So, I am inspecting and advising for a job that my firm designed a couple years ago now. This is one of those jobs that was just never meant to be. The general contractor is now on his 4th concrete sub and I consistantly find problems that have to be fixed. The project is now several months behind schedule and last week we learned of a doosie of a problem. Apparently one of the concrete subs decided to pour an extra 6" of concrete on some basement concrete columns to avoid having to use a 2" air space and place 4" CMU blocks below grade. When this was done a brick ledge was not left below grade thus the general contractor began cutting away what should have been excess concrete. It wasn't long before the general contractor discovered that there were major reinforcing problems as they discovered what is shown in the picture below. They found this condition at around 10 columns. The ledge is 4" wide at the moment and the height from the ledge to the top of the concrete is approximately 22". The reinforcement is placed such that there is almost no cover. The concrete columns are supports for steel tube columns which continue up to a two story height above grade. It is our suspicion that no reinforcing exists in the top 22 inches of concrete. Additionally, everything is already load bearing and all steel is up. I could go on, but I will stop for now and answer any questions as they come. We are looking for a creative solution for this problem that is "relatively" economical. Any advice/ideas at all will be appreciated.
What are the anchor rods specifications, (number, size, spacing, material, embedment and end treatment), and what is surrounding the columns location, (possible conflicts with an exterior "bandage")?
Ask if a testing company can nondestructively test the in situ structure and find out just what is there. I wouldn't even try to guess and take responsibility hoping they did what they should when you can see they didn't.
Anchor Bolts: 2 3/4" Dia. A325 Anchor Bolts Specified with 12" embedment, cast-in-place, 3000 psi concrete, spaced 5" apart on each side of a 4" tube column. (Note: contractor placed 4 anchor bolts in compliance with OSHA requirements)
Obstructions: Just the 8" CMU walls on each side, concrete column is flush with outside face of wall.
Column Load- exact from calcs unknown (not my design, but another person from my firm) however this is a library and there is one story and a roof supported by the steel column. Floor system is a steel beam and joist system with concrete deck 3" total height on 1" 26 GA steel form. Approx 200 SF of trib area. My quick approx is around 25psf dead and 150 psf live(floor) and 20 psf live(roof). Thus, a total load of approx 45 kip.
CMU walls: 8" block with #4 @ 48" o.c., grouted only at reinforcing.
See if the concrete column can qualify as an unreinforced pedestal by ACI 318 section 22.8 (assuming you are under ACI). Note the load requirements. This way you can disregard the faulty placed reinforcing and see if it still works. You could also dowel into the adjacent cmu wall, (connecting the concrete column to the wall) and then grout fill those cells to pick up additional strength.
If the piers fail as plain concrete, best bet is to shore the column, demo the pier and replace with correct pier. It will add about 7 days for demo and placement then another 7 days to cure and hopefully pass 7 day breaks.
I recently did a job that went along with what twinnell said. It was a very old building that used to house a chrome-plating operation. Hexavalent chromium (really bad stuff) had seeped around the foundation and into the soil and the current owner was compelled to remove the contaminated concrete and soil. We supported the column by installing a make-shift truss to share its dead weight with the columns on either side (no live loads - nobody was allowed on the structure while construction was in progress). We busted out the concrete, inserted the anchor rods through the hanging column's base plate, installed the reinforcement, and poured the foundation. After a few days (I forget how many - we used accelerators in the concrete) the contractor tightened the anchor rod nuts, dismantled the truss, and that was that.
Another alternative that we thought about using was to straddle the column with two horizontal beams near grade. The ends of the beams were to be supported on mats, one inside the building and one outside. This was nixed in our case because of real estate limits.
I have a cool close-up picture of the pedestal and footing formed up with reinforcement installed with the column and anchor rods in it just hanging there. I tell people that see that picture that we installed the steel before we poured the concrete...
aueng06, are those gravity columns because the column ties don't appear to have seismic hooks of 135 degrees at the corners? Or do you allow 90 degree hooks at the corners if they are alternated on different corners?
Very briefly, consider the following steps-
1. install adjustable screw shores under each spandrel to transfer all the floor and roof gravity load into the perimeter cmu walls. Space your jacks out beneath the spandrels as required. The steel column can now "hang" on shear connections to the spandrels.
2. Demo the as-built concrete pier down to the top of your footing. Keep col anchor bolts in place. (You might consider keeping the part of the pier below the brickshelf el, but it may be best for peace of mind to take out the whole pier).
3. Install new reinforcing for the pier.
4. Form and place concrete up to within 3/4"+ of the bottom of baseplate. Use foam blocks to form your brickshelf.
5. Place non-shrink grout under the baseplate.
Engineering and judgement are required at every step. Good luck to ya.
Sorry, but I don't know if those columns are designed to resist seismic. I am not the design engineer, but have been doing some inspecting and some thinking about possible solutions. There are around 12 columns that have this condition. All were designed with only 2 anchor bolts specified. Thus, my assumption is that they are gravity columns, but I can't promise that.
Thanks to everyone for the great responses! I expect we settle on a fix in the next few days. I'll be sure to post our solution so that you know how this turned out.
Aueng06,
If those are truely columns (weight of column > 300#) then per OSHA they have to have 4 anchor rods( anchor bolts). Only if they weight less than 300# can they have 2 anchor rods.
If the condition/strength of the column below is acceptable, it's possible to remove only the top of the column, splice the reinforcing with mechanical splices, and replace the top of the column and the concrete which has been removed at the face. This still involves shoring the steel above, of course.
