Is cmu truly pinned at the base? 2

[gb156]

If a cmu wall is unreinforced, the tensile capacity of the mortar prevents the wall from pivoting, thus, moment is transferred to the footing. If the mortar tensile capacity is exceeded, the joint cracks, pivots, and failure is surely to follow, so tensile reinforcing steel is provided and now the wall behaves similar to a reinforced concrete wall.

Is my logic correct? Is the cmu wall assumed to be pinned at the base to simplify calculations?

 

[dlew]

gb156,

When designing a wall as supported top and bottom, I would assume the wall pinned, to be conservative. But, as you said, the rotation of the wall is constrained to certain degree by the tensile strength of the mortar (if uncracked), by the own weignt of the wall, by the constraint due to the footing below and tie beam above, and maybe by others. If vertical reforcing is provided, it should extend into the footing and tie beam

If the wall is a cantilever and subject to relatively high lateral loads, then the base has to be considered fixed and rebars and dowels provided.

AEF

 

[Headran]

Pinned and Fixed are just idealizations, the actual condition is somewhere in between the two. The assumption that a wall is pinned at both ends gives you the maximum moments at the midheight of the wall. From this you can determine the amount of steel reinforcing is required. The nice thing about this is that the reinforcing can be centered in the wall to accomodate load reversal (i.e. seismic or wind loads). It also makes it easier for the contractor since he doesn't have to offset the rebars to one side.

If or when the base of the wall cracks, you will have sufficient reinforcing at the mid-height of the wall to resist the resulting moment.

In the case of a cantilever retaining wall, there is no "pin" or support at the top so you have no choice but to assume that the base is fixed in order to maintain equilibrium.

 

[Ron]

One reason to assume a pinned condition at the base of an unreinforced masonry wall is the significant stress imbalance. The tensile strength of the mortar is very low compared to the compressive strength of the masonry. Since there is no reinforcement to help balance, only very small strains are required to induce "failure" in the mortar (whether true tensile failure of the mortar or bond failure of the mortar-to-masonry or mortar-to-concrete slab).

 

[johnson2a2t]

In the case of designing the stem using pinned connections, is it still best to assume a moment transfer at the base for footing design? For example, in the case of a CMU basement wall with a significant soil retention (I'm looking at one now that retains about 24').

 

[UcfSE]

Many times the base bed joint will have sand over it or on it. That is one trick used when the footing is poured too low and the mortar needs to be stiffer to make a larger joint. That takes away any tensile strength the mortar might have. In addition, if you assume the base of the cmu is fixed or partially fixed you now have to design your footing for eccentric load. That will cause your footing to get bigger, possibly much bigger. I suspect it's not only easier to design cmu as simply-supported but it may well be cheaper to have a few extra bars in the wall than it is to dig and pour a footing twice the size it would otherwise be.

 

[miecz]

johnson2a2t

Did you say you have a twenty-four foot high basement wall made of reinforced cmu?

 

[jike]

It is interesting to note that if you assume pinned-pinned or fixed-pinned, the maximum moment (for a uniform load) is wl^2/8 for either case.

No connection in reality is perfectly pinned or fixed but you would have the problem bounded (stresswise) either way. You just may be a little unsure what exactly the deflection may be.

 

[johnson2a2t]

jmiec,

Yes, at least 24'. The developer is looking at some that could go up to 30'. It's a mountain development with slopes often 1:1 or greater. I've recommended for them to go with reinforced concrete, but the owner/contractor is a bit resistant to change.

 

[miecz]

johnson2a2t,

Then you're not talking about a conventional basement wall with supports at the top and bottom. Some kind of steel or polymer soil reinforcement must tie the wall to the soil?

 

[johnson2a2t]

jmiec,

No, it's just a CMU wall (they use 12" retaining wall block). They've been building them up to 20' (for 3-story cabins w/ a footprint of 26' x 42'- the 42' length being the retaining wall). The City's building official finally balked when they started exceeding 20' and asked for engineered drawings. I'm looking a a design which reinforces the wall with pilasters (acting like webs with the wall as a flange). Almost like a buttress wall design.

 

[miecz]

johnson2a2t,

I honestly don't think I could make this work without a pilaster every three feet, so, I must be missing something. How far apart are the (masonry?) pilasters? How deep are they? Are they attached to the floor above? Is the masonry wall then designed to span horizontally from pilaster to pilaster? Thanks for indulging me.

 

[johnson2a2t]

I'm having trouble getting it to work out too (thus my postings!). The way the contractor WANTS it to work is that the pilasters will end low enough so they can build a floor about 9.5' from the top of the wall (where there will be another floor).

I've looked at designing the wall to span horizontally from pilaster to pilaster, but I'm wondering if I can consider some two-way action in the wall to ease up some of the load on the horizontal "beam".

 

[miecz]

johnson2a2t,

To answer the simple question, I think you can consider two-way action. Even with two-way action, I don't think I could make this work. A sloping backfill can really increase soil pressures. If the soil pressure is on one side of the building, and not the opposite, don't forget to check sliding resistance of the entire structure.