I need to set some piers to support a 17′ wide x13′ deep 4/12 roof and a deck. The truss company is telling me that the left and right rear corners of the roof (where the piers/posts will be) represent about 3200 and 5300 pounds of live load respectively. Given the 1500-2000 lbs/sq ft compressibility of our soil, this translates into a pretty large pier. I was thinking of putting in four piers and four posts across the front of the deck to spread out the load of the roof. Any suggestions for the diameter of the piers to allow for a good safety margin and make my inspector happy (is that even possible)<?xml:namespace /><o:p />
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Dennis
Replies
30" square satisfies a lot weight, add rebar.
Spheramid Enterprises Architectural Woodworks
Repairs, Remodeling, Restorations.
Sphere,
Thanks for the reply.
Regards,
Dennis
1500 lbs/sf = 10.4 lbs/sq inch bearing available.
5300 lbs / 10.4 lbs/sq inch = 510 sq inches minimum bearing req'd.
30 in x 30 in = 900 sq inches = Sphere was right.
You could even get away with 24in x 24in if you wanted to.
Mike,
Thanks for the reply. I came up with about the same numbers as you for two piers If I go with four piers and four posts evenly spaced instead of two won't that cut the bearing area needed in half? My goal here is to keep the footing size down small enough that I can still dig the holes with a post hole digger from the local rental place.
Again, thanks for your help!
Regards,
Dennis
How deep do you have to go? If you are making the bottom 30 x 30, or even 24 x 24, it might be easier to simply use a shovel.
Al Mollitor, Sharon MA
MojoMan,
About four and a half feet deep.
Dennis
Well, my motto is: "A strong back is a terrible thing to waste."
Will the roof loads be spread uniformly along the wall, or focused on a few posts? Maybe you could put in a couple of big footings to carry the roof (Dug with shovel) and a couple of smaller ones carrying just the floor (PHD).
Al Mollitor, Sharon MA
Mojo,
My truss guy says the the load isn't exactly spread evenly due to some uplift. But if I go to 4 posts the loads will all be within a few hundred pounds of each other and I should be OK with 18" diameter piers.
While my back (and waistline) could probably use the workout, I will probably just rent an auger :)
Have a good weekend.
Dennis
I actually had to use caissons. Found a guy with a trailer mounted rig that ran a hydralic "motor". Went down 16'to "refusal". They were about 12" in diameter. He tied up at least 3 full lengths of rebar so they were about 8" apart. Poured concrete down those holes the minute the "bar" was ready. Poured up to about 3.5 to 4 feet below grade. Then another crew formed up "grade beams" with a real heavy chunk of cardboard at the bottom and we poured those on top of the caissons and around the rebar sticking out of them. Built a 2X6 wall in sections that went up about 9', added a glulam, floor joists, floor and went up another story. Baby hasn't moved while other additions in the neighborhood have sunk so the doors don't work. Tyr
DJJ: You only get to do this ONCE, and it has to be right, or sometime down the road you have settling, maybe unevenly, and a real expensive mess to repair, maybe even imposible - but in this forum, nothing is impossible! Rent an auger, bore several holes side by side & chip out the webs between them w/ a shovel. If anything - overkill. It's only money & your own time. I calculated that I needed four 3X3 piers 4 ft deep to support my steel shop bldg. In Nawth Jawja, w/ hard, dry, mid-July clay & crumbly, poorly formed granite. Took me the better part of two days to do it, but only half a day w/ a rented HD Auger to bore the holes, the rest chipping away in the best Chinese earth removal style w/ shovel, pick & baskets to move spoil. Been 6 yrs, now, & no noticeable settling or distortion in my bldg. Bldg inspector looked at them & Asked why so big. Told him & he went away shaking his head. Overkill on a foundation never disappoints an inspector, but it sure guarantees a stable base for everything else.
W/ that little vignette, let me tell you a true war story about foundation work. Back in the WW-II years, the Army built several medium sized bldgs at Pine Bluff, AR to manufacture mustard agent. The land was ancient river bed, 99% crappy clay w/ a bearing capability about what you claim. The reactor vessels were going to weigh in the 100,000 lb class, and dared not be unstable when things got wet. Fast forward to 1988 or so, when we wanted to put something new into the bldg. Needed new foundations for reactor vessels. Went to place new foundations. Core samples had not been taken, so we had no idea what was down there. In one area, Coring bits went into concrete of unbelievable hardness, and unknown depth. Finally found the PM from WW-II, who told us they just dug a HUMONGOUS hole & filled it w/ concrete - size unknown, not on as-builts. One core drill finally broke through at 14 ft depth. Concrete had cured for its entire life under absolutely perfect moisture conditions & was as hard as anyone had ever seen. We gave up & just made a clean surface on old foundation & attached to that. In other areas, we went 36 ft w/ new piers, 2 & 3 ft in diameter. Had to disassemble part of the bldg to get the drilling rigs in. Fortunately it was an engineered steel bldg.
Don