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Rafter connection

Rafter connection

Rafter connection

(OP)
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.
 

RE: Rafter connection

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.

RE: Rafter connection

(OP)
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.

RE: Rafter connection

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

RE: Rafter connection

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

RE: Rafter connection

(OP)
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.

RE: Rafter connection

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

RE: Rafter connection

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.

RE: Rafter connection

(OP)
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).

RE: Rafter connection

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.

RE: Rafter connection

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.

RE: Rafter connection

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.

RE: Rafter connection

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

RE: Rafter connection

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.

RE: Rafter connection

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.

RE: Rafter connection

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

RE: Rafter connection

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.

 

RE: Rafter connection

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)

RE: Rafter connection

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.

 

RE: Rafter connection

I suppose you could also cut wedges and slip them under the rafters. This would also allow you to drive the wedge in as far as necessary until tight. Then use some kind of Simpson sheet metal clip angle to connect the rafter to the sill.  

RE: Rafter connection

I know you guys will hate me, but what's wrong with notching the existing rafters to sit flat on the plate on top of the new steel beams?  I think a 1" max deep tapered notch would do just fine with toenailing and blocking.

Yea Toad, I know the contractors will bitch, but, frankly, contractors are paid to bitch.  It's part of their game.  bigsmile

I do not like the sloped bearing as it is imparting a lateral force to the system.  Flat is better, at least in this case.   

Mike McCann
MMC Engineering

 

RE: Rafter connection

I would agree with the contractor about that solution.  Too hard.  If you can connect the rafters to the beams for uplift, surely you can connect them for a bit of downhill force.  I don't see why this force would be any greater globally than with the sloped bearing.

RE: Rafter connection

I have a suggestion,

Why not put a channel on each side of the column and support the column directly thus avoiding the need for any new roof connections.

Not sure if you can get channels big enough but it is worth a thought.

RE: Rafter connection

csd-
Good idea.
Basically a transfer girder.
Seen this done in mills many times to remove columns and open a bay for new equipment.
 

RE: Rafter connection

Still not clear if existing beams are steel or wood. Either way I would be looking at connecting to the existing beams/columns and not the existing rafters.

If the beams are steel it would be easy to weld supports off the new beams to connect to the existing beams. If they are wood then csd had the best idea of attaching to the existing columns (maybe even if they are steel). You can add cross pieces each side of the existing columns that span between the new beams. Connect the cross pieces to the columns.

RE: Rafter connection

(OP)
4thorns

You basically have the section correct.  We were just thinking about spacing the beams closer to the original support beam.

csd,

Good idea but I believe clear height is going to be a problem,  At the lowest point we only have 10' or so to the underside of rafter.

When I originally toured the building I said (tear down)..... but again nobody listened.

Great suggestions from everyone.  I was originally hoping for someone to suggest a Simpson XXXX connector.  I have three major concerns.

1)Connection of the beam to the rafters to resist LTB of the beam.
2)Connection of the beam to the rafters for uplift.
3)Getting proper bearing for the rafters on the steel beam.

I guess ripping a new "sill" for the rafters would be the best.  I could then use a Simpson hurricane clip (H3) to attach the rafter to the beam

RE: Rafter connection

I would think they will most likely rip a 2x plate on top of the beam or cut a "birds mouth" in the joist.

EIT

RE: Rafter connection

Another option would be to site measure all the rafter spacings at each new beam, then fabricate cleats on the beams for bolting to the rafters.  Similar to purlin cleats.

RE: Rafter connection

I'm still perplexed by the idea that you would mess around at every joist bearing in the bldg., when they are nicely seated on an existing beams, which could be picked up (supported) from the new beams, on either side of the columns you want to remove.  Then remove the column and leave the old beam in place.  The idea of cutting a new birds mouth in the joists at each new bearing, would likely get someone shot if they ever showed up on the job site.   

In the OP you said you were going to nest two new steel beams alongside the existing beam, aren't these new beams as deep or deeper than the existing beam for the same load and triple the span and some reasonable deflection?  Now, all of a sudden you have a headroom problem with structure below the 2x jsts.?  Some sort of saddle off these two new beams would pick up the existing beam, I called that a 1' or 2' cross beam bolted to the bottom of the new beam webs.  Maybe this has to be a stiffened plate off the bottom flg. of the new beams to pick up the existing beam in bearing.  Then, at this same location, btwn. the 2x jsts. and over the existing beam, provide another cross beam, or two, to the new top flgs. to resolve the torsional loading problem, and again at mid span for top flg. stability.  My new beams are both at the same elevation and do not touch the 2x jsts., they relate solely to that existing beam.

Now go ahead and apply hold downs at the existing jst. bearings, if needs be, before you put in the new beams, so you don't have to reach over them.  And, then your top cross beams can be made to hold down the existing beam too.
 

RE: Rafter connection

dhengr, you just got me wondering. Is the original beam capable of carrying it's own dead weight in a span 3 times the original design? If not then it's load will have to be carried by the connections at the existing bearing of the rafters. Sounds as if the new beams might have to be attached to the old beam regardless of which design is used. At the very least, to prevent excessive deflection of the old beam.

RE: Rafter connection

hokie66 (Structural)    

"Another option would be to site measure all the rafter spacings at each new beam, then fabricate cleats on the beams for bolting to the rafters.  Similar to purlin cleats."

Missed this reply hokie. Didn't mean to sound repetitive.

RE: Rafter connection

No problem.  I think it would be possible to fabricate the beams with the cleats in place.  Needs careful measurement, but saves lots of site work.

RE: Rafter connection

dhengr - Brilliant!

I think I would go that route.

I guess I was think more along the lines of new construction. Cutting birds mouth though is not a huge deal - make 1 pattern, start tracing. But if the joists are already seated well then I agree, make use of what is there.  

EIT

RE: Rafter connection

After reading all of these posts at one time, dhengr's stands out to me as requiring the least material, least labor, and least modification to the existing structure.  One add on to this is could you somehow use the existing columns (cut off near the top) in the attachment to the cross beams.   

Jason A. Partain, P.E.
www.myconstructionfinds.blogspot.com
 

RE: Rafter connection

I agree with dhengr - don't support the rafters; support the thing already supporting the rafters.  

Slick solution.  Would love to see the final detail.

RE: Rafter connection

OK.  

Then use one steel tube section on each side of the existing beam and support the existing wood beam at or near the existing beam joints or at or near where the columns used to be.  Temporary shoring may be required here though and the OP's original idea was to not use shoring.  To get around the shoring, the beam supports would have to be installed to either side of the existing columns, not at the columns.  The use of steel tubes will take care of the torsional loading problem.

  

Mike McCann
MMC Engineering

 

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