Wood Joists on top of central Steel Support Beam

[Miacol]

I will replace an insufficient wood support beam with a steel W or H beam.
The joists sit on top of the beam. Because it is an old house there is a lot of electric and plumbing over the beam so I cannot raise the beam.
Currently the joists are nailed (actually ledger screwed) to the wood beam.
Note it is a central beam with the joists going from the beam to the outer walls on both sides.
Question 1: When I replace the wood beam with a steel beam, do I also have to attach the joists to the new steel beam, or can the joists "float" on top??
Question 2: To attach, can I drill a small hole in the flange under each joist and use a structural screw? (I guess not allowed!)
Question 3: Or should I weld an angle bracket to the top of the steel beam (How easy is this to do? It is not easy to access above the beam).
Question 4: To keep the welding effort to a minimum, can I weld the angle bracket on only one side of the joists, then add wood blocking between joists over the beam?
Question 5: Does the angle bracket need holes so I can drive a nail into each joist?
Question 6: Can any angle bracket be used or do I need a special kind that can be welded?
Gee that is a lot of questions! So I numbered them to make it easier to understand.

 

[RontheRedneck]

I would assume that the top flange of the beam has to be braced. (Unless you analyze it as un-braced) So you can't just set joists on top f it.

The most common detail I see around here is a 2X plate bolted to the top flange, and the floor joists nailed to it.

 

[SE2607]

I would assume that the top flange of the beam has to be braced. (Unless you analyze it as un-braced) So you can't just set joists on top f it.

The most common detail I see around here is a 2X plate bolted to the top flange, and the floor joists nailed to it.

I recommend a 4x nailer on top of the steel beam. If a 2x is used, the bolts attaching the nailer to the steel beam would project above the nailer and would interfere with the joists. With a 4x nailer, the nailer could be counter bored to receive the nut and washer so that the welded stud does not project above the nailer.

 

[Eng16080]

1. I'd consider it bad practice to not connect the joists to the beam. If you were going to do this, though, then you should design the steel beam as laterally unbraced for its full length. This will likely result in a much larger beam section, and any savings in not providing connections will probably be offset by the extra cost of steel.
2. I've used this detail a few times. It's unconventional and requires a high level of coordination if the holes will be shop drilled, but I see no reason why it can't work.
3. I would avoid field welding due to the fire risk. (I was involved in a renovation project once where a fire started due to field welding. The building was essentially destroyed.)
4. Again, I recommend not welding.
5. I wouldn't use a steel angle connection, although if you were to do this, I would have the angle provided with shop drilled holes and then have field drilled holes in the beam top flange, with bolts to connect them. Or you can shop drill the beam holes if you're really confident that you have the locations correct.
6. As mentioned above by others, a wood top flange nailer which would be bolted to the beam top flange is preferred in this situation. From there, you can make a wood to wood connection. I would probably have the joist locations field measured and then the beam fabricated with holes set in locations to miss the joists (and any other obstructions). That way you can avoid countersinking the bolts. If the total beam depth is critical, I would typically just use a 2x (1.5" thick wood nailer). I also wouldn't be overly concerned with countersinking into a 1.5" thick plate if you have to, as long as you're not removing more than half the thickness (0.75"). If the nailer will add too much depth to the assembled beam, then use the method from Question 2.

 

[SE2607]

1. I'd consider it bad practice to not connect the joists to the beam. If you were going to do this, though, then you should design the steel beam as laterally unbraced for its full length. This will likely result in a much larger beam section, and any savings in not providing connections will probably be offset by the extra cost of steel.

If the nailer was attached to the steel beam with a nailer and welded studs, and the framing had a positive connection to the nailer (A35s, TNs, etc), then I would consider the top flange of the steel beam laterally braced.

1. I've used this detail a few times. It's unconventional and requires a high level of coordination if the holes will be shop drilled, but I see no reason why it can't work.

That's why I suggested a 4x nailer. The hole can be counterbored so that the threads, nut and washer are at or below the top surface of the nailer.

 

[Miacol]

Thanks all. I do have a height requirement. I need the basement height from finished floor to finished ceiling under the beam to be 6'-6" for livable space requirement.
So a top nailer will probably not be ok. (The wood beam I am replacing is 8" depth nominal, sagging and is well under the 6'-6" ceiling height. I am hoping to replace it with steel H beam w/ 6" depth to just satisfy the ceiling height requirement. So a nailer on top won't do.
So the nailer is bolted onto the beam through the flange? Then the flange can be drilled through?
That was my Question 2.
I suppose I could drill on site, straight up through the 3" flange into the joists. I could alternate with left side of flange, then right side of flange, so the holes are 2x16" apart. Any idea what hole size or screw I should use? And I will need to research how these holes affect the beam calculation.
Is there any objection to this approach?

 

[XR250]

I recommend a 4x nailer on top of the steel beam. If a 2x is used, the bolts attaching the nailer to the steel beam would project above the nailer and would interfere with the joists. With a 4x nailer, the nailer could be counter bored to receive the nut and washer so that the welded stud does not project above the nailer.

2X nailers are used all the time without issues and saves 2" of beam height. They can counterbore carriage bolts. I have seen 3/8" carriage bolts used and just cranked down and they get pulled flush without counterboring.

 

[Eng16080]

If the nailer was attached to the steel beam with a nailer and welded studs, and the framing had a positive connection to the nailer (A35s, TNs, etc), then I would consider the top flange of the steel beam laterally braced.

I agree. (I wasn't meaning that the joists had to be connected directly to the steel beam.) I would probably use shop holes and bolts rather than welded studs. I called for welded studs on a recent project for this exact purpose and was told it's much more costly than drilling the holes and providing bolts. I imagine this might be shop dependent though.

That's why I suggested a 4x nailer. The hole can be counterbored so that the threads, nut and washer are at or below the top surface of the nailer.

FWIW, I often do the same with a 2x nailer and 1/2" bolts. I don't think it's an issue if it's the bolt head that's recessed (versus the nut end), at least for normal loading. If the bolt was in tension, then I'd probably use a thicker plate like you do.

 

[gte447f]

So the nailer is bolted onto the beam through the flange? Then the flange can be drilled through?
That was my Question 2.
I suppose I could drill on site, straight up through the 3" flange into the joists.

Of course the flanges can be drilled, but not easily in the field, and not by a low-end contractor. Typically this fabrication should be done in the shop by the steel supplier, so it has to be a real steel fabricator, not one of these on-line steel beam warehouses. The cost of such fabrication is usually not anticipated and accounted for by low-end contractors. Also, if the locations of the holes are critical and must be coordinated to align perfectly with the locations of the existing, in-situ joists, that complicates things and is an opportunity for error. Also, think about your entering and tightening clearances (even though we are talking about screws instead of bolts) for installing a screw through the top flange from the bottom. There is not much space between the flanges of a W6 for screw, cordless drill, etc. These are just some of the reasons that a wood nailer on the top flange is preferred. If you do drill the flange of the beam, then yes, you should analyze the beam for any reduction of bending capacity.

The architectural constraints of this beam installation sound a little unreasonable or unwise to me. Using a 6 inch beam to just barely meet a 6'-6" head clearance for habitable space? I would suggest trimming the joists and setting the beam up into the floor space so that you can use a reasonable beam depth and achieve a reasonable head clearance. Reroute MEP as needed, although I'm sure that will be vetoed due to the time, effort, and cost associated with the MEP.

I assume you have already ruled out an engineered wood beam as a viable alternative?

 

[XR250]

If you do drill the flange of the beam, then yes, you should analyze the beam for any reduction of bending capacity.

I have honestly never done this as every I-beam I have ever designed was serviceability controlled and not even close to yielding - especially now with 50ksi the norm.

 

[gte447f]

I have honestly never done this as every I-beam I have ever designed was serviceability controlled and not even close to yielding - especially now with 50ksi the norm.

Same here usually, and if you have a level of comfort based on your experience and engineering judgment, then fine, but based on the OP's questions, my impression is that they should do the analysis to make sure.

 

[SE2607]

2X nailers are used all the time without issues and saves 2" of beam height. They can counterbore carriage bolts. I have seen 3/8" carriage bolts used and just cranked down and they get pulled flush without counterboring.

Often, in my cases, the steel beam is either a drag strut for lateral loads or part of a moment frame. Therefore, I need the "meat" to transfer the shear from the sheathing, through the blocking in the joists, through the nailer, through the welded studs into the steel beam. By the time you c'bore a 2x to allow for the washer, head and a few threads, you would be lucky to have an inch of material to facilitate that transfer. If you are talking about a carriage bolt heads, then you have to drill the flange. Welded studs, in my area, are cheaper because the welding is automated.

 

[Miacol]

Thanks all again.
Yes, above the beam is all the MEP, plus HVAC tubing, all needing re-routing, plus a rough framed bathroom that will need to come down.
So it is costly to raise the beam flush with the ceiling.
I am hoping steel will save me from doing this.

I've got 2 new ideas though:
New Idea 1: Pack the H beam with 2 2"x5" LVL's, on each side of the web. Bolt both sides together through the steel web.
Since the 6"x6" has flanges about 2.75 inches on each side (because of the web thickness), the LVLs will stick out about 3/4 inch (1.75"+1.75"=3.5").
Under each joist I attach an angle bracket to the LVLs and joist.
So I get a higher beam, stronger than LVL, and attached to the joists.

New Idea 2: Sister two 6" deep C-Channels to the original LVL beam, bolted together through the LVL beams with 1/2" bolts. Since the LVL is already attached to the joists, that problem is solved. The double C-channels are likely stronger than the original LVL beam.
I'll have to rip off the bottom inch or so from the LVL before I attach the C-channels and so I'll need to support the beam with a temporary post.
So I get a stronger, higher beam attached to the joists! Plus I don't need to rip out an old beam!

Any thoughts?

Thanks again!

 

[bones206]

You can also consider PAFs for nailer attachment. contractors seem to love it.

 

[gte447f]

@Miacol What is the size and span length of the existing LVL beam?

Also, how are the existing joists connected to the existing LVL beam? You said "ledger screwed", but I don't know what you mean by this. Are they toe-nailed, but with screws?

I think both of your new ideas have some merit if you cannot set a new beam up into the floor space.

For New Idea 1, you will need a lot of holes in both the steel beam web and the seat angles. I assume you will get all those holes shop fabricated ($$).

For New Idea 2, to address in-situ sagging, you will need to jack up the existing LVL beam before reinforcing with steel channels, but it sounds like you are planning to do that already. Even so, when reinforcing wood with bolted-on steel, I am a little skeptical that you get the full effect of the reinforcing until some deflection has occurred, so actual deflection may be greater than theoretical deflection calculated for the reinforced beam section.