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block unit masonry enclosure of exit stair

block unit masonry enclosure of exit stair

block unit masonry enclosure of exit stair

(OP)
I am checking the structural adequacy of some steel beams installed about 1995 to support a reinforced concrete flat slab floor where a hole was cut in the floor to enable the installation of an exit stair between several floors of a multi-storey office building, including down into the 2 level parking garage below. The exit stair is surrounded all sides by an 8" thick block masonry wall. I imagine that provides a 1 hour fire rating between the stair area and the adjacent floor (i.e. 2 hours between floors).

Question:

Would this most likely be normal weight concrete unit masonry, or lightweight concrete unit masonry? 8" normal weight hollow concrete block not covered by any finishes weighs about 58 pound per square foot of wall surface.

RE: block unit masonry enclosure of exit stair

I'd assume normal weight unless there's a compelling reason to suspect that lightweight block was used.

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.

RE: block unit masonry enclosure of exit stair

Can you take a couple of core samples from the wall?

Or, drill a hole and look inside the cavity with a borescope: I doubt the blocks are filled with concrete, but that would make a big weight difference.

RE: block unit masonry enclosure of exit stair

The steel beams support the block infill walls around these stairs? Most of the CMU-block area-separation walls in this part of the country pass to the inside of the floor opening, and are continuous all the way up the stairwell to the roof cap. They are typically supported on the concrete foundation/slab below. This allows them to continuously separate the stairwell interior from the adjacent framing and floor construction, and it relieves the floor framing of the wall load. Of course, this only works for low- to mid-rise construction. High-rise is another story.

Thaidavid

RE: block unit masonry enclosure of exit stair

Depending on the situation during construct, the contractor may have chose 8" for that location because of weight savings and easier handling/logistics and labor.

Both Normal weight and Lightweight are loadbearing. The Normal weight would give a 1+ hour fire rating and the lightweight would provide a 2+ hour rating if fire ratings were desired.

Dick

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

RE: block unit masonry enclosure of exit stair

ajk1 -

Where did you get the 58 psf? That seems a little high for normal 8" Normal Weight or Lightweight units. Possibly unusual shaped units or if it was calculated, there must be some reinforcement and grout or just partially. There is such a thing as partially grouted masonry construction.

Dick

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

RE: block unit masonry enclosure of exit stair

(OP)
to Concetemasonry -- You are seemingly right about 55 psf being too high. Just took it from a very old table, but that might not be right for metric block in Ontario. According to the CISC Handbook, it would weigh 2.10 kPa (44 psf) for normal density metric block. So your comment was very helpful. Thank you.

To others: as to being grouted etc., I am certain this is not grouted because there is no reason for it to be. It is a 9 feet high interior wall forming the enclosure around an interior stair. The stair and enclosure (and hole thru floor and steel beams etc.) were installed in 1994 in a multistorey building built many years before that.

RE: block unit masonry enclosure of exit stair

Quote (ajk1)

as to being grouted etc., I am certain this is not grouted because there is no reason for it to be.

I'd knock on some cells to be sure. I know several engineers whose minimum block wall is reinforced with 15M @ 1200 regardless of demand.

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.

RE: block unit masonry enclosure of exit stair

(OP)
ok, will do. But I have never heard of that here in the Toronto area, and am intrigued why anyone would do that for an interior wall not subject to wind load and meeting code height to thickness empirical rules...are you thinking of a strong seismic area perhaps? By knocking on the wall, do you mean hammer tapping? Does a grouted block produce a clearly distinguishable sound from an un-grouted block, or do you have to have practice doing that to be sure.

RE: block unit masonry enclosure of exit stair

Nope, just for ordinary, non-seismic walls. You won't need much experience to distinguish between solid and hollow walls with hammer taps.

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.

RE: block unit masonry enclosure of exit stair

Are these new walls in an existing building? If yes, just call some local suppliers to see what is readily available.

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