Lateral deflection of beams/girders due to wind 7

[2134]

Hi all, Is there any way (the formula/any check/limit) to calculate the lateral deflection of bridge girders due to wind only.It will be very helpful if anyone gives me guidelines to calculate the latearl deflection.

 

[graybeach]

Why not calculate the moment of inertia in the direction of the wind, apply the load and calculate the deflection using the appropriate beam formula, based on your bearing situation? You need to include the concrete deck and all the girders in the I.

 

[jike]

The girder bottom flanges may deflect a small amount between diaphragms but I cannot imagine that it is significant. The main load will be taken into the deck diaphragm.

 

[civilperson]

There is no upper limit on lateral deflection so why bother?

 

[2134]

Thanks for your valuable reply and time.I have one more question how you will calculate the deflection during construction /(i.e without concrete deck)suppose we have wind speed of 100m/hr ,5 girders,bridge span
200ft,crossframe/diaphragm spacing 25ft and any girder section but no concrete deck so how we can calculate lateral deflection.

 

[Lion06]

Use the weak axis moment of inertia, but I am still failing to see the relevance. Deflection is a serviceability issue. I can't imagine anyone cares how much the girder deflects laterally before the deck is in place and the bridge is finished. I don't know too much about bridges, but unless this deflection is going to impact other components, I don't think it will matter. Once the deck is in place then you won't have the issue at all as one of the previous posts mentions.

 

[DWHA]

I spend over half of my working hours designing temporary construction supports and in cases like this I have never found it to be an issue. The reason being is that nothing is going to be connected or done to the bridge during the high wind speeds since there will not be anyone out on the bridge working on it. As long as the girders are stable and are not going to fall off the supports there is nothing to worry about. And since deflection is a servacibility issue only, deflection due to wind should not be a problem.

Sorry I did not answer the question, I hate it when people throw out answers on here that do not answer my questions.

So...
If you really want to check it I would recommend modeling it in a computer program. Personally I would use RISA 3D, but that is up to you. I believe that you can download a "demo or trial" version if you do not have it. You will not be able to save anything or get any printouts, but you should be able to check it that way.

 

[DWHA]

If you would like to do it by hand, StructuralEIT is correct. Since diaphragm spacing is 25ft, all the girders are going to have approx the same deflection. So take the load divided by 5 girders and apply it to the weak axis moment of inertia.

 

[jechols]

Depending on the x-bracing between girders it might act like a truss. For a quick estimate, the out board girders could be treated as chords of truss. Then (I approx.) would equal 2*(Ad^2). This would probably yield a higher (I) than the sumation of the individual (I's). However, I am not very familar with bridge design so the x-bracing might not be substantial enough to cause truss action.

 

[kelowna]

I think that you might be missing something here guys. What about lateral torsional buckling of the girders?

Composite bridge girders have very slender and small top flanges because once the concrete is set, they do not work that much. Furthermore bracing on bridges is usually provided for the bottom flanges but not for the top flanges because they are restrained by the concrete slab.

This means that during erection the top flanges of steel girders are working very hard and close to their limit, not because the stresses are high but because a flange under compression, long unrestrained span, has a high slenderness ratio.

Lateral wind loads might be enough to cause lateral torsional buckling failure. So although the wind loads themselves will be very small, they might be enough to puss the girders over the edge. The fact that 2134 is asking aboug deflection checks worries me even more. If the deflection is significant, then the girder capacity to support its own weight might be compromised.

Just a wild thought from my ignorance.

 

[UcfSE]

2134, what kind of work are you doing that you need to check this? Why aren't you asking the senior engineer in your office?