Another Bar Joist Repair

[EPCI-Steel]

So I have a bar joist where a contractor core drilled through the top chords, just like Kootk's post from a week ago. It's in an older office building with no readily available information on the joist in question. I had the idea that I could design a simple chord reinforcement to reinstate the strength of the original chord based on conservative assumption (50ksi) of its grade, and tie the web members together with a tie plate (the web was also cut right through the center of the node at the top chord). The joist has shear studs in the concrete slab, so it appears to have deflected little if any at all, but going to take measurements tomorrow.

Methodology:
Determine allowable compression of existing chord member at 50 ksi (P_chord).
Design reinforcement and connecting welds for axial + bending from P_chord and CG offset between members

Result:
Any shape which can be welded in the V of the angle fails due to load eccentricity, by a lot.
And while my mind is telling me this will work no problem with a nice hefty square bar nested in the angles and a horizontal tie plate between the reinforcements where the top chords were severed, my calcs do not show it.

Suggestions please!

 

[Ron]

It doesn't look like there has been any compreesion movement after the cut, since the cut hasn't closed. I would scab equivalent angles onto the top chord or chip out concrete enough to get a plate welded over the top chord to rstore. Don't over-think it!

 

[Once20036]

Typically when I`m designing top chord reinforcement, I do not consider the bending in the reinforcing steel due to the eccentricity from the centroid of the chord member to the centroid of the reinforcing steel.

 

[KootK]

just like Kootk's post from a week ago

Goddam epidemic. Did you check out my final details in the other thread? I feel that would be pretty much perfect for this. As far as bending and stability go, my strategy was to change the webbing configuration around the damaged web with externals to a) replace the damaged web and b) provide support near to the location where chord flexural stiffness had been interrupted. Contractor's set it out in the field for me in chalk. Looks to be coming together okay.

I agree with your methodology in general.



I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[EPCI-Steel]

Once20036 - I do not agree with you on this point. The Steel Joist Institute has "Technical Digest 12" on the modification of steel joists with design examples of reinforcements of typical joists and they do account for eccentricity between the CG of the original chords or webs and the composite sections of built up chords and webs.

 

[EPCI-Steel]

KootK - I was trying to go less invasive than rebuilding a section of the joist, but maybe I need to consider that approach. Time to put pencil to paper once again.

 

[Once20036]

EPCI,
You're welcome to agree or disagree as you see fit. I haven't see Technical Digest 12.
As a second data point to consider, Vulcraft publishes a document entitled, "Designing with Vulcraft, Steel Joists, Joist Girders, and Steel Deck".
p87 has reinforcing calculations for the top chord compression and this bending is not accounted for.

 

[KootK]

KootK - I was trying to go less invasive than rebuilding a section of the joist, but maybe I need to consider that approach.

Invasive?? Why, I never! Just kidding, I originally intended mine to be much less invasive too. I can only speak to my own experience but, in discussing it with my project team, it's shaken out like this:

1) "Invasive" in this context basically means anything that would require wholesale member replacement as that messes with existing service runs. My guys are thrilled at the prospect of having any repair that avoids member replacement.

2) Material cost is utterly negligible compared to people's time, project schedule, and the potential cost of having to reroute services. For all the work I'm requesting, it's still just 8' of rod and a couple of 6' tubes.

3) Welder time is not a big deal. It's one joist so whether you do a lot or a little, it mostly comes down to the mobilization and demobilization time for a guy that's spending his day on this one way or another.

4) Compared to more conventional work, my time is actually quite a large chunk of the repair cost. I'd not be doing anybody any favors by trying to shave off a few pounds of steel at the expense of a few more hours of pencil sharpening. My fees will be at least 1/3 of the total repair cost I think.

May I ask how much you'll charge for the repair? I got $1750 and sense that I left some on the table. GC is just passing it along to the offending sub-trade anyhow. If I could do one of these a day, I could have my house paid off by Halloween.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[EPCI-Steel]

Is there a private message function on this forum?

 

[KootK]

There isn't. If you're interested in contacting me, you'll likely find what you need in my member profile.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[racookpe1978]

In this case, why not try to sandwhich two 8x2x3/16 angle iron members to both sides of the truss.

2 inch leg "up" (against the bottom of the existing truss upper member), the 8 inch legs (6 inch may be sufficient) are on both sides of the joist round bars.

Bolt the two angle irons together clamping both sides of the truss. Don't know enough to estimate a length of the two members. Alt. Weld the inside of one angle iron to the rods with stitch welds. Bolt the second to the first.

 

[dik]

Can you just weld a couple of small channels to span across 3 panel points?

Dik

 

[Ron]

dik...racookpe1978...agree. Just needs a simple repair. Even round stock welded inside the angles spanning across the cut would be sufficient (properly sized and welded of course)