a nodal brace, but if each joist is considered to be a nodal brace, then JoshPlum is correct. The toe-nails would likely not provide the required lateral force at 16" centers.

I agree with bookowski. No need to use two beams or a larger beam. Provide a more substantial connection spaced as required for the beam selected.

BA

 

[DWHA]

I would take an angle and bolt it to the 2x10 and to to the W12. A couple of small pcs of angle and a few bolts will cost very little and with that, your connection will be much stronger and much more reliable.

I say this with out checking the AISC Spec. You should review the spec and see what you can do to meet the spec.

 

[apsix]

As the structure (joists) applying the load is also providing the lateral bracing you have the advantage of available friction increasing with increasing load.
Therefore I'd be satisfied with the toe-nailing.

I wouldn't be satisfied if the beam was supporting masonry, say, and relied on a couple of toe-nailed lateral restraints.

 

[hokie66]

Agree with apsix, although we may be in the minority in reasoning that increasing the load increases the bracing.

 

[BAretired]

Friction is of no use whatsoever if the live load is applied by vigorous activity such as dancing. It is better to provide a positive connection spaced as required by the beam.

BA

 

[apsix]

I'm not arguing about better, but about adequate.

If there's no friction there's no load.

 

[Lake06]

Thank you for all your helpful comments. It appears I will be using a more substantial connection at vaires points along the steel beam, such as angles attached to the nailer and floor joist, as required. Being new to Eng Tips I thought this was very helpful and plan on using Eng Tips in the future. For those of you who don't have access to Appendix 6 of the steel construction manual 13th edition, I have provided it as a PDF below.

 

[TK406]

Lake06 et. al. If you are looking for a quick online resource for steel beam design, check out WebStructural.com. You can quickly size steel beams for using AISC ASD or LRFD design criteria. I have found it to be a very helpful tool. Many some of you will as well. Check it out.

 

[a2mfk]

Seems simple enough, using APP D I would use every joist as a lateral brace, assuming as BA said that you have a diaphragm (floor decking) to ultimately transfer this lateral force into the shear walls. Using the force from those calculations, select a Simpson HGAM or similiar connection that meets your lateral force requirements to transfer the lateral bracing force into the joist and then into the diaphragm.

Or provide a top plate wide enough where you can HANG the joists off the side, which will also increase your ceiling height/head room. Then you can nail your floor decking directly into the top plate to brace the beam.

Or try a larger unbraced beam and figure $3-4 per pound (or whatever it is these days) of increased cost and compare the cost of installing those connections at each joist..