AASHTO Division I Sec. 3.15.3 Overturning Forces 2

[dabbler101]

Can someone help explain this section to me? I'm working on a 100' long single span bridge... Is this section pertinent to developing overturning forces on an abutment? The way I interpret the section is that for a simple span design this force will counter act some of the dead load of the superstructure, and depending on the load combination it may or may not lead to a more critical condition. Can someone explain what the theory behind locating the force at the "windward quarter point of the transverse superstructure width" is? And how locating at that point translates to an overturning force?

 

[bridgebuster]

The overturning force is not applied to the abutment only piers.

 

[Dinosaur]

I don't have my copy of the AASHTO handy, but based on your description I guess you are talking about the wind load acting on the superstructure.

In the past, engineers have observed that when wind flows over a bridge, an airfoil pressure differential may develop. In this case, a vertical force results that tends to lift the bridge. However, this force is dynamic and the point of application of the net force will not be in the center of the structures width.

In the code, an uplift condition of this kind is perscribed and acts at a distance W/4 from the mid-line of the bridge. The effect of this force can cause additional bending in piers, particularly hammerhead piers. Wide bridges with relatively short columns will see the most dramatic effect as a percentage of the total load.

Like many load combinations, this can't really be tied to a specific wind speed, angle of attack, depth of superstructure to width, and all that. But it is an attempt to make a reasonable conservative estimate of the potential effect of uplift on the supporting substructure.

 

[dabbler101]

Thank you much, that's very helpful...