Reinforcing Roof

[dsr729]

We have an existing flat roof 35' x 35' that was under designed and needs additional support. The flat roof is currently supported by 22k7 open web steel joists 5'-7" O.C (35' span). Based on structural calcs, the existing joist spacing is insufficient. We are proposing to add additional 22k7's between the existing joists (proposed spacing of 2'-9.5") and a W-shape at mid span for additional support. Can the W-shape support the 22k7 from its bottom chord or should the existing 22k7's be replaced to so that they are supported from their top chords? Also can it be assumed that the w-shaped has continuous support if the bottom chord of the 22k7 is welded to the w-shape?

Any recommendation or different ideas would be appreciated.

Thanks!

 

[MiketheEngineer]

HOW much capacity do you need? Right now it would appear that you meet a 50+ psf total load with at least a 30 psf live load. Doubling the joists will double the capacity. Just what are you trying to achieve??

 

[dsr729]

This roof is attached to another buidling which is aproximatly 5' taller..the snow drift load is 72 psf, roof live load is 20 psf, dead load is aprox 30 psf + 60 psf (future RTU's)

 

[hokie66]

Why not just take the whole thing off and start over? Probably less work than trying to reinforce underneath.

 

[JAE]

You can place a beam beneath the joists and also you can add more joists between existing joists.

Some things to consider:

1. The beam should be placed at a joist panel point and a vertical double angle welded between the joist chords directly in line with the beam.

2. Between the joists, in line with the beam you should add X bridging.

3. You should investigate the bottom chords of the existing joists since they will now go into compression. Might need additional lines of bridging to avoid buckling of these members.

4. The new joists can be added between the existing, but be smaller spanning members - not continuous over the beam. This might help in the construction as shorter joists are easier to handle and erect into place.

5. Weld the joists to the beam to create brace points against lateral torsional buckling.

6. Make sure your roof deck can span from joist to joist and support the higher snow drift loads.

 

[dsr729]

JAE

Could you please explain 1 and 4.

Thanks

 

[JAE]

A joist panel point is a nodal point where there is an intersection between the bottom chord of a joist and any diagonal/vertical member of the web.

You want to align your beam at one of those points along the span of the joist.

If you add a beam across your 35' x 35' space, say at the midspan, you would then have two 17'-6" spans on either side of that beam. Instead of adding full 35 ft joists between the existing, you could add smaller joists that are only 17'-6" long as simple spans from wall to beam.

 

[csd72]

dsr729,

Welcome to the forum.

As you have not posted before then this needs to be said.

The comments of the people above assume that you are a qualified structural engineer and have the necesary background knowledge.

I would also add that you should check the diagonals for the new shear as theses could be smaller at the mid span of the existing joist.

 

[JAE]

csd72 - good point. I forgot that one.