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RE: Lateral Torsional Buckling

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Joseph Yura presented a great discussion on beam bracing at the 2006 NASCC in San Antonio. He conducted research on steel bridges with timber decking and found that the timber decking provides lateral support under load conditions through friction. I suggest looking at his paper through AISC EPUB's.
 
Bill
-----Original Message-----
From: Padmanabhan Rajendran [mailto:rakamaka(--nospam--at)yahoo.com]
Sent: Thursday, March 09, 2006 9:19 AM
To: seaint(--nospam--at)seaint.org
Subject: Re: Lateral Torsional Buckling

Daryl,

Thanks.

I did not mention about the wearing surface. It is made of 2X12, each 2X nailed to the 6X6 underneath it.

Diaphragm weighs about 180 lbs.

As you may notice from the rudimentary sketch, I provided earlier, the flange of W36 is connected to the 6X6 only at the "outer" end of the flange. Therefore, the restraint to lateral relative movement between the flange and the 6X6 comes through the friction between the two surfaces. I cannot see how the warping or the twisting of the section could be prevented. If my apprehension is well founded, is it appropriate to assume an allowable bending stress of 0.66Fy?

Thanks.

Rajendran

Daryl Richardson <h.d.richardson(--nospam--at)shaw.ca> wrote:
Rajendran,
 
        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.
 
Regards,
 
H. Daryl Richardson
----- Original Message -----
From: Padmanabhan Rajendran
To: seaint(--nospam--at)seaint.org
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.

        
â-¬â-¬â-¬â-¬â-¬â-¬â-¬â-¬â-¬â-¬â-¬â-¬â-¬â-¬â-¬â-¬â-¬â-¬â-¬â-¬â-¬â-¬â-¬â-¬â-¬ <----- 6X6
               â"?â"€â"€â"€â"€                                       â"€â"€â"€â"€â"OE <-----L5X3
                      â"'                                               â"'
                      â"'                                               â"'  <------  W36
                      â"'                                               â"'
                      â"'                                               â"'
                   â"€â"€â"€â"€                                        â"€â"€â"€â"€
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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