wind load 3

[yjung]

Hi all,

please help me.

to calculate wind load on superstructure, what tributary area i should apply for longitudinal direction?
the same area as transverse wind load? or i have to use differenct area?
thanks

 

[Qshake]

Wind load on the superstructure may be calculated as

150 psf x A x L

Where 150 psf is the wind pressure (Check also your state or local requirements for different criteria), A is the area of the superstructure in elevation (Curb height, slab height, haunch height, girder and any superelevation), and L is the tributary length of the span at the bent under investigation. In that sense, L is the average of the two spans on either side of the bent. For a longitudinal force analysis, i.e., a total force applied to the frame, L may be taken as the total bridge length. This analysis is usually done to determine the distribution of the forces based on the stiffness of the frame (bent) members. However, if the bents are all about the same height and have similar properties and fixity then the result is the same as the tributary assumption.

 

[yjung]

hi,

thanks for superstructure wind load.

i am confused with longditudinal wind force calculation on substructure design.
for longditudinal wind load calculation, AASHTO said, i have to use 50 lb for transverse and 12 lb for longditudinal with span less then 125 ft. what is tributray area for longditudinal wind load ?
is it the same as the tributary area of transverse wind load calculation? or i should use transverse area (along the width) ?
AASHTO did not say anything about tributary area for substructure.

thanks again

 

[Qshake]

I explained it in the post above. The superstructure load has to be applied to the substructure. The 50 psf is applied to the tributary span length at the bent. This amounts to one-half the length of the span right of the bent and one-half of the span left of the bent. The height is exactly as I described in the earlier post. The 12 psf force is calculated in the same way but applied in the direction parallel to Centerline bridge. Don't forget that these forces are applied at the c.g. of the superstructure and need to be adjusted for application at the bearing level.

 

[bridgengr]

I have to make a correction here. The wind forces are not applied to the C.G of the sturcture like seismic forces are. Wind foces apply to the center of the vertically projected area, which is not the C.G of the structure. Actually, wind causes a torsion that is equal to the wind load times the eccentricity. There is another important load in the AASHTO LRFD Manual Article 3.8.2 and this is an up-lift wind load of 20 psf times the entire deck area. So, if the deck is 50 feet wide there is a continuous uplift linear force of 0.1 k/ft which is applied at quarter point of the deck on the windward side. This requirement introduces a much higher torsion (12.5 ft of eccentricity) on the cross section of the superstrucure and accounts for aerodynamical uplift forces..