For 3-138' continuous spans (P/S bu

[rowe]

For 3-138' continuous spans (P/S bulb tees), how can I show expediantly than intermediate diaphragms are not required. (If I can eliminate the intermediate diaphragms, I can save the contractor approx. $200,000.)

I appreciate any guidance.

 

[Qshake]

Good Luck. Most state bridge manuals require diaphragms for use with even moderate lengths (50-100). The single concern I would have outside of design of the bridge is the erection of those girders. Bulb tees are girders with relatively small bases, tall webs, and large flanges. This adds up to a critical situation with wind loading during erection. Or for that matter if something were to bump the girders during erection.

It seems that an ounce of prevention is worth more than a pound of remedy...

 

[curvbridger]

rowe,

I believe that you are stuck calculating horizontal bending in the bulb under wind loading. Much like a steel bridge, you can assume half of the wind loading is carried by the deck, and the other half in horizontal bending of the bulb.

For spans such as you describe, I would call for temporary diaphragms for erection at quarter points, and (probably)permanent diaphragms at piers and midspan only.

Have fun!

 

[rowe]

Qshake & Curvbridger - Thanks for your responses.

The P/S bulb tees will be cast with inserts to attach temporary lateral bracing (x-frames) at the mid-point. However, the DOT will not allow permanent steel bracing.

The DOT requires an intermediate diagphragm unless it is shown in calculations that there is no need for it.

I can calculate the torsional load on the section due to wind and compare it to the allowable torsion (ACI). But the stability equation for lateral torsional buckling only determines the critical bending moment and not a "torsional load". How do I "add" the effects of bending to the torsional load from wind?

 

[TpaRAF]

Greetings, Rowe :

I concur with QShake that lateral bracing during erection should be used -- especially with bulb tee's.

Since these appear to be continuous girders, your bottom flanges will not always be in tension -- hence, lateral bracing may be required anyway. If they were simple span, the bottom flange remains in tension (no bracing), and the top flange (compression) would be braced by the deck.

Is torsional loading going to occur during erection? If the beams are not anchored at the support, the only resistance to twisting would be the beam itself.

To check the horizontal bending, use the beam's properties about the weak axis, with the wind acting as a uniform load. If you exceed any of the compressive or tensile stress limits (0.60 f'ci or 3*sqrt(f'ci)), you'll need bracing (to limit L).

I hope this helps!