Concrete Masonry Wall Revisited 4

[JAS34]

Engineers,

I just recently posted about reinforced concrete masonry for a tall and slender building.

Now, the project has a short and wide masonry building. I am looking now at a 30 ft wide, 55 ft long, and just over 8 ft high building. The roof is wood trusses anchored into the wall.

My initial approach to this is that the base of the wall can be anchored into the base foundation, giving me a pinned connection at the bottom.

However, at the top, it is a free end. Roof trusses with roof sheathing - I just do not see this as rigid enough to be considered a pinned connection at the top. Also, when looking at ASCE 7 wind load cases where pressure is on one wall and suction on the other, all of the load on the given wall is resisted by the wall- a cantilever as far as a frame analysis goes.

So, is the only design solution to have a tie beam at the top of the wall, giving the wall a pinned at the top and pinned at the bottom beam analysis? The tie beams, to be a valid pinned connection point, would have to connect to tie columns that take the load to the foundation.

Basically, design the 8 ft vertical wall as pinned-pinned concrete masonry beams. The masonry beam loads are point loads on the upper tie beam. The end loads on the tie beam become the loads on the vertical tie columns. The tie columns are then designed to take the load to the foundation.

Anyone see other options when dealing with a short and long concrete masonry wall?

- JAS34

 

[jdgengineer]

Seems to me that you could consider that the roof framing braces the wall. There are significant limitations to using light-frame to brace a masonry wall. See Section 2305.1.5 of IBC and Section 12.11.2.1 of ASCE 7-05.

The detailing of this connection to the roof diaphragm should be done with care as this is a common building seismic deficiency (flexible diaphragms bracing concrete/masonry walls) but it's something that is done fairly often in tilt-up construction.

 

[msquared48]

I would definitely assume pinned-pinned here due to the stiffness of the roof diaphragm and the shear walls it spans too.

While it is true that the center of the diaphragm deflects horizontally, the ends do not, except for the amount that the shear walls deflect. That being said, there is a point where the diaphrqam deflects no more, assumint it does not fail, and all the load from the wall is transmitted to the diaphragm, giving a true pin-pin condition.

Don't over-think this. It's really a no-brainer.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[dhengr]

JAS34:
You would do a bldg. that size with 2x4 or 2x6 stick framed stud walls and call it a house, and with 8' high exterior walls it could be made to resist normal wind loads. So why not a conc. masonry wall to do the same thing. You might put a bond beam at the top of the wall to tie it all together, but otherwise the roof diaphragm, the roof trusses and/or some diagonal bracing at the level of the bottom truss chords, or a ceiling diaphragm, can be designed to take the lateral loads to the gable end walls. Obviously, the trusses and the roof diaphragm have to be properly attached to the top of the wall to take these loads. The wall is then supported at the top and at the floor slab or at the footing.

 

[BAretired]

Definitely a bond beam with a wood nailer on top of the wall . It is probably better to use "Hurricane Clips" to fasten the trusses to the nailer, although they are often toe-nailed. The connection to the trusses must be capable of resisting four feet of wind.

Walls spanning parallel to the trusses require a little more attention to detail. The wind force must be taken into the roof diaphragm with appropriate bracing members.

BA