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Lateral Torsional Buckling

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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?

Thanks.

Rajendran


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