CMU wall behavior as a deep beam

[JKStruct]

I have a load bearing masonry wall supporting a hollowcore plank floor system in a 3 story hotel. At the second floor, two of my bearing walls cannot continue down since the architect doesn't want any bearing element in his lobby for some strange reason. So I'm thinking of using a transfer element in the form of an inverted concrete tee beam (to pick up the plank on the flange) and using it as a tension boundary element to get the entire two story wall to behave as a deep beam. The tee beam will then be supported on columns away from the lobby.

I know to make sure I have bars to develop the shear flow at the concrete/CMU interface. But is there any reason I can't do this? I looked at the NCMA tek manuals and couldn't find much information on my first pass. Is there anything else I should know? A good reference or design guide perhaps? I've designed concrete deep beams like this but never masonry.

Thanks in advance.

 

[jike]

I would probably not rely on the masonry to act as part of the beam.

You need to consider whether the wall is running bond or stack bond, control joint locations, openings, etc. You also should consider the fact that the wall will be green for awhile until it can start carrying any load. Of what benefit then will the wall be: for LL only? Do you plan on shoring up the precast beam to allow the wall to cure properly so that it can help carry DL?

 

[JKStruct]

Thanks for the response jike. Good points.

FWIW, the wall is a room separator... 27' long, running bond, no openings. Although, the need to shore the precast tee is a great point. I don't know why I didn't think of that. The contractor may scoff at that notion... but I suppose it could be done. I'm looking at other options, but so far it's not looking good. Maybe someone else in my office has some bright ideas.

 

[Unionmason]

Start with a 16" or 24" deep bond beam above the precast beam. I'd consider specifying one piece horizontal re bars with hooks/termination nut that extends past the center line of the support columns. Maybe even a post tension cable in the precast beam?

 

[ataman]

Why don't you use a steel beam? I don't know your tributary areas but the 27 foot span supporting 2 storeys may be a bit much especially when you consider deflection. Can you possibly have a steel beam at each level and use the masonry units as block in fill or since it is no longer load bearing use some lighter material?

 

[concretemasonry]

I have seen some buildings with similar situations - for what it is worth -

They are apartment buildings in Brazil where the first floor requires more open spaces or larger rooms. The remainder or upper part of the building is 6" "partially" reinforced to "unreinforced" loadbearing masonry depending on the floor level. Because of the first floor wall removal, the open spans were in the range of 6 or 7 meters.

Because 8" and 12" block are not commonly inventoried, a double (grouted and bonded) wall is used on the first floor only. Where possible, a 12" wide beam was cast on the masonry wall up to level of the top of slab for the second floor. The beam was supported for abour several weeks while work above continued.

In some cases, the upper portion of the beam was formed 6" thick and cast to project upward an a additional 2 to 4 feet coincide with the 6" block walls used above. Above that, 6" reinforced walls were used at the second floor, with the remaining walls on the second floor (those without a beam below) were 6" partially reinforced walls.

In some cases, where the span was less, first floor ceiling was flat and the second floor walls were designed as reinforced concrete masonry beams.

The structures were load bearing 6" block walls with all exterior and virtually all interior walls loadbearing. The floor system was two-way cast-in-place slabs, 4" to 6" thick, so the structure was cellular and very robust. Because of the continuity(strength and material properties) of the constuction from concrete to masonry and the continuous first beam support, there were no major discontinuous structural elements. Typical construction rates were about one floor per week, with wall finishing and utilities fillowing closely.

The key was providing support to the beams while construction continued and not falling into the trap of using dissimilar materials and separating structural elements.

The masonry units were provided in 4 different hollow prism strength levels of f'm= 1750 to f'm=4000 psi. Grout was limited to 4000 psi maximum. Few prisms were made on site and strength verification was accomplished by

These buildings were typically in the range of 13 to 22 stories and constructed in groups of 3 to 10 buildings per site.

This example is for a higher building witha 6" wall, but some of the practices may be applicable to your hotel (granted you may not have the luxury of a floor plan or owner with an interest in engineering). The wall thicknees in the subject building is less, but the spans are also different. I have no clue to the load on the walls and the load distribution since most American hotels/motels are really not too structurally challenging.