Enlarge Existing Footings-Part 2

[BSE05]

This is a followup to a previous post a couple of months ago. The problem is to enlarge existing footings in place to support the addition of two new floors without underpinning (due to spacial constraints).

After running numbers on punching shear the existing footings fail under the increased load. There is room to pour on top of the existing footing. Has anyone had experience with this and how to best transfer shear to upper slab? I was thinking about welding angles to column.

 

[jike]

Is there a concrete pier on the footing or does a steel column sit directly on the footing?

 

[BSE05]

Most likely there is no pier, but I won't know for sure until we dig them up. For punching shear I assumed worst case, no pier.

 

[ironmon]

Can you temp support column, remove existing base plate and install oversize/stiffened base plate to resolve punching shear issues?

 

[BSE05]

The "preferred" method is not to have to hold up the column. The cribbing takes up too much floor space. The work will be on several footings in an occupied store. But if nothing else works that will be the solution. In that case I'll replace the whole footing. I'm preparing a couple of alternate details for pricing. Depending on what we find during excavation, I'll have to modify to suit. Unfortunatley I'll have a short turn around time for revised details during construction.

 

[jike]

The issue here is if you weld angles higher up on the column, how can you determine how much load is going to the angles and how much to the original base plate? Will the original footing fail before the upper part carries the load?

The possible approaches would be to widen the column bearing to reduce the punching stress as suggested or shore the column up and pour a new footing.

 

[jdonville]

SteelyLee,

Are you sure that you can't use micropiles to add capacity instead of enlarging the footing?

Jeff

 

[BSE05]

The soil is good sand and gravel. We have had borings taken and a bearing capacity established. Just need more footing area, etc.

 

[JLNJ]

You will want a very stiff weldment - I would look to something more meaty than angles. Maybe some heavy misc channels stiffened to suit.

 

[jdonville]

SteelyLee,

I just meant that adding capacity though piers or micropiles connected to the existing footing might cause less disturbance and proceed faster than enlarging the footing.

Many micropile contractors can work in low headroom conditions, if that is the major reason for not using an underpinning method.

Good luck with your project.

Jeff

 

[csd72]

Looks like you need to get someone to measure the footing so you know what you are up against. Otherwise you will be designing it twice.

If you have a pedestal (pier) then you have a great deal more options.

 

[BSE05]

I am designing it twice (with many boundary conditon notes)basically showing two detailed options to get pricing and a contract in place before we open up the floor. Depending on what we find changes will be made to the design during construction.