Designing W-Beam Supported by HSS Columns

[hdeleo46]

I've been asked to assist in the design of a 28' span W-Beam which would replace two existing 4" x 4" wooden columns supporting a residential second floor. Given the loading, the beam I came up with is an A992 W12x35. It seems on the large side, but after several back checks, I keep coming up with the same sized beam.

My new issue is supporting the beam. I am thinking a square HSS6x6x1/2 column (roughly) would suffice but I'm not exactly sure where to get started in the design, what the best option for beam to column connections would be, and what kind of issues it may cause with the existing foundation. Any tips or advice would be greatly appreciated.

Thanks.

 

[jayrod12]

Is this intended on being a moment frame as the forum would suggest or just gravity loads?

 

[JAE]

Usually the columns would either be "under" the beam with a cap plate and the beam bolted or welded to the plate -
or
you would run the columns up to near the top of the beam and have a single plate or double angle shear tab type bolted connection to the vertical web of the beam.

If you don't know how to design these - it would probably be good to find an engineer to assist you who does.


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[KootK]

KootK tips:

1) Grab an AISC manual and use the axially loaded column charts to pick a size. For two story residential, 6x6 is probably over kill. More like 4x4 typcially.

2) For the beam to column connection, I like single plate shear tabs with the beam typing into the sides of the columns. This is cheap, easy to erect in an existing situation, and eliminates rotational instability issues at the beam ends.

3) As for the existing foundations, there'll be some new loads. Depending on what you land on (sog/grade beam/frost wall), there may be some work to do there.

4) Be wary of lateral torsional buckling in your beam and try to brace the beam rotationally where the wood columns touch down.


I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[jayrod12]

I'll add that if gravity only connections (as my two colleagues above have assumed) that it's likely you'll need to ensure the top of your column is braced laterally.

 

[hdeleo46]

Thanks for the responses everyone..

It is intended to be a moment frame. And I did envision it being a column with cap plate and beam bolted to the plate. I don't typically do much structural design, so I'll definitely be getting assistance from a more experienced structural engineer.

 

[jayrod12]

I'm still a fan of running the column past the beam and fastening the top of the column directly to the floor system somehow to provide your out-of-plane bracing. Therefore I would recommend the below for the steel moment connection. By the way, what's the second floor framing consist of?

The pjp weld is the most cost effective if it gives you the capacity you need. The second most cost effective solution is actually a cut-out plate connection (not shown).

 

[KootK]

It is intended to be a moment frame.

Is it a moment frame because you want it to resist lateral loads such as earthquake and wind? If not, I question whether or not you really want this to be a moment frame. Making it a moment frame just to resist gravity load is generally not an economical choice.

If you really need this to be a moment frame, I'd recommend small wide flange columns and bolted end plate moment connections to the flange sides of those columns. I try to avoid welding around wood whenever possible. That said, people seem to do it all the time.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[HouseBoy]

IF you need it to resist lateral loading, I would not expect a 4" or 6" column to be sufficiently stiff. In other words, drift might be a problem.