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Old 04-28-2011, 08:22 AM   #1
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Terra cotta foundation load capacity

We are looking to open up a load-bearing wall on the first floor of a house that we just bought. The wall sits on a foundation wall in the basement that is made of terra cotta block. We want to expand a doorway into an 11'5" opening.

I had a structural engineer come and look at structure yesterday and he said that because terra cotta has much lower point loading capacity than concrete that we will need to pour pillars in the basement to support the end beams of the expanded opening. There is a half floor above and then the roof above that. He suggested one method was to cut into the terra cotta block, opening up the inside face of it, and use the other three sides as a form to pour concrete pillars in place.

Just curious about feedback on the general idea that terra cotta cannot support the point load and on his suggested method. I have no reason to doubt the fellow but I could not find a single reference through Google to the load bearing properties of terra cotta.


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Old 04-28-2011, 10:27 AM   #2
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I wouldn't impose any additional load on a terra cotta structure than that for which it was built, but that's just me. One of the weakest links in a hollow cell terra cotta structure is the mortar (terra cotta block units themselves have higher compressive strength than a unit of blocks assembled with mortar, and even that varies with the type of mortar). And I never have any data on the type of mortar used 100 years ago, so for the work I've done (industrial) it's not worth the expense of testing the mortar. Creating a different load bearing element is sound advice when you don't have 100% certainty on existing materials.

You should also take great care in making sure the new column is tied in structurally to the wall on either side. When you hollow out a stack of t-c blocks like that, you weaken the lateral strength. The new columns need to accomodate both the compressive load from above, and the lateral load of the wall, similar to a pilaster.


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Old 04-28-2011, 08:16 PM   #3
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Not a big help but you may find this interesting:


OK here ya go, there is a compressive strength chart in the third frame (page 195).

Last edited by Bud Cline; 04-28-2011 at 08:19 PM.
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Old 04-28-2011, 09:30 PM   #4
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Very often, terra cotta tiles are laid with the cores horizontally, so the units are carrying the compressive loads in the weakest directional and the strength of the units based on "book value" tests does not apply. In most masonry, the strength on the mortar is of little importance when it comes to vertical loads because the units are oriented properly and the units control the strength of the masonry assembly (units and mortar).

Since you are inducing concentrated loads by opening the exiting door, your engineer is correct, because the loads could require something more than the existing strip footings, especially if you have more than a minimal single story home above.

You did not say what the tile wall thickness was (4",6",8" or 10"), but that may not be important.

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