Rock Wall

[TrevorEIT]

Any idea how to approach the analysis of a 100 year old stone and mortar bearing wall? The wall is approximately 15" thick and roughly 4'-0" high and needs to support a 23 kip column load (total load with live load reduction taken) spaced every 7'-6".
Any ideas would be really appreciated.
Thanks,

 

[PMR06]

I would assume the wall gets torn out! Build a proper, modern foundation and then clad it with the same old stone if you wish.

For discussion though...

Consider the gravity bearing: Do you have any feel for what the old wall is bearing upon? Does it bear below frost depth? The additional bearing pressure will be at least 23k/(7.5ft * 1.25ft) = 2.5 ksf. That's if it distributed evenly, which it probably will not. Can the wall span without cracking?

Consider out of plane loading: I'm sure you've assumed the base of your new columns pinned. Can that old wall provide restraint out of plane?

 

[TrevorEIT]

Yeah, you're right, the best solution would be replacement of the existing stone foundation wall, however budget constraints are a significant factor here. This is residential. My present objective is to determine if the stone wall will work.
The wall is bearing upon a continuous strip footing (don't know the width...assuming 24", maybe 18", since it supports a 15" wall) which is located below frost depth requirements.
Can the wall span w/o cracking? That is the big question. If I analyze using the concrete modulus of rupture of 7.5xSQRTf'c as the max allowable tensile (bending) stress and analyze the stone wall like a continuous grade beam then do you believe that would be the correct approach? Maybe even assume lightweight concrete and use 75% of the modulus of rupture per the code?

 

[ishvaaag]

Safe design values to crush stone masonries have an ample range of variation. It would be useful to know the compression strength of the stone itself and that of the mortar. It can give 60 kgf/cm^2 for sound quarried granite over 1 ft thick taken with 80 kgf/cm^2 mortar to 6 kgf/cm^2 for rough weak sandstone or limestone taken with a 5 kgf/cm^2 mortar. And in between. Then, these are values absent of any consideration of buckling, and a proper evaluation can't be made but following some procedure in some book caring for the many aspects of structural strength and stability.

A general useful advice is to use a dormer beam atop the existing wall It can be a hidden one) to oblige the spread the column loads and forfeit localized modes of failure. At your load level seems can be quite feasible.

The main point here is general stability of the built structure. These structures cut at midheight are prone to failure at the cut, as the many casualties in failing badly conceived extended shorings prove. So if you have a plynth of stone masonry ensure you are not asking much of it and preferably derive everything related to lateral stability to new structural elements of reliable behaviour. You can so provide a lateral resisting system (notionally) entirely independent of the walls. In practice this can work but you anyway must check the ability of the walls of taking their share (that can be very important, because the very high stiffness of the stone walls) of the lateral forces. Designing this way you ensure you have your "new" lateral system in place, yet realize the truth that to forfeit problems at the midheight joint you need also be cautious there.

 

[ishvaaag]

The design values above are for a factored loads check.

 

[cntw1953]

I have never delt with stone masonry, guess it is controlled by motar bond, which could be minimal for out of plane forces.

Since the wall is not very tall, can't you dig and provide footing at base of the wall? If it is viable, it may yield more options that simplify the task. Pay attention to stabilization during construction, which could be crucial.