ajk1
Structural
- Apr 22, 2011
- 1,791
I am reviewing some aspects of a cottage that has been built in the last year. I had nothing to do with its design or construction. To avoid putting in a pump for the septic tank, the cottage was built so it is raised up, sitting on concrete block piers. The land slopes, so the piers vary from 1 block high to 7 blocks high. Each pier block is 10" x 16".
All piers sit on rock according to the contractor. The piers were built with no mortar in the bed joints, but the contractor says that the cells were filled with mortar (although drilling a hole into each of 3 piers seemed to indicate void in one pier and solid in 2 other piers -- I have not been to the site to see -- the contractor refused to drill the 5 exploratory holes I requested, saying 3 was enough. There are 11 piers under the cottage).
Construction photos show that there are 2-15M dowels per pier, and that they extend about roughly 4 to 6" up into the bottom block from the concrete base below. The contractor says that there is no other rebar in the piers.
QUESTIONS
How to check the structural adequacy of the piers to resist the wind lod on the cottage and carry it down to the footing?
I have tried two methods, as follows:
Method 1: Check the maximum horizontal force that can be applied to each pier, based on a saftey factor of 1.65 against overturning. Sum this load for all piers, and compare it to the wind load acting on the cottage, in each direction. If it is less, then there is no chance of the piers having adequate resistance against the wind, and determining how much wind load goes to each pier (by computer modeling) is unnecessary.
Method 2: Similar to methiod 1, except base it on stresses in the grout, using only the grout in the cells and the Code allowable stress for grout (if I can find that). Perhaps I could take the face shell on the cmpressive side of the block as working with the grout in the cell, in carrying out this check.
Both of these methods seem to indicate the piers are inadequate (based on 30 psi allowable tensile stress in the grout in Method 2, although perhaps I could use a higher stress).
Any comment?
All piers sit on rock according to the contractor. The piers were built with no mortar in the bed joints, but the contractor says that the cells were filled with mortar (although drilling a hole into each of 3 piers seemed to indicate void in one pier and solid in 2 other piers -- I have not been to the site to see -- the contractor refused to drill the 5 exploratory holes I requested, saying 3 was enough. There are 11 piers under the cottage).
Construction photos show that there are 2-15M dowels per pier, and that they extend about roughly 4 to 6" up into the bottom block from the concrete base below. The contractor says that there is no other rebar in the piers.
QUESTIONS
How to check the structural adequacy of the piers to resist the wind lod on the cottage and carry it down to the footing?
I have tried two methods, as follows:
Method 1: Check the maximum horizontal force that can be applied to each pier, based on a saftey factor of 1.65 against overturning. Sum this load for all piers, and compare it to the wind load acting on the cottage, in each direction. If it is less, then there is no chance of the piers having adequate resistance against the wind, and determining how much wind load goes to each pier (by computer modeling) is unnecessary.
Method 2: Similar to methiod 1, except base it on stresses in the grout, using only the grout in the cells and the Code allowable stress for grout (if I can find that). Perhaps I could take the face shell on the cmpressive side of the block as working with the grout in the cell, in carrying out this check.
Both of these methods seem to indicate the piers are inadequate (based on 30 psi allowable tensile stress in the grout in Method 2, although perhaps I could use a higher stress).
Any comment?
