Prestressed Tendon Damage 3

[Betzwood]

One of the double tees used in a precast prestressed concrete parking garage had three tendons nicked during the drilling for a post-installed anchor (yep, bad idea...don't ask!). A boroscope investigation of the holes reveals that each of the three strands had apparently lost less than 30% of their diameter. Reinforcing of the affected double tee has been designed and implemented to support the original loading as well as the weight of the reinforcing.

Are there references available that discuss the ability or lack thereof of tendons to continue carrying service loading despite being damaged?

Is it realistic to assume that the original double tee is no longer capable of supporting any load with this level of damage?

Thank You

 

[Zambo]

Your first sentence says 3 tendons were "nicked". Then you say that 3 strands have lost 30% of their diameter.

The first question is how many wires make a strand and how many strands make a tendon in thie beam. Then is it possible to determine how many wires have been cut?

Can you get the designer to check whether the beam is still capable of taking the load?

 

[Zambo]

This might be interesting although it doesn't answer your question.

http://pedago.cegepoutaouais.qc.ca/...GC-BON/Documents/ST088-Honorio Wight Erki.pdf

 

[Betzwood]

Thanks Zambo,
This is the typical seven-wire strand, and the investigation indicates two wires damaged in the bottom tendon, two wires damaged in the second tendon, and one wire damaged in the third tendon (all from the bottom). The report indicates that each of these five wires have lost less than 30% of their diameter. From this, it would appear that the tendons overall have suffered very limited damage.

I am acting as a third party to all of this, defending the person who drilled the holes based on direction from the EOR, so the lines of communication to the designer are gray to say the least. I do not believe any analysis of the tee was done to verify its capacity after the damage occurred.

The link you sent is helpful in that it indicates that there may have been a simpler means of repair of this member besides that which was performed; however, I am still interested to know if there is any documented guidance for whether the damaged member can still be considered useful, or is it to be abandoned from a design perspective.

 

[BAretired]

The question of damaged strands is addressed briefly in the following link:

http://pci.org/view_file.cfm?file=JL-90-NOVEMBER-DECEMBER-6.pdf

BA

 

[Zambo]

Good link from BAretired.

Betzwood I think you didn't quite get my point in my first post. Often in large beams the strand is made up of, as you state, 7 wire strands. But then a number of these strands are used to make one tendon. I gather that in your case this is not the case and for your beam 1 strand = 1 tendon.

Do not take my information as fact, but my opinion is that each of these damaged wires must be taken as not now contributing to the prestressing of the beam. That means that the overall loss of area of strand is too much and the strands need to be replaced (which is not possible) or other repair be carried out.

A design check would have been an option.

 

[Zambo]

"I am acting as a third party to all of this, defending the person who drilled the holes based on direction from the EOR"

Strictly speaking you aren't acting as a third party as you state you are "defending" one party. Your report has to be worded carefully as you are employed by one party but want your report to be seen as unbiased. To be truely a third party you should be jointly employed by both of the two parties to the incident.

I guess your findings will be that the drilling was carried out under the direction of the EOR who had fuller knowledge of the location of the strands. I expect the drilling also took a reasonable period of time in which the EOR had the opportunity to see the work being carried out.

Once the damage was identified (I suppose fairly obvious when the hole location was looked at from below) Then the decision must have been made to replace the beam without looking into repair options or a design check.

In fact replacing the beam, especially if available from stock, would seem the obvious solution. But the costs are another issue - no insurance?

 

[bridgebuster]

I was involved in something similar about 15 years ago: P/S concrete adjacent box beam bridge. There were Amtrak catenary lines below. When the contractor was grouting the shear keys, some grout hit a catenary, caused an arc, blew a hole in one of the beams; a few strand wires were broken.

We analyzed it for the reduced capacity. It was still adequate. A literature search indicated that high-voltage electricity doesn't aversely affect the tendons. At first, the owner wanted the beam replaced. This would have entailed dismantling the bridge because the transverse tendons were installed and grouted. It would have also delayed opening the bridge for months. We suggested to the owner that the contractor leave the beam in place, and post a 20 year bond in the event something goes wrong.

Everyone agreed; contractor posted the bond, patched the beam, and the bridge is still standing.

 

[Betzwood]

Bridgebuster,
Thank you for sharing your experience. Since there is no loss of bond between the strands and concrete, instinct tells me that the intended function of the strands could remain intact, but in the case of a double tee, the strands also act as primary reinforcing for strength. Any loss of section for primary reinforcing will certainly raise an eyebrow, but the EOR in this case did not even entertain the idea of checking available capacity once they knew the extent of the damage.

Your comment confirms my suspicion that it would be reasonable to verify capacity first then determine a course of action for reinforcing if required. What happened in this case is that the contractor skipped right to performing extensive reinforcing that involved encasing the webs of the tee with concrete and installing both mild reinforcing and prestressed reinforcing - all with the double tee in place

 

[doka1]

betzwood, how was the repair performed? I can't picture how you would install prestressed in the field to a tee in place. Could you provide a little more detail?

 

[Zambo]

Betzwood,

your point that:

"Since there is no loss of bond between the strands and concrete, instinct tells me that the intended function of the strands could remain intact"

seems quite persuasive and it is not easy to describe why this wouldn't be the case. Your point is that the wire couldn't slip so the prestress in the beam remains the same after the cutting of the wire as it was before. This is not the case, the cut end of the wire used to be under tension, now it isn't so the wire locally must have shortened. This has now transferred stress to the remaining wires. This will be fine until a load is applied in which case there may be a failure - so a design check is needed.

An easier example is this. If one wire was cut out of your 3 x 7 wire strands then you have lost about 5% of the area. This is still quite high I would normally say 2% is negligible, but having said that 5% would probably be fine. You have lost 30% and your theory says this is fine. In fact according to your theory all of the wires could be cut and the beam would still be fine.

 

[BAretired]

Wires damaged by drilling may have the capacity to carry static loads, but their behavior under dynamic loading is unclear. In the absence of overwhelming evidence to the contrary, wires nicked, even slightly, by drilling operations, should be excluded from the area of steel contributing to the strength of the member.

BA

 

[Ron]

Even though precast, prestressed double tees are "factory" produced, the tendon placement cannot and should not be determined by the design drawings. There are numerous ways to locate tendons in place to prevent cutting them when drilling holes or cutting cores. The one who cut the holes has the duty to avoid cutting the tendons and should verify their location (through nondestructive and semi-destructive means) prior to cutting. If they did not at least attempt this, then they were negligent. (I'm not trying to imply a legal finding of "negligence", just using the word in context)

 

[doka1]

I agree Ron, although the placement of prestressed cables in cable is far different than the placement of rebar. The end form that is made by the precaster SHOULD look exactly like the picture of the cross section of the beam in drawing. I doubt any precaster would be sloppy enough to locate the anchorage points differently than depicted in the drawings. When erecting a precast garage workers are constantly drilling and anchoring braces. While in the court your suggestion to use means to verify the locations is plausible, it just does not happen during new construction.

 

[Ron]

doka1... I've been called many times to locate rebar and tendons for contractors and others. Further, when we are coring concrete for further testing or analysis, we locate the rebar or tendons before we cut.

 

[Zambo]

doka1 has a good question about the repair method.

The earlier post says the repair consisted of additional rebar, prestressing strand and encasing the webs in concrete. Does this mean the repair is a post-tensioned addition?

I also can't imagine how this repair would be carried out and there must have been a design, which makes it a bit strange that the same designer didn't check the design of the beam to see if the cut wires still left the beam in a serviceable condition.

Just out of interest what is the depth and span of these beams?

 

[doka1]

Ron, maybe I was not clean enough. I agree, if you are retrofitting something into an existing structure and have to core a hole, cut or drill, by all means locate the tendons and rebar. But if one is erecting the structure you can easily see where the precaster patched where the tendons pass.

 

[Betzwood]

Thanks to all for very helpful posts.

To be clear, this is new construction in which a steel subcontractor drilled anchors into the webs of precast double tees. This was done based on a detail on the EOR drawings that did not give direction on the location of where the holes were to be drilled. The precaster drawings show the same detail indicating a minimum distance from the bottom of the web for which to locate the holes. The steel subcontractor produced shop drawings showing the distance for these anchorage points in excess of what was shown on the precaster drawings. The steel subcontractor was not given the precast drawings, although I am told he requested them several times. Despite this, it is usually not the responibility of the subcontractor to coordinate work with other trades, nor was the steel subcontractor directed to use any form of testing to locate strands, which would be a cost to include in his contract.

The repair performed is a prestessed addition, consisting of concrete encasement of both webs, the addition of four PT strands, and the addition of mild reinforcing, all of which is anchored to the member using stirrups doweled to the flange of the tee. Yes, I am also mystified as to how this could be done in the field, but it was, I saw the work myself.

I am not aware of an analysis performed on the member to account for the reduced area of the damaged strand, but I do have calculations for the repair which do not appear to account for the existing strands in the double tee. This a 12DT28 with a 4" topping that spans about 61'-0".