large second floor diaphragm opening

[nr123]

I have a two story building that will be located on the west coast of California so the diaphragm loads are fairly high (upwards of 500 kips). The building is steel frame with SCBF to resist the main lateral forces. The second floor which is 3" NWC + 1" form deck on steel joists and has two large openings that are in the center of the floor.

See attached for building layout. When loaded in the N/S direction I proposed to develop chords as shown, however because the openings are longer in the E/W plan direction I am having trouble developing a load path that seems to work. For this direction I have referenced to the AISC design guide 2 to try to emulate the concept used for web openings in steel beams but I am having a hard time.

I was going to use a similar path as I was planning for the N/S but the when I apply the uniform lateral load across the diaphragm in the E/W, find moment, uncouple and apply axial load at the end of openings as shown by the dashed lines, I get a very large force that does not seem reasonable to transfer thru the slab. When applying the uniform load, would I only calculate the moment based on the width of the openings instead of the whole plan width? Almost creating sort of subdiaphragm behind the openings?

If you can understand any my jibberish please feel free to give any comments or recommendations.

Thanks in advance.

 

[msquared48]

I would push for decreasing the size of the openings so that the extra chord ties do not have to be inserted - maintaining the steel framing that is there. Otherwise, I agree, there will be a problem.

I assume that the small remaining portion between the openings is to be used as a walkway?

Mike McCann
MMC Engineering

 

[nr123]

I don't think I will be able to get the openings any smaller as this has come up previously and was frowned upon by the owner. Yes the piece of framing between the openings will be a walkway.

 

[msquared48]

OK. Bummer...

Then consider saw-cutting in, maybe 2.5 of the 3.5" depth of the slab, but above the steel deck, and epoxy in your chord steel. May have to go 6" to 8" wide to get the steel in, breaking out between the sawcuts. Depending on the direction of the deck, you may need temporary shoring to do this.

Mike McCann
MMC Engineering

 

[nr123]

This will be new construction so there wont be any need to breakup any of the slab and epoxy in steel.

Where I'm stuck is when determining the amount of steel I need for the chord, which would invlove finding the moment created by the uniform load applied at the diaphragm, uncoupling over the distance to my chords getting to a tension compression load that occurs in the chord or the flange of beam, if you consider the diaphragm as a beam laid flat. I am unsure whether the length I use to get my moment would be taken as the entire plan length (approx 200') or just the length across the opening (approx. 40'). I can convince myself both ways. In referencing AISC DG2 it seems as though I would need to calculate the moment using the plan dimension which gives a larger moment and will require a larger amount of steel upwards of 6 sq. inches (7-8) #8s which seems unfeasible in a 3" slab. Thoughts?

 

[jike]

Maybe I am missing the point of your question, but why can't you locate a beam (to act as a chord) at the edge of the openings? Make it composite to act with the slab.

 

[nr123]

In th E/W direction, there is not a line steel continuous from SCBF to SCBF at the edge of the openings. To add a line of steel would be tough due to the joist direction.

Since posting my question I have done some more investigation and decided that I will use the steel at the exterior perimeter in the E/W direction as my chord to resist the full moment, and treat the opening no different than a hole in a beam web. Due to not having a line of beams at the perimeter of the opening, I will add slab steel to resist the secondary moments in the remaining slab (or web) above and below the opening. I will also be adding diagonal bars to aid with shear. Thanks to all for their comments.