Reinforcing open web bar joists 4

[INBCPE]

Hello,

I have a project where the owner wants rooftop units added to his building. Three of the spots have steel beam construction that can be reinforced with WT's (ala Miller) and additional purlins.

The fourth roof is a little more difficult, and he has no alternative position for the new unit. This roof is constructed of 18" bar joists 4 ft o.c., 27' long, with your classic gypsum built up roof. I checked my copy of the 60 year manual and the existing joist won't carry the additional load.

I don't have the access or room to get 2 new beams into position to carry the new load. I see two other alternatives. I can either reinforce the existing joists, or wrestle some new bar joists into position. New bar joists are low on my list due to space constraints in the layout of the building. I would need a crane to get them through a 3rd story window, and the pick would either involve a long run from a major street over power lines or block the local police department parking lot. Yes, reinforcing the existing joists is labor intensive, but in this case it's the less expensive alternative.

I am looking for the following advice:

1. Is there another alternative that I'm missing.

2. Is the following reinforcement plan viable:
a. Estimate current roof loads.
b. Produce shear/moment diagrams for existing joist.
c. Calculate shears and moments for the additional load.
d. Superimpose shears and moments.
e. Design reinforced supports at end reactions.
f. Determine the additional area of diagonal web members (angle or tubing shapes) required to satisfy additional shear requirements.
g. Determine additonal area of chord reinforcement (plate or shapes) required for additional moment.
h. Determine all weld lengths and cross sections required to satisfy all loads.

Any advice or story from personal experience is appreciated.

 

[mrengineer]

Are you able to install either a 2 piece joist or a 2 piece beam with a field splice? I have done done both in the past.

 

[INBCPE]

mrengineer,

To answer your question, no, but I'd love to try. We've performed field splices out the whazoo on steel beams, but not on bar joists. I'd really love to try it. It's workable. I know I could get a 14 foot section of bar joist up the stairs, and there'd be room to get it up in the ceiling and support it temporarily until it's in place. I like the idea. Is it a matter of simply designing a joint, fabricating it, and doing it in the field, or does the joist manufacturer have to do a special run? I'd love to see what one looks like.

 

[INBCPE]

mrengineer,

After considering splices more, do I merely have to design a field splice that develops the design stress of the original member (but no less than 50% of the allowable strength of the member)? That's right out of SJI. What does a manufactured splice look like?

 

[Ron]

It's a lot cheaper and easier to reinforce the joists in-place. I have done this on several projects with success for the same reasons you are doing it. It can be done in a variety of ways from enclosing the open web with plate to adding plate on the bottom of the angles. If you just need additional bending resistance, a channel, wider than the double angles, works well as it will be welded in a horizontal fillet position requiring the lowest level of welding qualification.

 

[mrengineer]

INBCPE:
We have called for a "bolted mid-span splice" many times where we added on to an existing building and were worried about snow drift conditions that we created. Often times the deck spans too far for the added snow load, and therefore you have no choice but to add members and reduce the deck span. You don't need to worry about detailing the splice-the joist manufacturer will take care of it. Add a note that you want full capacity of chords developed if it makes you feel better. As far as reinforcing existing joists, I think the relative cost difference between reinforcing the existing joist and adding a new one depends on the extent of reinforcing. If you are doing this project on a lump-sum design fee basis,considering that joists are relatively inexpensive compared to the overall cost of the project, you may want to give yourself the benefit of the doubt and just add a new joist. Also consider that you are at the mercy of the erector with regard to quality of welding to existing joists. Joists often have very light members, and it always makes me nervous doing much field welding to them.

 

[INBCPE]

mrengineer,

We are a design-build company, so we'd be doing the mods ourselves with our own certified welders.

I thought about it some more today, and I still think that with the beam pockets we'd have to make in the masonry, the piping and duct work that would have to be taken down and put back up, and the access problems to the room in question just makes it more expensive to put up the new joists. It justs adds up to more additional work than just reinforcing the existing joists.

 

[Taro]

Another option would be to install a platform above the existing roof deck. Post up from the column or wall locations and span new steel beams across to carry the mechanical unit without overloading the existing joists.

 

[epr]

INBCPE,
Reinforcing is not unusual. You can limit the reinforcing to just the portion of the joists that need it.

However, don't forget to check the end bearing conditions. I have run into numerous old buildings where the joists bear on a small plate on hollow CMU. It's the little details that always bite you in the ---!

Also, If you are reinforcing, consider jacking first to relieve the dead load stresses, so the composite (steel + reinforcing) section can handle all the loads.

 

[jheidt2543]

Two thoughts:

First, I think Taro's suggestion of a platform large enough that the legs can hit a number of joist with a wide spread would help distribute the load, two legs/joist for 4 or 5 joist.

Secondly, as far as getting material into the site, instead of taking it up the stairs or through a window, as you mentioned, why not cut the roof openging, temp. reinforce the edges and pass the new material through the hole in the roof? Then, patch it all up after the new, permanent supports are in place.

Just some more ideas to think about.

 

[KandG]

I am placing three roof top units on a tilt up building with non-bearing masonry interior walls. To support my roof tops I am planning to reinforce the bar joists if necessary. I have heard of it done but never tried it myself. Any suggestions in the design is appreciated.
kandg

 

[Ron]

KandG...read this whole thread...it contains some good info for you. You'll need to know your before and after analysis conditions and then proceed to reinforce the joists as needed. There are many ways to accomplish the reinforcing.

 

[UcfSE]

KandG and INBCPE

See my response to KandG's post in the TRUSS Engineering forum. I have outlined a procedure I have used many times for reinforcing bar joists for additional load due to RTU's and have a link to a book from Vulcraft that contains the procedure.

Reinforcing the joists can be done easily within reason and works much better than trying to get new beams or joists in there.