PEMB with full basement

[Ben29]

I am designing the foundations for a PEMB. The building will have a full basement and the PEMB columns will sit on pedestals. The column baseplate will be elevated to the first floor elevation. I am also designing the first floor framing. The architect originally wanted steel bar joist (non composite) floor system with a 4" thick concrete deck. But I am starting to convince myself that composite beams are the way to go.

I am trying to figure out how to brace the PEMB column base against the horizontal thrust. My thought is to have beams frame into the column pedestals via a steel plate bolted to the pedestal via epoxy anchors or CIP anchors. Is this a bad idea? Is there a better way?

 

[Ben29]

This discussion has been extremely valuable to me. Thank you everyone who contributed.

 

[Eng16080]

As others have said, I would try to provide a direct load path from the column base to the steel beam. Set the top of the column base plate at the same elevation as the top of steel beam (recessed into the slab). Make the base plate extra wide so that there's a projection of a few inches inches beyond the edges of the column. This projection would be going into and out of the page per the section. Once the column and beam are in place, provide a steel plate (or angle) per each side of the column welded to the top of the column base plate and top of the beam.

 

[Ben29]

How about this detail? The issue I am running into is that the beam I need has a small flange width, compared to the 10"X 14" PEMB column that I need to support. So I am adding a stub column (W14x61).

 

[phamENG]

I don't love that. I'd want to develop fixity between the stub and the MBS column and move the pin down to the pedestal, which could be problematic. Besides, fabrication is expensive. A bigger beam would probably be cheaper. I prefer a bigger beam running under the MBS column:

Put a steel embed plate in the pedestal with a couple of threaded studs. Take your bigger beam (preferably wider than 10") and put a pair of oversized holes in the bottom flange and a normal bolted steel-steel connection in the top flange. When the erect it, they can set the beam on the bearing plate and hand tighten a pair of nuts w/ washers. Then, when the erect the building, they can bolt it on and adjust the beam as required to align those connection. Once the building is erected, weld the beam to the bearing plate (and weld the column base plate to the beam if it still didn't align, but it should).

 

[Eng16080]

I might try to do something like this, which is a slight modification of your original detail:

Top of pedestal is equal to or slightly lower than top of beam. The red pieces are either a T section or a couple of welded steel plates (possibly back to back angles as well). Weld the red pieces to the PEMB column and also to the top of the steel beam. This could be modified slightly to use bolted connections as well, if that's preferable. Be sure that the red connector doesn't extend up too high such that it would stick out of the floor slab. I like that this detail requires no coordination between the different building components, aside from connecting the red piece.

 

[Ben29]

Eng16080: Thank you for taking the time to draw that up for me! I appreciate it. I will run this by the PEMB guy and see if he can get on board with it.

 

[Ben29]

pharm: If I switched all of the affected W18x35 to W14x90, my flange would be 14" wide (plenty of room), but this would add 19,140 lbs of steel to the job.

I could swap the W18x35 for W12x65. This gives me a 12" wide flange. This would add 10,440 lbs of steel to the job. But only leaves 1" on either side of the baseplate for the fillet weld. I guess that could work.

 

[phamENG]

Bern - is this just a running joke between us now?

Those seem like big numbers, but they're also missing context. An extra 5 tons of steel is about $5k + shipping from the mill, throw in an extra markup if it's passing through a service center first. How many columns do you have? You'll eat up $5k-$10k in fabricating and field welding your stub columns pretty quickly I imagine.

(Somebody feel free to correct me - I haven't had to price a steel job in a while and I may be a bit off.)

 

[Aesur]

Did you by chance check the floor for vibrations? I wouldn't be surprised if you find that you will have fairly large size beams if you need to check floor vibrations.

 

[jerseyshore]

Steel prices I saw last week (in NJ) from a client were somewhere between $1.5-$2 per pound for fabricated beams and columns ready to be assembled.

Steel plate prices are highest, $2 per pound minimum.

I'm with you pham, give me the bigger beams, on-site fabricating and welding prices are insane around here. A few extra pounds is almost always well worth it.