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RE: Lateral Torsional Buckling

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How are wind loads (on the structure and on vehicles) being transfered to the abutments? Will the timber deck function as a diaphragm? Do you need some angle bracing at or near the top flange?
Jim K.
-----Original Message-----
From: Daryl Richardson [mailto:h.d.richardson(--nospam--at)]
Sent: Thursday, March 09, 2006 11:08 AM
To: seaint(--nospam--at)
Subject: Re: Lateral Torsional Buckling

        The top flange of the W36, as you've presented the structure, does not seem to me to be adequately reinforced laterally.  You have presented no means of transferring shear laterally between the 6x6 wood members.  Now if the structure has a three inch wear surface consisting of 3x8 or 3x12 wood planking spanning parallel to the W36 members and securely fastened to the 6x6 would provide adequate lateral support for the top flange.  It would also help distribute any point loads to more 6x6 members.
        If you're seriously concerned about actual twisting you could decrease the spacing of the diaphragm bracing from 17' to say 10' by only adding three more braces. What do they weigh? 100 pounds per diaphragm?
        I have another concern that you might want to address.  With only two main girders the span of the 6x6 seems rather long.
H. Daryl Richardson
----- Original Message -----
Sent: Thursday, March 09, 2006 9:14 AM
Subject: Lateral Torsional Buckling

The allowable flexural stress of a rolled W-Section can be assumed to be 0.66Fy if the laterally unsupported length of the compression flange is less than a certail span, Lc, which depends on the section properties.

A 70' span, steel beam/wood deck, bridge has come for my review. The bridge is 14' wide. Two steel beams (W36X160), 9' apart support 6X6 wood members, laid continuously across the top flanges of the steel beams. A continuous angle member (L5X3) is bolted to the underside of each of the 6X6 wood beams. The outstanding leg of the angle (pointing downwards) is bolted to the W36 at 30" spacing. Hope the following schematic sketch explains it.

▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬ <----- 6X6
               ┐────                                       ────┌ <-----L5X3
                                                                     │  <------  W36
                   ────                                        ────
In addition, There are transverse diaphragms spaced at 17'.
The design engineer considered that the above arrangement provides effective restraint against lateral torsional buckling and applied an allowable stress of 0.66Fy. I contend that, even if the deck effectively restrains the lateral moveemnt of the top flange of W36, a lateral brace should prevent rotation of the cross section. On this basis, the laterally unbraced span should be 17' which is the spacing of the diaphragms.
I recall reading somewhere that in order to qualify for an effective restraint against lateral torsional buckling, the brace should restrain, at least, the upper third dedpth of the W-section.
May I have some opinions?



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