Underground Parking Structure

[ajk1]

I have some questions on unheated underground parking garages, as follows. If anyone can answer even some of the questions, I would be most grateful.

1. Are expansion joints commonly used in below grade unheated parking structures?

2. If expansion joints are used in underground parking structures, how is the horizontal earth pressure resisted? Is it by horizontal beam action of the floor slabs which carry the horizontal load to the parallel perimeter walls? If so, is extra steel added in the floor to make it act as a horizontal beam?

3. What is the maximum distance that should be used between expansion joints in unheated underground garages?

4. Where pour strips are used to relieve shrinkage stress, what is the maximum distance that should be used between the pour strips?

5. Given that according to Troxell & Davis, the average amount of concrete shrinkage at 28 days is only about 30% of the long term shrinkage, how long should the pour strip be left open (we find that it is a struggle to get the contractor to keep it open even 28 days)?

6. Should the pour strips be continued down the perimeter walls as well?

7. In calculating the temperature movements, what temperature range should be used (southern Ontario)?

8. Is there any basis for believing that the temperature range in an unheated underground garage with air intake shafts and exhaust fans etc. is significantly different than an above grade garage, and if so, by how much?

9. If the roof level (i.e. the grade level of parking) of an underground garage is exposed to the sun, would this be expected to undergo approximately the same thermal expansion and contraction as the roof level of an above grade open air garage?

10. What is the maximum length oh unheated below grade garage that you have seen designed and constructed with good results, not excessive cracking?

11. Is there so much movement restraint from the perimeter walls of underground garages that engineers just take the approach that expansion joints will not sufficiently reduce cracking and they will just live with the cracking (provided of course that it does not start ripping brackets off walls, cracking columns, etc.)?

 

[MotorCity]

Here are a few thoughts....

The fact that the garage is unheated is not really relevant, its pretty much a given for parking garages. I have never heard of a heated parking garage and I am in snow country. I suppose its possible if the client has pretty deep pockets.

Once you get more than a few feet below grade, the temperature is relatively constant (this is the principle behind geothermal heating......and wine cellars). Would not expect significant change in temperature.

The design of any joints in the concrete should be based on the drying shrinkage of the concrete, not expansion due to changes in ambient air temp

 

[FishTheStructure]

I currently have a project in an existing unheated parking garage below grade (in Southern Ontario!) and when I am down a couple floors below grade it is definitely cold, with a considerable difference in temperature from the summertime. The temperature of the earth itself a few feet down may be constant year round as MotorCity suggests, but the open drive isles from the ground floor to the lower levels allow a lot of cool air into the parking garage which "sinks" to the lower levels.

This particular parking garage (circa 1990) was constructed with control strips but no expansion joints.

Great spirits have always encountered violent opposition from mediocre minds - Albert Einstein

 

[MotorCity]

Agreed FishTheStructure. I was assuming that by "underground", the OP meant that there was something on top of the garage such as earth cover or a (heated) building. If the top of the garage is exposed to the elements, then there will certainly be a change if temp from season to season.

 

[VTPE]

We have done several underground garages and had to learn a lot through them. We are in Buffalo and have built in Boston (6 and 3 floor down), Milwaukee (4 floors down), and elsewhere. Here are a few suggestions:
1.) I would not use expansion joint unless you have unbalanced earth laods. If you have a structure completely underground, the slabs will always be in compression from the earth pushing in on them.
2.) You need to review the axial load in the slab due to the earth pressures and add it to the streeses induced from PT and from bending. I have had to thicken my floors so they dont buckle under the earth pressure loads.
3.) I try to limit my pours to 100'-150'MAX. 150 is a long way to go if you have a single stressing end. You also start to run into some major losses in PT.
4.) We specify 90 days to place the pour strips, but have always had to give up to Contractor push back. We poured them once at 28 days and had some cracking. 40 days seems optimal. We have also had projects where we surveyed adajcent points against a fixed point to measure the rate and amount of shrinkage to determine when to pour.
5.) It is tough to have the pour strips at the exterior walls if you need the slab to resist the earth pressure loads. If the contractor wants to backfill right away, you can not have a pour strip there. You also need to analyze the structure for the loads on the portion of the slab until the pour strips are placed. We have negotiated partial backfilling until the pour strips are in place.
6.) If you have a PT deck, you need to consider how the PT can slide against the wall, still provide lateral resistance, and not bind against shrinkage or you will end up with several cracks, especially at the corners. I have a chunck of concrete on my desk from a project where we missed that little detail. It is a reminder....
7.) We have taken to seperating our main entrance ramps from the first undergorund level to try to mitigate the temperature variations. After that, the tempratures pretty much level out once you are down a couple fo floors.
8.) One other tip, try to get the ramp in the middle of the building. Transferring earth pressures across a ramp is tough, especially if it creates a scissor at the middle. You can get some huge moments in your columns from offset earth pressure loads. With the ramp in the middle it becomes a natural pour strip and allows you to dead end at the walls.

Good luck.

 

[ajk1]

Thanks for the responses.

To MotorCity: I can tell you from actual winter temperature measurements in a multistorey underground garage last winter, that there is little temperature variation from floor to floor, and it gets down very close (plus or minus 2 degrees Celsius) to the freezing point in the garage when the outside temperature is about minus 10 deg. C. Also, I can tell you for a fact that not all underground garages are unheated. The office buildng that we are in, was constructed within the last 10 years and has heated underground parking.

To Fishthestructure - thank you. What is the length of the garage that you are currently investigating?

To VTPA: I should have made clear that this is a reinforced concrete garage, not prestressed. We have always avoided prestress below grade. My fault for not being clear on this in my description.

My feeling is that many engineers are assuming seasonal temperature effects are not applicable to underground garages, but I cannot see the justification for that.

If the pour strip is not included in the outside wall, then it sees to me the walls will restrain the slab shrinkage and diminish the effectiveness of the slab pour strip. But I agree that contractors scream about a pour strip in a wall.

Thanks again all. I would be interested in any further comments.

 

[hokie66]

1. No.
4. Depends on the structure geometry, but 30 metres is common.
5. As long as possible.
6. No.
10. From memory, about 100 metres.
11. Where slabs are rigidly connected to walls, the restraint should be considered almost infinite, and cracking is inevitable. Therefore, slabs must have sufficient reinforcement to control (not prevent) cracks to an acceptable width.

 

[ajk1]

To Hokie66: thanks very much for the precise and useful information. I think we are in basic agreement, particularly your point 11. Regarding item 5, what in your experience is the length of time that pour strips are kept open?

Since I believe you live in a much warmer climate (Australia?), you may have a much smaller temperature range to deal with than here in Toronto, but maybe the conclusions are the same anyway, namely that there is so much restraint below grade that there is going to be a lot of cracking.

 

[hokie66]

Yes, you are correct that the temperature range here is much less than in Toronto. But the main contributor to direct tensile stress, and thus cracking, is shrinkage.

About pour strips, you are certainly correct that resistance in delay will be encountered. I have seen 60 days specified, but rarely complied with.

 

[ajk1]

to hokie66- what do you take for your long term shrinkage strain?

 

[BUGGAR]

To prevent the contractor from resisting time periods for curing and between pours, put those times in the Specification so they are part of the Contract, as they should be. Then if the contractor objects, the Owner doesn't have to pay him. It works.

 

[ajk1]

to Buggar - yes I am very fastidious about the specifications and the time that the pour strip is to remain open are in the specifications, but that does not really help things when the contractor tells the owner that he cannot meet the schedule if he has to abide by the specifications. Those are simple magic words that every contractor learns early in life...that he cannot meet the schedule if the engineer really wants to enforce the specification...then the engineer becomes the bad guy in the owner's eyes. After 51 years if structural engineering practice, I have never seen it fail to sway the owner if the contractor says he cannot meet schedule...even if it s clearly in the spec.

 

[KootK]

@VTPE: that was an excellent summary of PT underground, even if it wasn't 100% applicable to ajk's situation. I actually printed it out for future reference. There were a couple of points that I was hoping you could elaborate upon:

You need to review the axial load in the slab due to the earth pressures and add it to the streeses induced from PT and from bending. I have had to thicken my floors so they dont buckle under the earth pressure loads

My understanding is that PT forces, while adding to compression stresses in the slab, do not contribute to compression buckling. Do you see this differently? I'm assuming conventional unboded PT systems as are typical in the US market.

7.) We have taken to seperating our main entrance ramps from the first undergorund level to try to mitigate the temperature variations. After that, the tempratures pretty much level out once you are down a couple fo floors.

You mean seperating the exterior portion of the ramp from the rest of the structure to create a thermal break, right? If so, I'm contemplating the same thing on a project at the moment. What does the joint between the exterior ramp walls and the basement walls look like? Any issues with differential movement, eiher vertically or laterally?

@ajk: It sounds as though this thread is winding down and that you've pretty much gotten what you need. Hopefully you won't mind my mining it for a little informaiton for myself.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[CELinOttawa]

Ajk1: Having worked as both a structural engineer and a project manager, I have a simple solution that I regularly use to fantastic effect:

For the contract:
1. Order of precedence puts Specification above all correspondence.
2. Schedules are required to be accurate and up to date within 24 hours of any change affecting the end date.
3. The owner's payments may be tied to the progress of the work as required by the original schedule submitted by the Contractor.
4. Contractor must submit a schedule prior to mobilisation.

For the spec:
1. All schedules shall be prepared using the CPM method.
2. All schedules shall include, and specifically show, all required curing, pouring and operation wait times as show throughout the sections of this specification.


This only works with major jobs where you have "serious" contractors, but normally these are the jobs which require such fitness.

And if you do it, it works!

 

[ajk1]

to hokie66 - this morning when I got up it was minus 26 degrees C here in Toronto (what is the winter temperature where you are?). The high for today is forecast at minus 20 degrees C. Although this is unusually cold, it is not unusual for it to be minus 15 degrees C in winter and in summer to be plus 35 degrees C. Giving a range of 50 celsius degrees. There is a little time lag for the interior of a 250 mm thick slab to reach the air temperature. Assuming say 80% (just a guess) of this range to allow for the slab interior being warmer than the exterior of the slab, would result in a thermal strain of about 450 microstrain. I am doubtful of your comment that the shrinkage strain renders the thermal strain insignificant...

 

[hokie66]

You are doubtless correct. Where I am, the temperature in winter occasionally dips to +4C. In summer, it occasionally goes to +38C. I don't have experience in dealing with your climate, and frankly don't think I could tolerate it.

 

[ajk1]

LOL Neither can I.

 

[ajk1]

to PTKE - I do not understand how you conclude that "the slab will always be in compression due to the earth load pushing in on them". If that were true, why do we always see so much more floor cracking in underground garages than above ground garages?

Let's do some calculations, as follows:

For a 10 foot high storey, and active soil pressure Ka=0.35, and unit soil weight gamma of 130 pcf, the earth force is:
Ka gamma H²/2 = 0.35 x 130 x 10² /2 = 2275 pounds per foot length of the wall.
The horizontal reaction force at the top of the wall then is 2275/3 = 758 pounds. This causes a compressive stress in the floor slab (say 9" thick 2-way flat slab) of 758/(9x12) = 7 psi unless I am missing something. The restraint provided by the perimeter basement walls will far overwhelm that stress and you will end up with tensile shrinkage stress. Although the soil pressure may eventually increase from "active" to "at rest", and the floor height may be more than 10 feet, it is still not going to cause a significant compressive stress in the floor slab when compared to shrinkage stresses.

 

[ajk1]

to VTPE - I do not understand your first point (#1). Why would you introduce expansion joints if you have unbalanced horizontal earth loads? I would have thought that expansion joints are undesirable if you have unbalanced horizontal earth loads because then you have to design the floor slab to act as a horizontal beam to resist the horizontal earth force.

 

[ajk1]

I am still waiting for someone to respond to the following questions:

- what spacing should pour strips be placed in unheated reinforced concrete (not pre-stressed) underground parking structures? Any accepted engineering practice for this spacing? If not, what spacing have you people used, how long did the contractor really leave it open (note that on average only about 30% of the long term shrinkage has occurred at 30 days according to Troxell and Davis), and how has it worked out in terms of reducing shrinkage cracking when you looked at the slab a year or two after construction?

- do you agree that if a corresponding pour strip is not included in the wall, then the effectiveness of the pour strip in the slab is diminished? (I agree that a pour strip in the wall will delay the back-filling and therefore is usually not practical).