Existing Property Line Footings
Existing Property Line Footings
(OP)
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
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.






RE: Existing Property Line Footings
RE: Existing Property Line Footings
RE: Existing Property Line Footings
Can you chat with the original designer? Could be a great source of info... Mistakes like these often get made, only occasionally are drawn, and rarely get built after being drawn.
Think of it this way: You have positive evidence that the current screw up (if in fact field verifications show it was truly built this way) can take existing loads. That means you can extrapolate from current with the corrections and potentially carry new, added, loads.
Adding loads presumes the building is old enough to justify existing capacity through performance (minimum 30 years, 50 much better - see similar resources including CBD 230). Fixing the existing with plate can be done regardless of age, but you'll have to do much more work for justifying new loads.
Field scan, test pit, and core... Verify, verify, verify!
RE: Existing Property Line Footings
RE: Existing Property Line Footings
Question #1
Is it because you don't have enough development length in the footing or not enough rebars ?
In my opinion an hook fully developped is enough to transfert tension resulting from moment ? Can you develop on that, I am curious to see your opinion.
Question #2
Is you footing pressure ok under the wall segment
I have a lot of problem with these L-Shaped wall due to the high pressure under the footing. (e >> L/6 -> triangular pression under the footing where the resultant is just below the wall ! t_wall / 2 = L'/3
RE: Existing Property Line Footings
RE: Existing Property Line Footings
Rebuilding and/or reinforcing the strap may well be the way that I go with this. However, the version of the strap beam that I initially shared was a simplified idealization of what actually is. I presented it that way to keep the discussion focussed on the issues that I'm most concerned with. The attachment to this post shows the real, more complicated, thing.
Reinforcement of the strap beam sounds great in the macroscopic. However, I suspect that working out the details will be tricky. In particular, satisfying code provisions for anchorage of positive moment reinforcement into the supports (columns) will be rough.
@CEL: the building is less than a year old so there's no basis for taking past performance into account here. It's built on fairly plastic clay so settlement would surely be the "failure".
@Jike: I have drawings issued part way through construction. I'll dig deeper into the as-built situation.
@Pico: I do not consider fully developed rebar -- hooked or straight -- to be automatically capable of developing full moment capacity (As x fy x jd). There is research out there, several decades old now, to show the fallacy of that assumption. Google "efficiency of opening and closing concrete joints". I design the L-shaped footings for uniform (q_max) soil pressure. That's the point of the wall/footing moment connection really. In the elastic range, most of the soil pressure will be concentrated under the wall. As the soil gives under the wall, however, the net reaction will move out towards the centre of the footing. In truth, it gives me the heebejeebees a bit too. It's just one more aspect of structural engineering dogma that I've come to buy into.
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Existing Property Line Footings
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Existing Property Line Footings
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Existing Property Line Footings
RE: Existing Property Line Footings
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Existing Property Line Footings
RE: Existing Property Line Footings
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Existing Property Line Footings
At least the designer did put in a strap beam even though the steel is in the wrong place.
What about doing a WSD to check the tensile stress in the concrete top face and if its below max permissable then maybe its ok as is ?
RE: Existing Property Line Footings
I have always used "L" shaped footings for wall loads with a max width of about 2.5' before I get concerned. I design them assuming a uniform loading under the footing; the free body diagram works out if you consider lateral force reactions from the soil pressure, soil friction and the connection into the slab above. I assume a full moment connection with a good lap splice and a hook the full width of the footing.
I don't love the idea, but that is how it has to work. Otherwise, another engineer gets the job and does it this way. I don't see any other realistic or practical detail to do edge footings.
Column loads get a strap beam or combined footing connection.
RE: Existing Property Line Footings
RE: Existing Property Line Footings
RE: Existing Property Line Footings
RE: Existing Property Line Footings
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Existing Property Line Footings
@ 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.
RE: Existing Property Line Footings
RE: Existing Property Line Footings
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Existing Property Line Footings
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.
RE: Existing Property Line Footings
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.
RE: Existing Property Line Footings
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.