cable structures and fire protection

huangyhg

cable structures and fire protection
i am currently designing an 77' spanning steel underslung roof truss system with an arched top chord with a total depth at midspan of 18'.  yes, i am pretty damn excited because it is my fault that this structure is now replacing the 3 times more in weight steel proposed by the arch. i am considering a cable for the bottom chord but i haven't been able to really justify the complexity of prestressing compared to just using a tubular steel bottom chord.  either member still looks really thin spanning 77'.  currently, my concern is how to us this if the structure needs any fire rating.  the arch's code guy is looking into it at the moment.  any thoughts or experience would be great.  (beleive it or not, the arch wants a dropped ceiling.  another battle...)
thanks
  
vato,
if the structure is more than 25 feet above an assembly area it usually doesn't need fire protection.
attached is a bowstring truss i once did - used (3) 4" dia. bridge strand as a bottom chord. (span was 375')
this worked pretty well for this application.  with an arched top chord it might pose some problems - lateral stability of an arched truss with a cable bottom chord would be a challenge.

here's a pic of the bottom panel point of the truss:

jae,
what a magnificent structure!  congratulations!
ba
jae,
wow, impressive. so what stops the bottom of the truss from buckling sideways?
for my application there will be diagonal bracing to brace the bottom chord.  one of the difficulties i am having is orienting the bracing in such a way that it braces instead of transfering load that the bottom chord should be taking care off.  i can't really make it fit with the arch requirements if i run it horizontally everywhere.  anyway more fun for me.
i took a short trip from denver to chicago midway and saw a lot of exposed steel under 25'.  maybe it was coated with a fire proofing but the swages were not.  i'm not sure if the arch is going to be able to help me much on this issue.  i did  review the design for fire section of the new steel code and that would be really interesting, but there is no way they are paying me for that analysis, if there is another option.  there is some redundancy in the system as the "curtain" wall would support the roof, till the wind picks up, if a truss was to fail.  so, were (3) cables required on the alamo dome or is one for redundancy?
ps i think i'm going to avoid the wire rope issue and go with rods.  i think they are little more predictable and familiar to the contractor.
csd72, just like vato mentions, there were diagonal braces from the adjacent roof   
jae:
are the grooves in the bottom of the plate lined with teflon or some other material to allow slippage of the cables?
mike mccann
mmc engineering
i would guess no movement or slippage of cable, but 375' might require some movement.  let's see....  i'm still guessing one cable was "extra".
no, they were grooved to match the cables and the two plates actually connected with pre-tensioned bolts in order to clamp the cables.  in any truss there is a net change in tension in the bottom chord (the cables) across that joint/node so you don't want your diagonals and vertical to slip along the bottom chord cable length but work as a "fixed" truss would.

makes sense.  thanks.
mike mccann
mmc engineering