Reinforcing Joist Floor

[jjeng2]

I have a steel joist floor that I did not design but discovered was underdesigned while analyzing for additional loads. 14k3 spanning 27.5' at 2' c/c. 4-1/2 slab on 1/2 deck. First Floor Retail Load. The entire floor needs to be reinforced.

It is a strip mall so adding joists/beams or reinforcing each joist would be extremely difficult and expensive.

I am proposing adding a vertical strut at midspan of the joists and a new dropped girder to cut the span in half, along with new columns and footings. I realize that the bearing/strut needs to be designed and the entire joist needs to be analyzed for stress reversal issues and each member checked.

Does anybody have any experience on this type of project? Any better ideas? Any thought on my proposed plan?

 

[gtaw]

Adding footings and columns isn't inexpensive and it reduces the clearspan between columns which might not be well received by the owner.

Consider adding a cover plate to the bottom chord which can be secured to the chord member using intermittent fillet welds. It will push the neutral axis toward the bottom chord, but you can live with that.

the amount of reinforcing required is influenced by the error in the initial design. Depending on the increase in the floor capacity required, other options may have to be considered.

Best regards - Al

 

[OHIOMatt]

Do you have the original drawings? It is not overly common, but I have seen composite joists in the past. Perhaps that is why the joists appear undersized.

 

[JRGse]

Depending on the amount of overstress, one option may be to and bars to the top and/or bottom chords with intermittent welds. The web may also need to be reinforced, angles can be used for this. Another possibility would be to cut the joist near mid-span and add a beam line, in lieu of adding a strut as you had proposed. You would still need to rebuild the joist ends and check the joist for the new span. However, with the reduced span the joist will likely not require any more reinforcing other than rebuilding of the ends.

 

[msquared48]

jjeng2 and OHIOMatt:

By "1/2 deck", do you mean form deck, or is the decking covering only 1/2 of the joist?

If it is form deck, there is no composite action, and very little, if any, if the deck covers only half of the joist.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[jjeng2]

The joists are not composite. It is 1/2" form deck. The basement space is storage only so I dont think adding columns will be a big issue.

 

[msquared48]

And just to be extra sure regarding any compositeness, the joist tags do not say "Hambro", do they?

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[jjeng2]

No-building just built. I pulled tags and spoke with Vulcraft whi has all design info.

 

[a2mfk]

When you spoke with Vulcraft did you ask them for suggestions? I'm no joist expert but they are and may have some less expensive and obtrusive fixes... Would not want to be the other engineer, yikes...

 

[Brad805]

What percentage underdesigned are they? I am assuming the original contract drawings have the wrong spec.

If the owner is okay with all the extra columns (unlikely if rental space?), your plan is about as cost effective as possible. With joists at 24" o/c you end up with far too many local members to fix if the joists are more than say 15% under. All the site welding, inspections, access, finishes, and mech/elec costs make this a costly problem to fix.

If the joists are under, are the beams too?

This all smells like a lawsuit and seems to be a problem for either the EOR or Vulcraft to fix. I would stay far far away from this unless you are being hired as the expert witness. I have had too many problems getting paid in a timely manner when working for developers in this situation.

Brad

 

[jjeng2]

Brad-
They are good for 98 psf +-. The live load is 100psf. Then there is the pesky weight of the slab. So its not even close. The beams are OK. There is one shorter span bay and I think they designed the joists for that bay and coipied over and didnt realize.

I am going to call Vulcraft and I am going to require payment in full before I release anything.

Thanks for the advice.

 

[Brad805]

Sounds like quite the problem. If they forgot the slab dead loads, and they missed the partition loads also, this could become a big problem.

Brad

 

[JAE]

jjeng2 - you might also try to purchase this document from SJI as this is essentially what Vulcraft will do to verify/upgrade the joists for the new configuration:

TECHNICAL DIGEST No 12 - EVALUATION AND MODIFICATION OF OPEN WEB STEEL JOISTS AND JOIST GIRDERS 2007 (February 2007)