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Pinned or Fixed Suports

Pinned or Fixed Suports

Pinned or Fixed Suports

(OP)
I am designing a masonry building. It is 12" thick on a strip footing. Would support be pinned or fixed?

JO

Thank You

RE: Pinned or Fixed Suports

I'd call it pinned with the height being from grade to the roof. People have been known to take advantage of some degree of fixity however.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

RE: Pinned or Fixed Suports

I always call it pinned how do a single line of dowels placed in the centre of the wall constitute a fixed boundary

RE: Pinned or Fixed Suports

I always use pinned. You can lap splice your vertical reinforcement with dowels into the foundation to develop a "fixed" condition if you absolutely need it, but the lap lengths can be very long if you have bars at the face (not centered in each cell) or large bars (#7 and larger) in a shear wall. I will only fully lap splice my vertical reinforcement when I have uplift in the wall or it is cantilevered.

RE: Pinned or Fixed Suports

In the past, I've considered a single line of dowels placed in the center of the wall to be fixed.
As long as you have a tensile force and a compression force with some distance between them - there's your couple.
With a single line of dowels down the middle, you have a very small arm between the two, but it's helpful in some cases.

In this case, I assume we're talking about out of plane loading.
In order to use any fixity, your need to develop the couple, and the foundation/soil below the soil need to resist the moment couple.
For the sake of simplicity, I'd treat it as pinned.

RE: Pinned or Fixed Suports

(OP)
I have two lines of dowels, It looks like I can say fixed then?

JO

RE: Pinned or Fixed Suports

Yes, but make sure that your foundation can handle it. You will find that getting that fixity means a wider wall footing sometimes.

RE: Pinned or Fixed Suports

(OP)
Slickdeals,

Ok I get it thanks.

RE: Pinned or Fixed Suports

It is neither fixed nor pinned. You could consider the footing to be providing a rotational restraint which would be dependent on the properties of the soil.

BA

RE: Pinned or Fixed Suports

I remember my masonry instructor telling us about a job where he walked out for inspection and grabbed a tall masonry wall from the scaffold. He and the wall nearly fell over. The wall was apparently quite easy to move because of the large cantilever arm and leverage effect provided by the easy access of the scaffold. He said he never designed tall masonry walls with a fixed base again.

RE: Pinned or Fixed Suports

That's a great reference bigmig. Makes me glad I'm not being overly conservative when I'm assuming a pinned base.

RE: Pinned or Fixed Suports

Pinned. The amount of rotation required to reduce it to pinned is very small, so unless it is socketed into solid rock, it is pinned.

RE: Pinned or Fixed Suports

I've lost track of it now but used to have an excellent paper on concrete walls that showed that, from a buckling perspective, it takes very little rotational flexibility before a fixed-ish wall becomes effectively pinned.

Here, I thought that any fixity would come from 4+ feet of embedment into compacted soil. I'd put far more stock in that than I would rotational stiffness of a strip footing. Is that not the case here?

If you want to base fixity on the restraint provided by the footing, the soil has to work, the footing has to work (transverse steel), the starter dowels need to be developed into the footing and lapped with the vertical bars, and the dowel hooks need to extend across the wall rather than away from it. It takes a fair bit to get the job done right.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

RE: Pinned or Fixed Suports

Pinned is the conservative assumption, but it is neither pinned nor fixed. The connection is rigid, not fixed, but the amount of rigidity is dependent on a lot of things.

RE: Pinned or Fixed Suports

I vote for pinned also unless there a reason to justify the time trying to figure out a reasonable rotational spring or subgrade modulus to account for the footing/wall interaction.

RE: Pinned or Fixed Suports

Pinned here too. Although I have seen instances when engineers assume fixed because their rebar was undersized or the wall too thin. But they don't carry their assumptions into foundation design.

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