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Re: Pedestrian Bridge Live Load

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David,

AASHTO published in 1997 "Guide Specifications for Design of Pedestrian Bridges". 

1.2 Design Loads:

1.2.1.1 Pedestrian LL:
Main Members shall be designed for 85 psf.  
If the area to which the load is applied exceeds 400 sf, the pedestrian live load may be reduced:

w=85*(0.25+(15/A^0.5)), but never less than 65 psf. 

Secondary Members (Floorbeams etc.) shall always be designed for 85 psf. 


1.2.1.2 Vehicle Load:
If no agency design vehicle is specified use

deck width 6-10 ft: 10,000 lb (H-5 Truck)
deck width over 10 ft: 20,000 lb (H-10 Truck)

No combination with Pedestrian LL necessary. 

Since you have a 70 ft simple span and probably about a 10 ft clear deck width you could design your mainmembers (truss chords/diagonals?)for about 70 psf. 
Some engineers prefere to design with 85 psf, what is too conservative for longer spans. We recently designed a single-span inclined rib steel bow truss in Tempe, AZ with a span of 200ft. We used 65 psf for the mainmembers. 

I recommend to get the AASHTO guide specification. 

Oliver Mueller
PARSONS
Chicago, IL
Oliver.Mueller(--nospam--at)parsons.com
www.parsons.com
 
 




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