Footing Enlargement ?

[1SEngineer]

We are designing a metal frame building which will abut an existing metal building.

In order to maximize floor area we'd like to place the the steel columns of the new building directly next to the slab edge of the existing building.

The existing metal building foundation is a thickened edge 12" deep x 18" wide.

We have a few options, but we feel the cheapest would be to add a new rectangular shaped footing next to the existing thickened edge. The new footing would be 4' wide x 6' long. 1.5' of the width would be from the existing footing.

We'd calculate the bearing pressures and use these numbers to design epoxy embedded steel reinf. to be connected to the bottom of the existing footing to transfer tensile (and shear) forces. We be enlarging the spread footing. This approach is shooting from the hip a bit but we are curious to know if anyone has done something similar (and had no problems : - )) ?

The other options would be to offset the column far enough from the exiting thickened edge to create a completely isolated footing. This reduces the amount of interior space and creates other space problems.

Another option is to use helical or pin piles to pick up the new loads. Of course this adds more cost to the project.

Cutting into the existing buidling is not an option.

Thanks in advance for any comments!

 

[msucog]

consult a geotech to take a look at the soils and to evaluate what may happen to the existing building (ie. settlement for instance). that may remove the need for special foundations and they might be able to help head you in the right direction while saving some money. i've seen plenty of times not needing anything special and the new columns were right against the existing building. i've also see some ugly stuff happen when the existing building line/columns settle due to the new loads.

 

[msquared48]

I would consider excavating deeper and underpinning the footing of the existing building with the footing for the addition. This would be able to be done easily if the existing building is owned by the same individual. Not so otherwise.

In the latter alternative, you could consider an elongated, eccentric footing too, integral with the slab. Allowable soil bearing will determine the footing sze and orientation.

Mike McCann
MMC Engineering

 

[oldestguy]

If the old and new building main bays are typical frames, perpendicular to the wall, what will a little settlement do to the stresses in those frames?

If you have differential settlement along the wall, will it cause you any grief with the roof system.

I've seen some amazing differential settlemnt in the roof systems perpendicular to the main frames with no difficulty structurally.

What can your job tolerate?

Then, you have some basis for changing your ideas or adding reinforcing, etc.

 

[1SEngineer]

Gentlemen,

Thanks for your comments.

We're getting a soil boring and I'll ask the geotech what he'd recommend.

In order to keep the current column layout we'll have to use helical piles. If the owner doesn't want to pay for them, then we'll use new isolated footings(and encroach on the interior space).

Regarding connecting to the existing footing, it is the most practical thing to do. But differential settlement is likely. I think the only thing that would occur is some concrete cracking in the existing slab-on-grade/mono footing system. The movement might create some new stresses in the existing metal frame, but I don't think these would be critical as the loads are very light. This is probably a case where attaching to the existing footing might not be a big deal, but in conclusion, I think it is shooting from the hip a bit too much.

Cheers!

 

[escrowe]

Yeah. Given the likelihood of settlement of the new foundations,
I generally advocate joints to separate all new and existing construction.
I might reconsider that in the case of a very good foundation support condition.

 

[civilperson]

Try locating the first frame four feet into the building with the girts and purlins cantilevered out to the exterior wall corner. Thus the footing is not in conflict with the existing building and only a thickened edge is needed for the end wall support.

 

[InDepth]

1) Even though epoxy doweling into the existing footing sounds like a good idea, it certainly won't help you develop continous bottom tension reinforcement...you would literally have to drill and epoxy a lap splice into order to develop the existing bottom reinforcement.

I would try and keep a gap, this way if the adjacent building needs to be demoed, it can.

2) You could use an eccentric footing with a grade beam connected to it to take out the moment. (Cantilever Footing...take a look at old texts).

 

[1SEngineer]

CivilPerson:
Due to site specific issues, I don't believe this would work for us.

InDepth:
Sure...we wouldn't be developing the bottom reinforcing of the existing footing. One could argue, though, that epoxy doweling of bottom bars to an existing footing could resist the tension forces in the bottom bars created from the new bearing pressures. You'd have to look at the existing footing as one system and the new footing as another system. You develop the steel by concrete failure cones.
Yes...this is creative...and I would not use it where loads are signifcant, but there is no reason the combined systems should not work. There is, of course, a differential settlement issue that was mentioned above.

Thanks for the comments!