Timber Beam Stability Factor

[gnault]

I am trying to check the bending capacity of a 39"x7" glulam girder on a bridge. The bridge deck is composed of 2x6 deck lams in the transv. direction which are toe-nailed to the top of the glulam girder. Diaphragms are provided at the 1/4 points of the span as well as at the supports.

My question is in regards to the stability factor that I should be using to determine the bending capacity of the girder. Is the toe-nailing of the deck lams to the top of the girder considered a continuous lateral support for the girders? Or should I be basing my stability factor on the distance between diaphragm lateral supports?

AASHTO Standard Specification of Highway Bridges 13.6.4.4.2 states that "...when lateral movement of the compression zone is prevented by continuous support and points of bearing have lateral support to prevent rotation, there is no danger of lateral buckling and CL = 1.0."

The points of bearing on the bridge do have lateral support from the diaphragms. However, is the toe-nailing of the deck lams to the girders strong enough to classify as a "continuous support" for the girder?

Any thoughts or references on this is much appreciated. Thanks in advance for the help.

 

[beton1]

Not a fan of using toenail for lateral support. You may require something more substantial.

 

[BAretired]

The toe-nailed deck does provide some lateral support, but like beton1, I am not a fan of it. Depending on the span and width of your bridge, it may not be adequate to resist lateral loads.

If the deck is not considered a lateral support for the beam, you cannot count on the quarter point diaphragms either as they do not provide lateral support to your beams without a horizontal diaphragm keeping them in alignment.

Best is to form a horizontal truss from end to end using the glulam beams as chords and diagonal members designed to resist wind and lateral loading.

If you need additional lateral bracing for the glulams, you can arrange the web members of the horizontal truss to provide it.

BA

 

[MiketheEngineer]

The old rule of thumb was that you ahd to resist about 2% to 5% of the axial loads to give lateral support.

While toe-nailing can supply this - I also am not a fan. But are the quarter points not good enough?? Might be / might not be??

The installation of a simple clip at each 2x6 and a couple of screws might allow you to sleep at night.

 

[BAretired]

What is the span and width of the bridge?

BA

 

[gnault]

I am load rating an existing bridge. Thus, I am just dealing with existing conditions. The bridge is a 60' long simple span bridge with steel cross-braces at the 1/4 points.

I searched around a bit more on the internet and found some information in a Timber Bridge Manual published by USDA Forest Service. They have a passage in the transverse bracing section that states "Although some lateral beam support and load distribution are also provided by the deck, these effects vary with the type of deck attachment and are normally neglected in design." Thus, I will use the cross-braces at the 1/4 points to determine the lateral stability of the girders.

Thanks for all of your input.

 

[BAretired]

If the existing bridge does not have some type of diagonal bracing, you are not entitled to consider the 1/4 point bracing as lateral support. The reason is that both beams can buckle together so that the bracing simply keeps them a constant distance apart.

BA