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






RE: Is cmu truly pinned at the base?
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
RE: Is cmu truly pinned at the base?
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.
RE: Is cmu truly pinned at the base?
RE: Is cmu truly pinned at the base?
RE: Is cmu truly pinned at the base?
RE: Is cmu truly pinned at the base?
Did you say you have a twenty-four foot high basement wall made of reinforced cmu?
RE: Is cmu truly pinned at the base?
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.
RE: Is cmu truly pinned at the base?
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.
RE: Is cmu truly pinned at the base?
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?
RE: Is cmu truly pinned at the base?
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.
RE: Is cmu truly pinned at the base?
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.
RE: Is cmu truly pinned at the base?
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".
RE: Is cmu truly pinned at the base?
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.