Lateral Bracing for Long Span Beam
Lateral Bracing for Long Span Beam
(OP)
Hi,
I have a question about lateral bracing for a beam I'm designing. It's an existing 2 story structure, built 2 years ago. The setup is repetitive. Anyway, they want to remove the 2nd story mezzanine and and 2 interior columns from the interior of the building and hang the roof from a new beam stubbed up from the roof. The span is 75', with 2 point loads (interior columns)acting symmetrically. I sized it as continuously braced, but have questions about the lateral bracing. I am going to attach another W section horizontally to the top flange of the beam. This horizontal section will resist the lateral forces due to Lateral torsional buckling. I'm sizing it for lateral load of 2% * half the reaction of the beam.
The point loads are equal to 35 kips. The reaction of the beam is 35k + .262k*75'/2 = 44.9k. 2% of this is .8985 kips. This is applied per foot so the moment is .8985*75^2/8 = 632 k*ft. I also designed it for L/360 -> Ix required =8823 in^4. I get a W24x250. Does this sound reasonable for this kind of span? Or is it 2% of the reaction from self-weight only? I don't think that's the case, buy it yields a much smaller section, W21x93. I know it's a 75' span so the lateral loads are large but I just want to justify my calc.
Thanks
I have a question about lateral bracing for a beam I'm designing. It's an existing 2 story structure, built 2 years ago. The setup is repetitive. Anyway, they want to remove the 2nd story mezzanine and and 2 interior columns from the interior of the building and hang the roof from a new beam stubbed up from the roof. The span is 75', with 2 point loads (interior columns)acting symmetrically. I sized it as continuously braced, but have questions about the lateral bracing. I am going to attach another W section horizontally to the top flange of the beam. This horizontal section will resist the lateral forces due to Lateral torsional buckling. I'm sizing it for lateral load of 2% * half the reaction of the beam.
The point loads are equal to 35 kips. The reaction of the beam is 35k + .262k*75'/2 = 44.9k. 2% of this is .8985 kips. This is applied per foot so the moment is .8985*75^2/8 = 632 k*ft. I also designed it for L/360 -> Ix required =8823 in^4. I get a W24x250. Does this sound reasonable for this kind of span? Or is it 2% of the reaction from self-weight only? I don't think that's the case, buy it yields a much smaller section, W21x93. I know it's a 75' span so the lateral loads are large but I just want to justify my calc.
Thanks






RE: Lateral Bracing for Long Span Beam
I am unsure about what you mean in the first sentence. Are you setting a second W section on its side, atop and parallel to the main section?
If so:
Using the simplified method you are suggesting, I would size this for 2% of the compressive force in the compression flange, not 2% of half the beam reaction. With the second section spanning the entire length of the main section, this will result in a varying force with the same shape as the main beam's moment diagram.
However, an analysis of the actual moment resistance of the new section may result in a smaller section being required, as the combined section will have greater lateral and vertical stiffness than the two added together. I would go this route, as you will need to design the connection between the two members for shear flow anyway (even if you aren't counting on it, it is there and not designing fot it could fail the connection).
And it should be 2% of the total worst case loads.
RE: Lateral Bracing for Long Span Beam
RE: Lateral Bracing for Long Span Beam
I think you'd be better off in your circumstance to use torsional bracing to laterally stabilize the beam.
It will make your profile smaller as the strut beams will frame into the side of the girder (versus sitting on top)
This allows for a cleaner moment connection to the girder too. Also, you can design the other end as a pin connection and don't have to worry so much about adding any additional loads into the original building components(aside from the vertical reaction)
Appendix 6.3 of AISC 13 does a fine job of walking you through the process.
RE: Lateral Bracing for Long Span Beam
What you are doing is creating a new section comprised of the two wide flanges. Simple calculate new section properties and design the beam accordingly as a unit.
With LRFD, you'd use appendix F, calculating all the required λ factors and using the beams new cross sectional properties (you will still have a singularly symmetric shape).
With ASD, you now have a different rT factor (a very large compression flange) and design according to the beam formulae given.
RE: Lateral Bracing for Long Span Beam
RE: Lateral Bracing for Long Span Beam
Pretending that the new capping beam is somehow an independent thing that laterally braces another beam is pie-in-the-sky simplification.
The two will work together as a single flexural element in reality.
By adding the WF on top, you are greatly altering the cross sectional properties, and the neutral axis location.
Design it as a combined section. Calculate the horizontal shear using q = VQ/I and design the longitudinal connecting welds accordingly and in accordance with Chapter B in the AISC Spec.
RE: Lateral Bracing for Long Span Beam
RE: Lateral Bracing for Long Span Beam
RE: Lateral Bracing for Long Span Beam
The problem with using a horizontal beam to resist the lateral restraint force is its lack of stiffness. It may have the strength to resist the 2% of flange force but, unless proven otherwise, I would assume that it doesn't have sufficient stiffness.
RE: Lateral Bracing for Long Span Beam
RE: Lateral Bracing for Long Span Beam
RE: Lateral Bracing for Long Span Beam
Incidentally, you might want to review how shear flow is calculated....the shear is definitely not zero at the interface between the 2 sections.
RE: Lateral Bracing for Long Span Beam
I'm understanding this differently than you are. If I understand faromic, he has abandoned the idea of providing lateral support to a 75 foot beam with a horizontal beam attached to the top flange, and wants to use a hot rolled shape with no lateral support, and design it for deflection of L/360. He said that deflection will govern over strength, and I disagreed with that.
RE: Lateral Bracing for Long Span Beam
RE: Lateral Bracing for Long Span Beam
faromic - I think that is just plain wrong. There is no prescribed design method in any specification (AISC or otherwise) that suggests you design a compound beam by first designing the "vertical" beam as fully braced and then providing a "horizontal" beam to brace it using some rule of thumb 2% rule.
I honestly, and humbly, suggest you design it as a compound beam using the AISC prescribed methods for single symmetric shapes.
Just my opinion. But do show me where AISC suggests you can do it your way...I'm all ears.
RE: Lateral Bracing for Long Span Beam
I'll keep you posted. Thanks a lot.
RE: Lateral Bracing for Long Span Beam
If you're not going to design the member as composite, you must design the connection between them to not carry any shear. do not weld them together. you'll require some kind of angle configuration butted against the top flange of the lower beam and welded to the (I assume)web of the top beam rotated horizontally.
It can be done but is probably not as efficient as using a combined section. the top member will have to be designed as fully unbraced along the 75 foot span as well and you need to limits its deflection to prevent twist in the lower beam from horizontal deflection as ctcray suggested.
your best bet would be to find a way to laterally brace the lower beam with perpendicular members bearing somewhere along a column line or building perimeter. torsional restraint pinned at the far end will put the least additional load on the existing building.
RE: Lateral Bracing for Long Span Beam
Guess you had it right. I must be going crazy.
faromic-
Sorry, misunderstood you. Research has shown that lateral bracing needs a certain stiffness as well as strength. The 2% rule is not always adequate. It may provide the required strength, but not the required stiffness, for the type of bracing you are proposing. As others have said, you need to refer to Appendix 6 for the stiffness (and strength) requirements. I expect you'll find that you cannot meet the stiffness requirements, and need to design as a combined section, as JAE has suggested.
RE: Lateral Bracing for Long Span Beam
RE: Lateral Bracing for Long Span Beam
RE: Lateral Bracing for Long Span Beam
RE: Lateral Bracing for Long Span Beam
RE: Lateral Bracing for Long Span Beam
RE: Lateral Bracing for Long Span Beam
RE: Lateral Bracing for Long Span Beam
Combined section properties:
W36x260 and W21x93(laid flat on top flange of W36)
St = 1362 in3
Sb = 1054 in3
IT = 2614
AT = 51.1
Sx(top) = 1361.9 in3
Sx(bott) = 1054 in3
rT = 7.15"
l = 900"
l/rT = 98.4
Cb = 1.0
Fy = 50 ksi
Fb(comp) = 17.51 ksi per AISC Eq F1-7
Fb(tens) = 30 ksi per AISC Eq F1-5
M = 1123 ft-kips (2 - 35k loads plus 355 plf self weight on 75 ft span)
fb(tension in bottom) = 12.8 ksi OK - less than 17.51 ksi
fb (comp in top) = 9.89 ksi OK - less than 17.51 ksi
Forget the 2% stuff.
RE: Lateral Bracing for Long Span Beam
Use v = VQ/It to check the welds between the sections under longitudinal shear. Usually a staggered half on half off will do the job.
RE: Lateral Bracing for Long Span Beam
RE: Lateral Bracing for Long Span Beam
RE: Lateral Bracing for Long Span Beam
RE: Lateral Bracing for Long Span Beam