huangyhg

design of continuous or strip footing
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?
it is very rare to provide shear stirrups in any footings, especially so for residential construction.
#10 bars? is that a typo?
#10 bars probably (i would hope) is metric.
yes! metric, 10m bar, i took a look and can see why some eye brows would raise for imperial #10.
when you say "a civil engineer", does that mean he actually does this kind of stuff regularly?
i'm "a mechanical engineer", so by licensing, i'm qualified to do hvac design...but i don't know a thing about it.
by the way, the attitude you describe isn't too helpful. generally, if an an engineer can do $100 worth of engineering and save $200 off the cost of the project, you figure you're money ahead. so if you can save $1500 and still meet customary practicies and building codes, it's not unreasonable to ask the guy why not. and if he's knowledgable, he wouldn't mind giving you a straight answer.
i recommend you ask him to evaluate the footing as unreinforced concrete. this is typically how a residential strip footing works - the longitudinal bars are just for temperature. if you design as reinforced concrete, then minimum reinforcing ratios must be satisfied.

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