Rafter connection 1

[SteelPE]

We are currently in the process of removing some columns from an existing building. The client wants to remove the columns in order to increase the amount of open space. The existing columns are spaced at 15’ o.c and the client would like to have a 45’ open space (so we are removing 2 columns). This happens at multiple locations. The columns are supporting a wood roof.

The idea we have come up with is to place new steel beams under the existing joists on each side of the existing columns. Place the steel and then remove the existing columns (to eliminate shoring). We would have a wood nailer placed on top of the steel beam.

I have a little bit of a detail problem. The roof slopes and average of 1.75/12 (it varies but this is the average). How would you attach the sloping joists to the top of the steel beam? Is this something you would just fill with blocking? I was looking at using Simpson VPA connectors but they require a minimum slope of 3/12.

 

[MiketheEngineer]

If you have a sill plate on the sill, Simpson sells a number of ties and/or straps that will work. Look under "Hurricane" clips.

 

[SteelPE]

I was looking at those for my uplift concerns, however, I also have bearing concerns as I don't quite have proper bearing with the new condition.

 

[ToadJones]

can you sister a new block to the rafter such that it bears flat on the new beam?

 

[hokie66]

Not sure I understand the problem. Wouldn't you rip the wood plate to match the rafter slope?

 

[SteelPE]

Toad

That is a decent idea.

hokie66,

This is kind of what I am talking about, Ripping a member to place on top of the wide flange beam to match the slope of the joists. Seems kind of labor intensive, especially over a few hundred feet and I am looking to see if there is a better way to solve the problem.

 

[hokie66]

It's not labor intensive with a table saw. You should be able to get them ripped in somebody's workshop. Maybe a cabinetmaker.

 

[woodman88]

What is the existing rafter connection and why can you not keep the same type of connection?

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[SteelPE]

I am not really concerned with the existing bearing condition as my new bearing will be 1'-0" away from the existing (placing new beams either side of the existing).

 

[dhengr]

SteelPE:

You’re keeping plenty of important detail info. secret, but presumably there are beams there now, spanning 15' and supporting the existing jsts., don’t mess with those existing attachments. And, you plan on bringing in two beams, one on either side which will now span 45', and be considerably deeper than what’s there now, to make that new span work. Make up a bunch of small cross beams 1' long, with welded end plates for bolting to the webs of the two deeper side beams; these to support the existing beams at 5' o/c, or some such. Bring the large beams in on either side of the columns, bolt the cross beams in place, lift this assembly up to support the existing beam, and remove the columns.

 

[woodman88]

Okay. If the rafters do vary in pitch then sloping the sill plate to match them all will be a problem. I would look a ToadJones idea of a block. A 2x(depth to allow the spacing the nails evenly into the rafter)x12" or 18" long with the top rip a 1.75 pitch would be easiest to make work in the field.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[dhengr]

Better yet, you only need four of the cross beams, 2' long, at the 15' points on the new beams, one on each side of the existing columns, to pick up the existing jst. support beams. This is also a more favorable loading on the new beams, and less fitting and bolting in place. If the existing jst. bearings and support system is functioning well don’t change it, or mess with it, just support it (the existing beams) from below.

Talk about labor intensive and expensive, new top nailing plates bolted to your new beams, and special hardware at every joist, ain’t cheap either. A good shop will rip sloped 2x4 nailing plates about as fast as you can load them on your truck.

 

[MiketheEngineer]

You got a lot of guys willing to help - but understanding your problem is well "problematic". A pic or drawing will be quite helpful.

 

[hokie66]

Yes, I am sure we are all drawing different sketches. Mine showed the existing beams to be wood, while dhengr has them steel. SteelPE said they were irrelevant.

 

[TXStructural]

It sounds like you will be putting the bottom chord in bending with supports "a foot away" from the current (panel point) supports. This is a very bad idea, since truss members are barely strong enough to take the intended axial loads.

Also, the trusses are probably much stiffer than your proposed steel beam will be. You would need to lift the existing members to camber out the deflection and avoid damaging connections in the trusses.

A sketch or photo will help us. I have done a good bit of this kind of work, and every instance is different.

 

[SteelPE]

Not at my desk now, I will post a sketch tomorrow

 

[4thorns]

This is the way I'm seeing it. Close?

 

[JAE]

4thorns, that is the way I pictured it - but with the two new beams actually adjacent to the existing and not spread apart as you suggest.

Ripping a blocking on top of the beam is the way to do it - least expensive and most logical.

 

[ToadJones]

the way i'm talking about doing it would probably involve no special ripping at all. The type of blocking I am talking about is fairly common on what I call "pole barn headers" in which case a block is slid down between the two barn headers and sticks up high enough to nail into the side of a rafter or truss.

Another thing that might be worth looking into since your slope is not very steep is using the support beam on an angle and designing it for bi-axial bending like a roof purlin (I can already sense that I am going to be chastised for this suggestion)

 

[4thorns]

SteelPE (Structural)
"I am not really concerned with the existing bearing condition as my new bearing will be 1'-0" away from the existing (placing new beams either side of the existing)."

This quote is the reason for the spacing of the new beams in my detail. I'm guessing it's to allow room for it's placement etc.