Existing Property Line Footings

[KootK]

Please refer to the attached sketches.

My client has requested that I evaluate his existing building for its ability to support an additional story. Upon review of the drawings, I discovered a couple of foundation detailing errors that I believe to be quite serious. So serious, in fact, that I doubt that the foundation could be shown to have enough capacity to support the number of stories that the building currently has.

The detailing errors that I'm concerned with both involve "property line" footings along one side of the building. Specifically:

1) Some of the load delivered to the foundation system along the property line is delivered as concentrated loads (columns). The original designer placed eccentric pad footings beneath those concentrated loads and balanced things by providing strap beams back to interior columns. The trouble is that, according to my understanding of things, the strap beams have been reinforced bass-ackwards. They've been provided with bottom steel but no top steel. In my mind, this error renders them pretty much useless.

2) Some of the load delivered to the foundation system along the property line is delivered as uniform loads. Unfortunately, as detailed, I have little confidence in the ability of the footing / wall connection to transfer moment. To me, it looks like just a regular wall plopped down on a regular footing. It'll have some moment capacity, no doubt, but I feel that it will be unreliable and difficult to assess.

So my questions are these:

1) Do others agree with my assessment of things here? Might there be something that I've overlooked that would improve matters?

2) Does anyone have any clever ideas for a repair? I've got exactly one so far and I'm not terribly enamoured with it. See the bottom detail in the attached sketch.

Thanks for your help.

KootK

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[KootK]

Thanks for all the fine suggestions.

@ CivEng80: I also have considered using unreinforced flexural capacity. Certainly, when I have the numbers available, I'll check it for sport. In general, however, I'm reluctant to take advantage of unreinforced concrete flexural capacity. I worry about reliability. If you get one transverse crack, it's all over. The strap beam will be in an environment where it may be get wetted and dried, exposed to temperature fluctuations, and perhaps even heaved from below. While I haven't procured the soils report yet, the project is located in an area where expansive clays are common.

@ Manstrom: agreed on all accounts. In my market, it is also common to assume that concentrated loads spread out across the foundation walls and make their way to the eccentric strip footings. No strap beams or isolated footings. I know... questionable. Damn market pressure.

@ Jayrod: interesting concept. The drawings indicate a space between the strap beams and the slab on grade however. That might be something that I could use in the fix.

@ CEL: I'd like to know more about the system that you've described but I'm confused by the terminology. Probably regional differences in jargon. What are "trapezoidal floorings" and "main wall beams"? Can you elaborate? Feel free to speak to me as though I were a child.

@ PicoStruc. I disagree. Soil isn't an elastic material that goes to hell in a handbag when q_max is exceeded. Usually, geotechs provide shallow bearing pressures governed by settlement concerns. If the average bearing pressure beneath your footing is within the allowable value, it's often acceptable to have the peak value go over. I've done this numerous times with shear wall footings and the blessing of my geotechnical minders. It's also important to realize that, even under a regular column footing, the concept of a uniform bearing stress is fiction. Your true soil stress pattern will be convex or concave depending on the cohesiveness of the bearing stratum. Even when you think that you haven't overstressed the soil locally, you usually have.


The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[CELinOttawa]

Lol... Trapezoidal floorings are my auto-correct's idea of trapezoidal FOOTINGS. The reference to wall beams is a nod to howI learnt to apply the trick - To think of the wall to floor interface as a confined beam.

 

[KootK]

I must have this technology. Is the attached sketch right CEL?

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[CELinOttawa]

Yup... Plus if it gets bad, you can actually thicken the floor into another trapezoidal shape in plan, though I never have as I dislike it intensely.

The sketch you've drawn is very common construction practice for New Zealand. Lots of tight-space infills, and tilt-up is quite popular so above the "pin assumption" you show would be drossbach ducts to connect to a panel.

Also, it need not be a pin. Two lines of drossbachs, or other forms of steel construction, can do the same.

 

[KootK]

Neat. Two more questions:

1) Is it ever a problem that the slab on grade needs to be constructed before the superstructure?

2) Are any measures taken to ensure that the floor slab is sufficiently stiff? In this, I'm kind of flip flopping and siding with PicoStruct. With a skinny slab on grade providing rotational stiffness, I'd be worried about its ability to get the job done convincingley. Obviously, it's been working in NZ.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[CELinOttawa]

1) Occasionally, but the benefits far outweigh the cost(s).

2) Almost always works with a thin slab - You get to choose how far out the trapezoid occurs, after all. On larger structure and other difficult situations the slab is not always thin. Concrete is relatively inexpensive.