pedestrian bridge

huangyhg

pedestrian bridge
here is an easy question. aashto section 9.7.5.1 states that precast deck slabs shall be 7" thick or more.
i'm designing a pedestrian bridge and want to employ precast concrete deck slabs, but i am having a hard time swallowing the requirement for 7" slab for a modest deck span that could easily work at 4 1/2 to 5" in thickness.
i've looked in the aashto pededstrian bridge publication and this topic isn't addressed.
has anyone come across this? is 7" the golden number regardless of analysis?
have you thought about 8" hollowcore?
mike mccann
mmc engineering
i'd be careful about hollow core in exposed conditions.  as for the arbitrary 7" for pedestrian bridges, i'd contact the associated dot.  they are usually not very flexible.
do you anticipate the use of salt or chemical de-icers on the slab, as this could be driving the aashto minimum thickness for reinforcing protection.   
mike mccann
mmc engineering
the aashto commentary mentions something about shear keys breaking off when the slab thicknesses are to thin, essentially eliminating the tied together aspect of a bridge. in short, the precast panels need to be tied together to transfer forces from lateral loads to a perimeter chord that acts in tension. if traffic eventually wore the bridge to the point that the shear keys disintegrated, the panels would no longer function together and the bridge would be unstable.
my argument is that i don't have hs20's rolling over things, i have an 85 psf (once in a blue moon) foot traffic, with the occasional maintenance vehicle.
mike, can you precast hollow core with notches for shear keys and tension chords? or is it a nominal, off the shelf panel?
i was designing one going from a stair tower over to an amusement park. the span was pretty far b/c it crossed over the main road into the park.  i sat down with the owner and convinced him to get a quote on a prefabricated bridge.  it came in much less than what the original cost estimate was.  i did my job by saving him money, and still covered my behind by having the liability fall on the manufacturer.
this is who he bought it from:
the precast panels we have here can be prestressed to accommodate the load, and the ends grouted if required for additional shear and rebar embedment.  they are special order, not like a glulam off the shelf.  
you might try contacting your local prestress panel manufacturer to see what they have.  are you in boulder or denver?
mike mccann
mmc engineering
chip
i see those bridges and think, "how the heck is the compression chord braced?"
we argue here about if or whether tension chord of a truss needs to be braced and these bridge guys are laughing at us all the while.

jlnj
compression chord bracing is likely provided by the the stiffness of the 'vertical post - deck beam - vertical post' u frame.
the last ped bridge i designed had a 4" cip concrete deck with 2" metal form. the section you're citing is intended for vehicular bridges. however, with that in mind, you could end up with 7" based on the vibration criteria of section 1.3.2 of the ped bridge guide spec.  
mike, i am in the mountains of colorado.
jlnj, think "space frame"
bridgebuster, you are correct. we will be looking at frequency per the aashto spec.
chipb, the bridge is a retrofit. we are replacing the existing concrete filled metal deck with something less corrosive. so we have to be concious of how much additional weight we add. matching what is there would be ideal.
i've made a reasonable assumption that the 7" aashto requirement is for vehicle bridges only. like bridgebuster mentioned, i just have to check for frequency. the controlling load so far is a maintenance vehicle (h5)introducing shear.