Masonry Wall Moment Connection

[sedesigner06]

I have some very long (approximately 80' with no perpendicular walls framing in to support) 2 story corridor walls. The floor to floor height is 16'-0'' for the 2nd floor and roof. The second floor will have a composite slab spanning over steel beam that frame into the cmu wall, and metal deck spanning the 10' from corridor wall to corridor wall at the roof.

My approach is to design the masonry wall with a moment connection and the second level and cantilever beyond the second level. (reinforcing ends up being #5 @ 16 which I can live with)

My question has to do with the attached detail and trying to avoid a construction sequence delay. Below is the construction sequence as the detail is shown.

1) Build the wall up to the top of concrete
2) place the steel beam in cmu pocket
3) deck and composite slab to be placed to secure dowel bar into cmu wall below.
4) Finish building of block.

The problem I have with the is the masons must wait on the steel beam, deck, and concrete to be placed in order to finish building the wall.

Thanks in advance for any suggestions.

 

[mike20793]

Is the wall not braced by roof deck at the roof? One option to avoid construction sequence delays is to use a plate with post installed anchors to pick up the steel beam and an angle running along the wall to pick up the edge of composite deck. That depends on the reaction on the beam though and can be hard to achieve if it is anything relatively high (which I'm sure it is being composite deck). Alternatively, you could just provide a beam pocket in the masonry and have the deck supported by a continuous angle so they can come back and pour the composite slab after the fact. You will be okay cantilevering your wall with simple lap splices instead of that mechanical coupler on top of the beam. The reinforcement spacing is 16" max so there won't be any issues with bar placement as long as they plan it out ahead of time.

 

[KootK]

Cantilevering the wall is an unusual choice but I'm sure that you have your reasons. I wouldn't bother trying to achieve continuity in the vertical wall reinforcing at the beam connections for two reasons:

1) You'll never manage a detail that convincingly accomplishes that and;
2) You should be able to get whatever moment capacity you need from the segments of wall on either side of the beams.

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.

 

[SteelPE]

That is one heck of a detail. Too complicated for me.

I don't quite understand why you need a moment connection here. As mike states, it is usual for the decking the brace the tops of the walls. From your detail, I can't tell if you have this or not (the floor will certainly brace the wall in your detail).

 

[sedesigner06]

Agreed, it is complicated for my taste and trying to simplify.

See attached section of the corridor, hope this clarifies. These corridors go for a long ways (approximately 80') the connection shown is for the second level to create a frame with the wide flange. I don't thing the metal deck at the roof will to much to resist bending of the cmu wall. I think it will go along for the ride as there is a simple 4x4 angle the roof deck will be supported on.

I guess my main concern is the dowels that go into the composite deck at the beam AND between the beam connection. In my mind the dowel bars keeps the "moment connection" continuous between the wide flange members spaced at 10'.

If I eliminated the dowels that would the phasing, but the dowels seem like they are an essential part of the system to me.

 

[KootK]

Are there not shear walls or braces at the near and far ends of this thing at both floors?

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.

 

[sedesigner06]

There are perpendicular masonry shear walls that frame in at each end of the corridor walls.

 

[KootK]

Does the corridor portion pop up above the rest of the building at the second floor?

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.

 

[sedesigner06]

The corridor does not pop up, but there is an expansion joint within the particular run of corridor. This would prevent any chance of assuming the walls could be braced by the perpendicular walls.

 

[SteelPE]

So basically you have a little structure that is 80' long by 10' wide. Even if the walls at the end of the corridor were shear walls, you diaphragm has a 8:1 ratio. Not pretty at all. I don't think I would rely on the walls cantilevering up from the ground to resist lateral loads (like you are trying to do), just doesn't seem right to me. I would try to resist the load another way either by transferring them to other LFRS or adding frames withing the corridor to resist the loads and cut down on the 8:1 ratio.

 

[wannabeSE]

Is this basically a freestanding structure with no diaphragm on either side to provide lateral support? So, it has +/-10' x 80' diaphragms and the design intent is to have the floor beams and walls act as moment frames. Also, the walls above the second floor will be designed as a cantilever because a 10'x 80' bare metal deck diaphragm will not work. You got a love architects.

I am not in love with idea of having the CMU walls and steel beams acting as a moment frame, but that is besides the point. If construction sequencing is important, have you considered embed plates designed to fit in the wall and have the beams attached to the embeds with a bolted or welded moment connection. The embed plates could have perpendicular plates inside the wall welded to reinforcing.

Have you considered using 10" or 12" CMU walls that cantilever from the foundation.

 

[sedesigner06]

Is this basically a freestanding structure with no diaphragm on either side to provide lateral support? So, it has +/-10' x 80' diaphragms and the design intent is to have the floor beams and walls act as moment frames. Also, the walls above the second floor will be designed as a cantilever because a 10'x 80' bare metal deck diaphragm will not work. BINGO! Not ideal but its what I got to work with

You got a love architects Ugh. Don't get me started at this point

I am not in love with idea of having the CMU walls and steel beams acting as a moment frame, but that is besides the point. (agreed, not sure I have any other options at this point besides going to this wall as suggest below.) If construction sequencing is important, have you considered embed plates designed to fit in the wall and have the beams attached to the embeds with a bolted or welded moment connection. The embed plates could have perpendicular plates inside the wall welded to reinforcing.

I do like this idea, I will try and draw this concept and see how it works itself out. Will keep you posted

Have you considered using 10" or 12" CMU walls that cantilever from the foundation. I am afraid this is not an option at this point due to likely increased cost, I think I can get the concept to work and have a very stable system, just trying to avoid construction phasing issues.

 

[KootK]

Don't do this. You'll never be able to accurately quantify the strength or stiffness of those joints. You could strap brace your roof diaphragm to utilize your perpendicular shear walls. You could also just install two story steel moment frames inboard of your block.

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.

 

[bookowski]

This seems pretty crazy - you are a brave man. How did you analyze this, I'm assuming that you took some effective width of masonry and modeled those as frames.... with a d of < 4"? The stiffness must be almost nothing - probably so much so that p-delta really matters.

Is this interior and are you looking at seismic?

I vote adding steel columns cut into the masonry, if it's interior why can't they be built in same plane as the wall? I'd take the smallest steel column they make over this.

 

[BAretired]

The most attractive option is to get rid of the expansion joint in the 80' length and use a 10' deep horizontal truss at 2nd Floor and Roof spanning 80' between shear walls; and you could provide expansion joints at each end of the trusses.

The architect can still have masonry control joints in the 80' length, but your truss would have to be continuous.

BA