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New Column Footing at Existing Column FootingHelpful Member!(3) 

Ntollinger (Structural) (OP)
6 Jul 04 17:15
I'm sure this has been discussed before but I can't find the magic "keywords" to match my question...so please bear with me.

Given:
1. An existing pre-engineered steel building supported by reinforced concrete pier/spread column footings.

2. New pre-engineered steel building addition to be constructed immediately adjacent to existing.

3. Addition columns set in line with existing columns to maximize clear (aisle) space when existing wall is removed.

4. Addition columns are set within 12" of existing building wall.

None of this is a reinvention of the wheel, I know. With the "givens", it's obvious that the new column's pier and spread footing MUST share space with the existing's. Let's say that the existing column has an 8'-0" square spread footing centered beneath it...this leaves approximately a 3'x8' pad of concrete, ( 4' below grade...welcome to northeastern Pennsylvania) extending outside of the foundation wall.

The new footing should bear at the same elevation as the existing, so as not to induce lateral stresses, etc., but how can these two separate entities share this space. I've seen designs where the new column pier simply bears on the existing footing and "extra concrete and rebar are scabbed on" to increase the bearing area. The bar is sometimes drilled into the existing footing, but I don't know many contractors who would not tear my head off when I spec a 24" deep drilling to generate the required development length.

In the past I've seen a concrete bridge beam placed under the new column, which piers down to two footings on either side of the existing, but offset footings can get so big since they're forced up against the existing wall...let alone compensating for the pre-engineered horizontal shears and induced moments that come with them.

Has anyone "thought outside of the box" successfully for this seemingly common, yet head-scratching, problem?

Thanks.
Helpful Member!  SlideRuleEra (Structural)
7 Jul 04 16:29
Consider this possibility: To "scab on" the extra concrete, forget about developing the rebar to design strength. Use Hilti chemical anchors (or equal) to anchor a large number of rebar dowels into the side of the existing concrete footing. Hilti (and other manufacturers) have good data on allowable loading on rebar anchored with their product. Keep in mind recommended edge distances & spacing. The hole diameters are reasonable (a little larger than the bar diameter) and embedment depths are on the order of 10" (or so). I prefer to add the thickness of the existing concrete cover to the embedment depth to make sure the existing concrete cover does not spall under load.

Have used this general approach to enlarge concrete footings for conveyor belt foundations. You wind up with a lot of drilled holes - but the diameter and depth are reasonable for typically available tools and personnel skill levels.
Helpful Member!(2)  DaveAtkins (Structural)
7 Jul 04 16:57
SlideRuleEra:
The problem with what you are proposing is that although this develops the new reinforcing bars, it does not develop the bars in the existing footing.  In essence, the new footing can pull the concrete off from around the existing reinforcing bars, because they are not developed.
Ntollinger:
I did a project once where I used a grade beam which ran along the existing building, bridging across the existing footings.  I even specified styrofoam be used over the existing footing and below the grade beam.  I modeled the grade beam on RISA-3D, using soil springs.  And regarding the horizontal thrust -- it should be balanced by the existing horizontal thrust.

DaveAtkins

SlideRuleEra (Structural)
7 Jul 04 20:36
DaveAtkins - Thanks for the correction - I went too far "outside the box". In the situation I described the new concrete was more or less a "bracket" added to a massive existing footing and subjected to mainly shear loadding. With your clarification I now see that my suggestion does not solve Ntollinger's problem.
pmkPE (Structural)
15 Jul 04 20:05
I have added on to existing foundations (before and after checking the stresses, existing and proposed,) and have found that they were adequate for the increased stresses.  The important point is to check the existing concrete and reinforcement before dowelling in with new steel.  

In regards to a contractor complaining about doweling in 24" or more, as long as you are confident in your calculations/assumptions and design it is not up to them to decide.  After all they bid their work based on what your contract documents call for, if they don't like it they don't have to bid.

(I specify HILTI adhesive anchors quite often as the majority of my industrial work is retrofitting/expansions).
themooker (Civil/Environmental)
12 Jan 05 19:43
I am new on this site, so i may not know your protocol but i am working on a project and i have a question.
I have an existing foundation for a 100' x 40' block bldg.  Building was knocked down. I want to use the existing foundation, but only 3 sides of it and tie a new foundation fourth wall to make a foundation for a 50' x 40' single story block bldg with a floating slab.  The existing foundation walls are 10" thick and step from about 12 foot to 6 foot below grade along the 40' side.
How would i go about tying steel into the existing walls? I have used Hilti products before but i am unsure if i should dowel into the face of existing wall or the ends so i can use a corner bar "L" to tie into my new wall? Any suggestions?

themooker
DaveAtkins (Structural)
14 Jan 05 17:25
If I understand your question correctly, it really doesn't matter how you tie the new foundation wall and footing to the existing, because a continuous wall footing puts load directly into the soil beneath it.

DaveAtkins

themooker (Civil/Environmental)
17 Jan 05 14:48
Thanks, but do I need to go all the way down to the existing footings,or can I have a more shallow footing on the new wall as long as my material and compaction meet requirements? If i was doing a completely new building i wouldn't need such a deep footing but i don't know if it would be better to tie new footing into existing footings. It would definitely cost more.
SperlingPE (Structural)
25 Jan 05 13:04
Are the uplift loads large?  Are the lateral loads large?
Do you have overhead clearance?
Consider using shallow drilled piers.  Depending on loads and site conditions, a truck mounted drill rig (like those used by utility company or electricla contractor) might be able to drill holes for piers.  I have no idea what your soil's conditions are.  So I cannot estimate depth or soil values.

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