Slowzuki
Mechanical
- Mar 27, 2003
- 137
Good afternoon,
Being a mechanical engineer, I retained a civil engineer for our house when the building inspector said he required stamped drawings for the post loads on our foundation.
The posts are 6x6's spaced 8 ft on center carrying 6600 lb of design load (this is a factored type load based on the local building code) upon review of the code, a 0.4 m2 (4.3 sqft) footing 6" thick is the minimum requirment.
Since we have a strip or continous footing, this is not the same and they asked for the engineers approval.
Our engineer for a modest sum provided a design. His design involved a 12" wide by 18" high footing, 4 x #10 rebar longitudinal and #10 ties on 18" centres.
The footing bears on a 2 ft deep x 3 ft wide drain tiled trench of compacted crush rock (3/4-1.5 laid in 2" lifts) which in turn bears on undisturbed till which is very firm. Typically an 8 ft x 8" basement wall in this area bears on a 12" footing with no reinforcement directly on this till.
I was surprised at the ties and discussed and the engineer and he said the design needed no reinforcment but the minimum reinforcment requirements kick in. He also said he based his analysis on a lintel beam with distributed loading flipped upside down.
I looked into this further and found the ACI standards and found the slab/footing calculations seem more appropriate.
I followed through and found the footing needed no reinforcment but would need longitudinal rebar top and bottom for shinkage cracking and to maintain aggregate interlock. This is what the minimum reinforcement requirment is for. I could find no requirment for minimum number of ties in the footing design.
The footing by my calcs is much over designed for 1 way and 2 way failure without ties. The longitudinal rebar meets the min reinforcment criteria and provides a significant cushion for bending failure.
I tried to discuss this with the engineer to get the ties removed and I didn't get more than a couple of words out past that before he became very upset and refused to look at the design again and nearly hung up on me after suggesting I was being cheap. I phoned later and apologized on the answering machine for questioning his design in the interest in maintaining good relations.
The ties represent about 1500$ cost in material and labor so I won't fight too hard about but I hate over designing things just because. If we always did that there would be no need for engineers.
I work in the building mechanical field and clients generally won't settle for overdesign, especially when it incures 20% budget overruns.
Any opinions here?
Being a mechanical engineer, I retained a civil engineer for our house when the building inspector said he required stamped drawings for the post loads on our foundation.
The posts are 6x6's spaced 8 ft on center carrying 6600 lb of design load (this is a factored type load based on the local building code) upon review of the code, a 0.4 m2 (4.3 sqft) footing 6" thick is the minimum requirment.
Since we have a strip or continous footing, this is not the same and they asked for the engineers approval.
Our engineer for a modest sum provided a design. His design involved a 12" wide by 18" high footing, 4 x #10 rebar longitudinal and #10 ties on 18" centres.
The footing bears on a 2 ft deep x 3 ft wide drain tiled trench of compacted crush rock (3/4-1.5 laid in 2" lifts) which in turn bears on undisturbed till which is very firm. Typically an 8 ft x 8" basement wall in this area bears on a 12" footing with no reinforcement directly on this till.
I was surprised at the ties and discussed and the engineer and he said the design needed no reinforcment but the minimum reinforcment requirements kick in. He also said he based his analysis on a lintel beam with distributed loading flipped upside down.
I looked into this further and found the ACI standards and found the slab/footing calculations seem more appropriate.
I followed through and found the footing needed no reinforcment but would need longitudinal rebar top and bottom for shinkage cracking and to maintain aggregate interlock. This is what the minimum reinforcement requirment is for. I could find no requirment for minimum number of ties in the footing design.
The footing by my calcs is much over designed for 1 way and 2 way failure without ties. The longitudinal rebar meets the min reinforcment criteria and provides a significant cushion for bending failure.
I tried to discuss this with the engineer to get the ties removed and I didn't get more than a couple of words out past that before he became very upset and refused to look at the design again and nearly hung up on me after suggesting I was being cheap. I phoned later and apologized on the answering machine for questioning his design in the interest in maintaining good relations.
The ties represent about 1500$ cost in material and labor so I won't fight too hard about but I hate over designing things just because. If we always did that there would be no need for engineers.
I work in the building mechanical field and clients generally won't settle for overdesign, especially when it incures 20% budget overruns.
Any opinions here?
