huangyhg

temporary roof supports
when replacing a roof joist support beam (as in the case of permanently removing an existing column and the two beams attached to it, and spanning the gap with a single larger, properly sized beam), is it best to support the adjoining web joists from below or above? there's a debate raging here as to whether we should support the web joist roof from above with a crane and spanners, or build a temporary structure on either side of the beams, jack the joists up a smidge, and swap beams.
either method is ok, provided it is properly designed and detailed. the conventional solution would be to provide temporary support from below eg scaffolding etc and then remove and replace the structure. in m experience, this is also the cheapest option and hence most usually adopted.
if the structure is up high, the temporary support may need to be very elaborate and then crane (or helicopter!) support can be considered.
if you support from below - just be sure to consider the lateral stability of the joists that are designed for top chord bearing. also - look at your end reactions on the joists with the in-place dead loads and check web buckling as the end web
inbcpe
my understanding of your question is whether or not to support the roof joist from below with vertical shoring or from above by lifting the ends with a crane and spreader beam. i did not interpret your question as whether to support the open web joist on their top or bottom chord. my comments are:
1. i would never install temporary shoring of open web joists on their bottom chord.
2. vertical shoring against the top chord of the joist is most certainly the least expensive choice. select the shoring beam locations as close to web panel points as is practical and check vertical shore bearing. in a multi- story building you may need to spread the load to one or two stories below the work level.
3. i would be careful how much upward jacking was used (a smidge sounds about right) because that jacking will be back some distance from the original joist bearing and could damage the joist and the floor/roof above.
4. generally the biggest problem is how to get the new, longer, larger beam into the building and up into place. wheeled carts, rolling scaffold and ductwork jacks work, along with many hands, make light work!
good luck!
a crane should never be used to lift against a fixed load - there is risk of overloading the crane with all the attendandant safety hazards.
furthermore, the lifting force applied by a crane is very difficult to control - there is a probability that the crane will lift the whole roof off the building.
the slow, sure, well proven shoring and jacking methods as expounded by jheidt2543 above are by far the safest way. such methods also allow you the time to sit back and take stock when something doesn't quite fit - consider the cost of keeping a crane or helicopter standing by while the new beam is being reworked!!
jheidt2543,
i should have included more info, but you hit the nail on the head.
a few responses to your comments:
1. one of the old ironworker salts here is trying to talk me into doing exactly that: support from the bottom chord. i know what my answer will be.
2. we're at grade, and the web joists support a simple flat roof.
3. a smidge is just that. just enough to make sure the ends are free from the existing beams before we start lowering them.
4. our field crew's specialty is getting steel like this into position. we've done it before, and it's just a matter of muscle and planning. it was the aspect of dealing with open web joists that threw me. my past experience is with open industrial steel structures, not commercial buildings with open web joist roofs.
bottom line: i now know what to tell the scaffolding sub.
thanks everyone.

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