Fastening wood column to steel beam

[jimtheengineer10]

I am working on a project where a new hvac unit will be installed on a roof. The contractor would like to cut through the roof membrane and install 6x6 wood columns on top of the existing steel beams. An lvl would then be installed on top of the columns to support the hvac unit. What is the best way to fasten the 6x6 columns to the steel beam? Or would it be better to install steel columns and have the lvl's rest on top of the steel column? Thanks.

 

[MiketheEngineer]

How tall are the 6x6 posts??

Simpson and others make a variety of clips that might work - Strong-tie.com

I guess you cold drill holes through the I-beam flange and lag screw the columns.

 

[jimtheengineer10]

Mike,
The 6x6 will only be about about 1'-2' tall. Are you saying lag screw through the flange into the bottom of the columns? I was thinking of using steel angles and bolting the angles thru the top of the flange and to the column. What do you think?

 

[msquared48]

I would use steel columns instead of wood. Otherwise how will you provide for the lateral for the unit?

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[MiketheEngineer]

YES.

And your idea is OK by me. Mine might be cheaper - but if you only need a few - who cares. Just make sure any moments can transfer if need be.

 

[focuseng]

What Mike said. Improvise with some standard clip or fabricate some angle hardware for cheap. Whatever you can do to get a positive connection.
Biggest issue may be thermal and moisture though. With steel columns they will be big thermal heatsinks to the interior of the building (depending on climate). With wood you need to seal and boot the wood to prevent water intrusion properly at the roof interface.

______________
MAP

 

[jimtheengineer10]

msquared, so would you weld the columns to the steel beams? I was thinking of using a steel angle to fasten the wood column to the steel beam. I could also install a cross member form the top of one column to the bottom of another column. This would provide the lateral restraint.

 

[msquared48]

I was thinking of four bolts, but welding would be OK if there is no threat of fire - remember the roofing.

The diagonal bracing would be OK too. Just have one omore area of roof penetration tyo deal with using the diagonal, and one more potential for a roof leak...

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[Eldorado]

Jim, it sounds like you are basically building a platform (decked or otherwise) for the RTU. It would be good to know the footprint of the platform. Are the beams spaced more than 20' apart and are you intending to frame the LVL's from one beam to the next (e.g. a 20' x 4' platform). Are the members between the beams open web joists or other steel components. I generally run into two issues when mounting onto an existing structure

(1) The contractor doesn't want to weld to the existing roof beam
(2) The platform columns need knee braces and I have to elevate the platform more than intended (steps may be needed).

This is also assuming the existing framing can support the added loads.

Can you provide more specifics about the construction needs.

 

[jimtheengineer10]

You are correct that we are basically building a platform. The beams are 18' o.c. and I am not sure what is between the beams (I will be checking that out soon, we are just coming with up ideas right now). One issue with the knee braces is that it is a historic building and we are supposed to keep the unit as low as possible so it doesn't stick up above the roof.

Will steel angles on all four sides f the columns be sufficient or do I need to elevate the unit to provide knee braces?

 

[JAE]

Is the LVL exposed? Typically they don't do well in wet environments.
Is it acceptable to use wood as a structural member in this building? Check on the fire rating requirements.

By using steel columns (short stub columns) and perhaps rigidly welding to the existing roof beams, you might get by without any knee braces if you can laterally brace the supporting beams against the twist or bending in the columns. Essentially using vertical cantilevered columns as your lateral bracing system.

Also, for RTU's set above a roof, it sometimes makes a lot of sense to allow some space below for maintenance on the roofing materials in the future. We usually use at least 24" clear to allow access underneath.

 

[jimtheengineer10]

Ok, I have some more information. The architect emailed me a section view of the building with the unit on top of the roof which I attached to this post. I have some questions for him and am waiting for a call back. There is a wood stud wall built on top of the steel beams. In my next post I will attached another section view from the other direction. I am thinking I can just have the wood column go through the stud wall and then provide cross members from column to column for lateral support. Then drill holes in the steel beam's flange and install lag screws thru the flange into the bottom of the columns.

Any suggestions or input? Thanks in advance.

 

[jimtheengineer10]

Attached is the cross section view from the other direction.

 

[msquared48]

Lag screws into the end grain of the column is a BIG nono. Yse metal straps nailed into the sidegrain of the columns.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[jimtheengineer10]

msquared48, I thought lag screws into the end grain seemed like a bad idea but I thought you said to do that in one of your above posts. I must have misunderstood your post. How do you fasten the metal straps to the steel beam?

 

[jimtheengineer10]

acutally instead of the cross braces I am thinking of using a plywood gusset.

 

[msquared48]

Use a strap material thick enough to weld. 1/8" or thicker should be OK.

Alternate would be to use a bent plate and bolt it to the top flange of the steel shape.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com