Reinforced Concrete Masonry Wall 5

[JAS34]

Hello Engineers,

I have a rather unique structure.

It is 13 ft wide, 19 ft long, 30 ft tall, concrete masonry, 140 mph wind, and exposure category C. It is basically a tower for the local firefighters to train maneuvering throughout a smoke-filled stair way.

As far as load and load combinations on this relatively tall and slender structure, the out-of-plane load analysis on the 8” reinforced concrete masonry walls fail at around 15 ft tall.

Now the question is if I am on the right track. I am putting reinforced 8”x16” concrete tie beams spaced at the tower base, roof, and 10 ft on center vertically. This breaks the 30 ft wall up into three 10 ft sections. I can now analyze each 10 ft vertical concrete masonry wall for out-of plane loading as three separate pinned-pinned beams. The pinned-end reactions of the vertical masonry wall become the loading on the horizontal reinforced concrete tie-beam for its design. The reactions of the pinned-pinned horizontal tie beam become the load as far as analyzing the adjacent shear wall that takes the out-of-plane wind loads into the foundation.

Does this analysis sound acceptable? Anyone having performed concrete masonry design ever get involved in finding some design to get a single wythe 8” reinforced concrete masonry wall to resist high lateral loads?


Thanks to all for your input,
JAS34

 

[ron9876]

Sounds right. Design tie beams for trib widths. Design tie beams to span horizontal. Design shear walls for applied loads. 10' c/c seems excessive unless you can't get the tie beams to work.

 

[DST148]

The design concept is fine.
Is the structure covered at roof level? How about seismic loads? Are there any other loads to be accounted for?
You mentioned about providing concrete beam. Why bring in another trade? See if you can live with a masonry beam at a lesser spacing. You may also provide reinforced bond beams all around, spaced at about 4 feet o.c. vertically. This would help the four walls to be tied together and behave as one unit.
For lateral loads, 8" thick wall can span horizontally for 13 feet.See if a 10" thick wall can span horizontally for 19 feet.

 

[concretemasonry]

Sounds reasonable and similar to many I have seen (although some were slightly higher), both with concrete stairs and steel staircases. The aspect ratios between height and width/length)are not impossible. These were total masonry with no concrete beams, but judicially used bond beams (8" or 16" high). Some even ended up being totally reinforced and fully grouted because it was cheaper once you start pumping grout.

Some used lightweight block for better fire resistance in case there was potential direct fire exposure.

The real key is the continuity/connections and whether the landings can be tied into the structure.

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.

 

[hokie66]

I would design the walls just as any slab supported on 4 sides. With the 30' height, most of the bending will go horizontally, so the horizontal reinforcement will be more than the vertical. Forget bond beams, just reinforce the whole slab as required, and fully grout the wall.

 

[JAS34]

Thanks for the input guys.

DST148, to answer your questions, right now I am looking at load combinations with wind loading and also, the roof is covered. I do like your comment about concrete pours in with concrete masonry block. The concrete tie beams, an extra material, will add extra cost. The architect just included them in the prelimanary design that I am analyzing. So, can I design a horizontal reinforced concrete bond beam in the same matter as the reinforced concrete tie beam? That is to let the vertical reinforced concrete masonry act as vertical beams that tie into the horizontally reinforced masonry bond beam. The horizontal bond beam then can be designed as a horizontal beam that takes the lateral loads to the shear walls?

- JAS34

 

[Gumpmaster]

You can just span the whole structure horizontaly. No need to look at any vertical span at all. A 19' horizontal span may be a bit of a stretch, but you could maybe make it work with bond beams at 16" or so. If not, as DST148 said, 10" CMU would probably work for the 19' span.

 

[JAS34]

Now hokie66, you seem to have another good idea and it is, overall, a different approach. Since the concrete masonry wall is narrow but tall, loads will transfer through the shortest (most rigid) load path. I should look at the load transfer as horizontal bond beams spaced vertically on center. Each horizontal bond beam takes its tributary lateral load. These bond beams then carry this load to it ends where it connects to the adjacent wall, giving point loads all along the adjacent shear wall. Of course, the whole wall is grouted but no vertical reinforcemt is needed for out-of-plane loads. Basically, the shear wall receives a point load at each horizontal bond beam connection. Correct?

- JAS34

 

[hokie66]

Similar, but I don't use the term "bond beams". The wall is just reinforced horizontally and vertically (two way action) and fully grouted. Think slab, because that is what it is. Yes, the horizontal moments, thus the reinforcement, will exceed the vertical. Agree with others that a thicker wall should be used for the 19' span, but you may be able to cut that down by using the stair landings.