H JOIST REINFORCEMENT 1

[RWW0002]

Does anyone know if James Fisher (or another author) has any recommendations for reinforcing H-series joists with double rod bottom chords in any of his joist publications? I am looking for some practical detailing guidance similar to that contained in TD12 or several of his other publications before I start re-inventing the wheel. I have several ideas bouncing around in my head, but thought I would check if there has been anything published that I am not aware of first.

Thanks.

 

[jike]

Do not know of any publications. How about a plate as bottom chord reinforcement? Make sure the connections between web and bottom chord are adequate.

 

[EngineeringEric]

why not another rod or plate or channel? i don't think there is a perfect solution as long as you account for the existing stress in the chord and the load sharing for future loads of the reinforced section.

 

[RWW0002]

The connections are my concern as well. I am currently hoping to use additional rod web reinforcing so that it can still fit between the double rods at the bottom chord and (hopefully) no chord reinforcing, outside of any steel needed for connections of chords to webs. I have yet to dig into the details too much yet but was just curious if Fisher had covered it in a seminar or paper. He has published so much good information on the topic I thought there might be a chance that there was something out there.

 

[KootK]

I've got most of Fisher's stuff and I can't say that I've seen anything on this in particular. The connections are certainly a problem. Should you need to reinforce the bottom chord, the sketch below shows the options that I'e encountered. The right one depends on:

1) The skill and preferences of the welding team.
2) How much capacity you need.
3) How concerned you are about the impact of shifting the bottom chord centroid.
4) Available headroom.
5) Services below, above, or attached to the bottom chord.
6) Potential interference with bridging.
7) Material quantities not at all.

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.

 

[RWW0002]

Thanks for the sketches KootK. Hopefully I can keep the reinforcement limited to the web, but if not those are all good suggestions.

 

[KootK]

You're welcome. You might wonder why I bothered to include sketch #4. At first blush, it would seem to be the most obtrusive and inelegant of the bunch besides the nice centroid alignment. You may want to do #4 even if the bottom chord does not ostensibly require reinforcement. Here's why:

1) Geometry will likely conspire such that your connection reinforcement will introduce eccentricities into the joint.

2) The eccentricities will create a tendency for joint rotation that will be difficult to counter because none of the members coming into the joint possess any real flexural stiffness (all rods).

3) The angle increases flexural stiffness considerably and rectifies the joint. Or so we hope.


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.

 

[RWW0002]

Yes, I think the connection eccentricity might be the tricky thing here. If I use rod web reinforcing to keep it within the web space to simplify connections, the simplest solution would seem to be to add the reinforcing rod "under" the webs and weld to chords directly. This would tend to lower the workpoint (which is probably already lower than the centroid of the chords due to the bend radius of the continuous chord).

I will have to put some thought into this if the job comes through. For the one case that I ran through quickly, only the end 6'or 8' of the joist web will need reinf. and the chords are OK as is. I would hate to add a full 32' chord angle just to combat connection eccentricity, but it might come to that.

 

[KootK]

If I use rod web reinforcing to keep it within the web space to simplify connections, the simplest solution would seem to be to add the reinforcing rod "under" the webs and weld to chords directly.

That's what I was envisioning too. If the job comes through, check back in and we'll see if we can't dream up some manner of concentric reinforcement.

I would hate to add a full 32' chord angle just to combat connection eccentricity, but it might come to that.

Well, if you're only adding the chord reinforcing to deal with the joint eccentricity, then there would be no need to go full length. Basically just in from the ends up to the last joint requiring reinforcement.

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.