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.

 

[TLHS]

Could you just build a new beam above the old one with proper reinforcing and tie it into the column?

 

[jayrod12]

Or how about install a steel section with appropriate anchoring to develop the tensile resistance required to make the tie beams work?

 

[CELinOttawa]

Yup. Never have had this problem myself, but if you reinforce (gasp, shock, horror) the existing floors with flat plate developed as tension steel, I believe it should be relatively easy to both make the condition work and potentially add the additional floors.

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!

 

[jike]

Do you have AS BUILT drawings? Could the mistake have been corrected before it was built? Is there a way to verify it?

 

[PicoStruc]

Question #1

I have little confidence in the ability of the footing / wall connection to transfer moment

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

 

[Brad805]

How is the structure performing?

 

[KootK]

Thanks for all the suggestions gentlemen. Keep 'em coming. I will indeed speak with the original engineer and the geotechnical engineer to get a better handle on what was actually built. The strap beams were definitely an 11th hour add-on.

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.

 

[KootK]

@Brad: fine but it's unoccupied and less than a year old.

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

 

[KootK]

Don't get me wrong, if I had to put money on it, I'd wager that the footings will support the revised structure just fine until the end of time. However, what I suspect and what I can demonstrate through rational analysis are quite different. I get paid for the latter. My accountant brother could manage the former.

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

 

[CELinOttawa]

Positive connection to the existing columns is easy, if anything but trivial; Do you have access to a Hilti International manual, or the bridge reinforcement work manual from the UK Highways? I don't have electronic copies of either, but they are great (and a constant reminder that a lot of the "good stuff" is not provided to us in North America as the manufacturers are too scared of liability to actually tell us clearly how to use their products.

 

[KootK]

Thanks for the references CEL. I'll do some Googling. I'm not convinced that the connections will be easy but I'll have to reserve judgement until I've got some numbers to play with. I'll post some details for review once I've got my ducks aligned to my satisfaction.

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

 

[hokie66]

Your last attachment shows how inept the original engineer really is, so I wouldn't expect much help from talking to him. The strap beams reinforced in the bottom, and then cranked around a re-entrant corner, make me cringe to the point where I would be concerned not just about the footings, but about the whole structure.

 

[KootK]

We shall see. I'm betting that the absence of the strap tooting got picked up in checking and the fix got handed off to a rookie. Visually, it's a pretty tight set of drawings. It's just that one atrocious detail that draws the eye.

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

 

[civeng80]

Footing on boundaries are almost always a problem due to eccentric loads.

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 ?

 

[manstrom]

Interesting topic.

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.

 

[jayrod12]

Does the slab on grade tie in to the interior columns footing? maybe you look at it as the tension member?

 

[CELinOttawa]

When designing from scratch I like using trapezoidal floorings with moment capacity (typically introduced through elastic soil grade beams) at the inside edge. They are easy to form, easy to reinforce, provide positive connection to main wall beams without trouble, and are able to provide very reliable service.

 

[PicoStruc]

I design them assuming a uniform loading under the footing

May be safe for 'cantilever' footing design.... but REALLY not safe, in geotechnical point of view... You may miss a large soil overstress under your wall doing so !

 

[KootK]

@ 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.



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