How to Enage This Moment Frame to Resist Lateral Load?

[azureblue83]

Doing a building extension design for a 4-story building (40ft/12.2m height).

Existing structural is a closed CMU building 33ft x 20ft (10m x 6m) dimension.

Extension is to be done by knocking down one side of the CMU wall and extend into the back yard for 14ft (4.3m). A steel moment frame is to be used instead of CMU, because the openings in the back yard wall are very big; one of the windows is 16ft x 16ft (4.9m x 4.9m), which would left only 2 ft (0.6m) or less of CMU pier on each side.

See attached rough drawing.

I'm not sure how to engage the steel moment frame to the CMU building. The floor diaphragms do not seem to be able to transfer the lateral load, since the joists are running in the same direction of the lateral load.

Any ideas?

 

[KootK]

It's your floor deck rather than the steel joists that will engage the moment frame. You'll need a shear transfer connection between the deck and moment frame and another between the new deck and the existing deck.

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.

 

[msquared48]

You said that I'm not sure how to engage the steel moment frame to the CMU building. The floor diaphragms do not seem to be able to transfer the lateral load, since the joists are running in the same direction of the lateral load.


Does this not also apply to the existing endwall where there is a connection? What would be the difference?

Mike McCann, PE, SE (WA)

 

[JAE]

Joist direction doesn't have anything to do with diaphragm shear and getting the load over to your moment frame.

You have to do a few things here:
1. Analyze the new diaphragm and determine the shear values across its width.
2. Also determine the moment in the diaphragm and determine the axial forces that develop in the chords of the diaphragm.
3. For the shear, the diaphragm must connect to the original end of the old diaphragm to transfer the shear that you determined at that point in item 1 above.
4. For the chords - I would imagine that the original building may have relied upon either a bond beam in the top of the CMU wall or an edge angle of some sort for the chord.
5. If the bond beam was the chord, then you have a huge discontinuity there as you extend further out - some type of chord splice will be needed but without knowing what you have I can't suggest anything.
6. The new diaphragm will then be tied into a collector along the full depth of the diaphragm - directly over your moment frame - the collector may simply be the top roof beam that also forms part of the moment frame.




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[azureblue83]

JAE,

Those are some good points.

1) I plan to overlap the diaphragm between the new and existing floor.
2) Chords shall be bond beam and designed for the shear and axial forces due to moment caused by lateral forces.
3) Item 1)
4) When you said "top of CMU wall", you meant on each floor level, right? Because this is 4-story building.
5) I plan to splice the bond beam in the new wall with the existing wall using steel plate installed inside the building anchor bolted to the bond beam / wall.
6) Do you have some example/drawing of diaphragm-to-collector-to-steel frame? Any products to recommend that can do this?

 

[KootK]

@azure: what kind of floor system will you have? Concrete toppled metal deck? What are the dimensions? Is there a deck edge angle at the edge of the existing deck? If so, does the vertical leg of that turn up or down? Will your perimeter beams be composite with the concrete deck using Nelson studs? We can give you better details if we understand yours.

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.

 

[azureblue83]

@KootK

The floor system is diaphragm rated plywood sheathing supported by cold form steel c joists spaced @ 16" O.C. Plan dimension is 47ft x 20 ft, 14 ft of the 47 ft is the extension. No sure about the edge angle, I would assume no.

 

[KootK]

Ugh. Is the plywood actually the walking surface? That's a pretty light duty floor for a four story masonry building. Must be a utility building of some sort.

I'd bolt a double wood top plate on to your moment frame beam and nail your new plywood sheathing to that for shear transfer. At the existing diaphragm, I would cut the existing plywood back to the face of the nearest existing joist. Place your first new joist right next to that existing one and stitch the two joists together somehow for deck to deck shear transfer.

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.

 

[JAE]

Top of CMU wall - yes at each level.

Diaphragm to collector (plywood to steel beam) - what KootK said.



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