Pour Strips and Constructability

Sometimes, in a multi-story building, the pour strip will not extend through every level and may only be present at lower (larger) levels. In these cases, it is tempting to assume that the backshoring will only be required to carry the floors that have a pour strip. There are many situations that would make that assumption incorrect. The backshoring needs to be stripped from the upper-most level and then down to the lowest level. Therefore, just because the lowest level of pour strip has been poured back does not necessarily mean backshores can be removed once they have been loaded. Another example is if the structure is 9 stories and the pour strip is in the same bay for all nine levels, the lowest level cannot be removed until the 9th level is cast and cured. If the construction sequence averages one floor per week that puts the stripping operations out a minimum of nine weeks, plus the 30 to 60 days the pour strip is required to remain open. This could mean that other trades will not be able to access the lower levels for a minimum of 10 extra weeks.

Additionally, if there is any delay on a pour at any level of backshores, it is possible that a floor above will be poured before all the pour strips have been placed and backshores stripped. The reshoring designer needs to carefully consider these types of situations and communicate closely with the contractor with regards to scheduling, as that is a critical path item. Once a backshore plan has been developed and construction has begun, it is difficult to add backshoring for additional floors. While it can be done, some backshores are preloaded; it is difficult to add backshores to take a similar load and to anticipate how the loads will redistribute.

A better option is to widen the pour strip and balance the backspan so that the large cantilevers will support their self-weight. This will allow the floor to be stripped and reshored without accumulating several floors of dead load. Reshores will extend to slab on grade, as there is no live load capacity in the floors to resist pour loads. Once the formwork under the cantilevers is stripped, then reshores are installed to carry construction loads from upper levels. Having reshores extend to the slab on grade in these areas may limit access in the affected bays, but likely will not close them off entirely. With careful planning, it is often possible for the reshore designer to provide access points through the reshoring to allow other trades to move from one side of the building to another. Please note, if the engineer designs the backspan in this manner, it is imperative to strip the backspan before stripping the cantilevers in order to eliminate the risk of large tip deflections (Figure 2).

The best option for the construction schedule is for the contractor to ask the Engineer of Record to widen the pour strip and balance the post-tensioning forces so that there are short cantilever slabs that will support their self-weight and some live load. This allows the reshoring designer to design the area as he/she does any other reshored area. Other trades will have full access to the floors below. While it is possible to design these cantilevers to support some superimposed live load, it is often not as much as the typical bays; therefore, the reshoring designer should take extra care to only utilize the slab capacity that is actually there. Again, the backspan should be stripped before stripping the cantilevers (Figure 3).

If it is not possible for the pour strip to be widened, then the Engineer of Record has the option of staggering them in different bays at different levels. This is especially effective if the schedule is such that lower floors are poured and have live load capacity to support the cantilever and construction loads from levels above. Using this option, the schedule can often be worked so that the reshores only need to support the loads from one or two poured floors instead of stacking several above each other.

With a little planning, pour strips need not be a major headache for anyone involved on a project. Once a project is awarded to a general contractor, he/she should sit down with the design team and discuss pour strips. The main questions to ask are:

1. Are the cantilevers on either side of the pour strip self-supporting?
2. If so, will they support any superimposed live load?
3. If not, would the Engineer of Record consider widening the pour strip and balancing the backspan so that the cantilevers will carry their self-weight and, ideally, some superimposed live load?

Once these questions have been addressed, the reshoring designer can get involved and construction can commence in an organized fashion without any surprises creeping up on the owner, the contractor, or the other trades as construction progresses.▪

Post tensioned slab blow out

Post tensioned slab blow out

(Structural)

(OP)

Hi,
I have an 8" post tensioned slab under construction, the contractor called and stated that the slab anchorage area was blown out, it crushed the concrete slab edge for about 3'x5' area.  This happened in 6 hours after stressing.
I have checked the design and it was satisfactory, the end anchorage details were just typical post tensioned slab detail provided by the PT supplier, i.e. backing bars, and some top bars.  The reinforcement was inspected by building inspector before pour.  

In your professional opinion, what would have caused the slab blow out?  Please shed some lights on this, because I am quite nervous under this situation.

Thank you in advance for your kind advice.

RE: Post tensioned slab blow out

(Mechanical)

LUTEIN:  I assume you are saying that concrete was crushed and/or buckled around one of the post tensioning jack or anchor.  My first guess would be that the concrete was not the right mix or not fully cured or both.  Try to find out what the applied load was.  Is there a concrete coupon available to test?  Was the concrete properly installed ?

I sympathize with yu, I would be nervous also.  I suggest you get to the site as quickly as possible and see for yourself.

Good luck
Dave    

RE: Post tensioned slab blow out

(Structural)

(OP)

Dave
Thank you very much for your response.
You are correct, the concrete was crushed and/or buckled around one of the post tensioning jack or anchor.

The concrete strength appears to be ok, i.e. 75% strength @ stressing.  

I suspect that the anchor may not be installed exactly @ the mid depth of slab, do you think this might be the problem?

RE: Post tensioned slab blow out

(Structural)

Issues of professional liability are not clearly defined in such instances.  Fingers, typically, will be pointed in order to recover costs for physical damages and contract damages for delays.  But you can be prepared.

1. Lock up all your project files and restrict access.
2. Make a list of all contractors, subs, suppliers, vendors, and inspectors.
3. Notify your insurance carrier.
4. If you’ve working bare, have an attorney review your project files for an opinion of your exposure.
5. Go on-site and photograph the damage and any areas that were successful.  Do not discuss, offer opinions or speculation to anyone while on-site.  If pressed, say you’re looking into the incident.
6. Don’t offer anymore opinions on this forum or any other forums.
7. Notify the owner of the incident – offer no opinions.  Say you’re looking into the incident.
8. Have an outside SE review you project contract, calcs and documents for standard of care.
9. Were you on-site prior to the incident?  If so, review your notes and contacts as well as any opinions expressed and to whom.
10. Try to do everything from this point forward in writing.  If you must talk on the phone or to other directly, make a contact report and keep it with your files.
11. If this becomes a mess consider arbitration rather than litigation.

Don’t be nervous.  Prepare for the worst and expect the best.

RE: Post tensioned slab blow out

(Mechanical)

LUTEIN:  OLDDOG04 has some good ponts.  Were all the rods tensioned, or was this the first and only?  Is the 75% strength based on tables or is it actually calculated from a coupon?  How far off center is the post tension rod?  Is there other rebar in the slab?  How about the actual applied load as opposed to the specified load?  is the post tensioning system equipment in good condition and claibrated?  Is only onme side dmaged or is the opposite side also damaged? Call me old fashioned, but I still suspect a problem with the concrete and/or the methods.

Out of curiosity what are the parameters such as the concrete area, concrete type/strength, the applied load, the cure time, etc?

Can you post some photos?

Regards
Dave

RE: Post tensioned slab blow out

(Structural)

(OP)

Thanks for the advices
I will be on site Friday and take some photos.

RE: Post tensioned slab blow out

(Structural)

This is one more reason for me to dislike post tension.

Lutfi
www.cdeco.com

RE: Post tensioned slab blow out

(Structural)

Lutfi - isn't is something - each of us has a pet peeve structural system that we just can't quite embrace.  Mine is hollow core slab systems.

RE: Post tensioned slab blow out

(Structural)