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Increasing headroom to existing PT beams to parking structure
6

Increasing headroom to existing PT beams to parking structure

Increasing headroom to existing PT beams to parking structure

6
(OP)
This was an interesting project that I thought I would share:

New multi-level parking structure, constructed with PT beams spanning 60' (Cunningham form system) with transverse PT 1-way slabs spanning 25'. Structure got topped-off last month and it was discovered during a round of inspections that 6 beams to lower levels had insufficient headroom, by magnitude of 4".

The contractor was expecting to have to demolish the beams full-depth and re-form and re-do to shallower and wider dimensions. Given the beams are post-tensioned (14 unbonded tendons in 2 bundles/groups of 7 each), the tendons would have to be first de-tensioned before demolition could be accomplished.

De-tensioning is not difficult (albeit somewhat dangerous), however, what do you do with 14 cut tendons that have to be spliced to new tendons for the new re-configured beams? Splicing hardware for unbonded PT is typically 2" dia x 4+" long. With 7 tendons in a bundle, even if they splicing hardware was staggered, accommodating the spices was near impossible,

Removal and replacement of the existing strand tendons over the full 60' beam length would also be difficult. Although access to the live-ends was possible (even though the stressing tails had been cut off), access to the fixed-ends were inside RC columns so significant concrete removal would be required to the column face, and some anchorages may be 'hidden' behind longitudinal column rebar making access even more difficult.

We developed an alternative beam-reduction method. Maintaining the existing PT, we re-profiled the tendons whilst 'live' and fully-stressed, via concrete chipping and removal approx 8" of beam soffit concrete, then using vertically-orientated hydraulic jacks we re-profiled the live tendons by raising them by 4", creating a new 2-point harped tendon profile. We used XX-strong concrete-filled pipe sections at the harp points to act as deviators of the PT forces. 
Given the beam was to be now 4" shallower, the beam was widened by 8" (4" each side of existing web) and 6 new/added tendons were installed in the new 4" thick concrete 'beam web composite jacket' that was placed using self-consolidating concrete (SCC).

Here it is in photographs:

Controlled concrete removal of 8" depth x 16" width to center 1/3rd span:


Concrete removal at temporary 'concrete node' with temporary steel tube packing installed:


Temporary bolt-on brackets and hydraulic set-up for re-profiling live tendons:


Resulting beam after tendon re-profiling by 4":


Concrete-filled steel pipe deviator grouted into existing beam web:


Additional PT tendons were added to the 4" thickening each side, along with embedded rebar dowels and additional longitudinal rebar:


Added PT tendons were anchored using through-column drill holes and partially embedded bearing plates for stressed-ends:


Fixed-end anchorages were custom stiffened steel plates through-bolted and epoxy injected through the existing beam web:


When all completed it visually blended with the other beams:



This alternative method resulted in significantly less concrete demolition (8" depth vs 25" depth) and hence time savings.

There is significant risk associated with concrete chipping around live and exposed PT tendons, however, I am pleased to report that no damage to tendons occurred. Do NOT attempt such work unless experienced.

RE: Increasing headroom to existing PT beams to parking structure

Interesting project, and clever solution with excellent construction crew. Congratulation and thanks for sharing.

RE: Increasing headroom to existing PT beams to parking structure

Good to see it all worked so well in the end. What was the final decision on radius for the deviators?

RE: Increasing headroom to existing PT beams to parking structure

Steel's much easier...

Dik

RE: Increasing headroom to existing PT beams to parking structure

Far out man...

nice work.

RE: Increasing headroom to existing PT beams to parking structure

Dik,

Do you think the steel beam is even required, if the wall below was there before?

RE: Increasing headroom to existing PT beams to parking structure

Definitely not... masonry infill... I did add a strip footing at the bottom. Top flange was lintel... about 50 years back when I was a rookie.

Dik

RE: Increasing headroom to existing PT beams to parking structure

dik,

You know, I think it would be much fancier, if you can cut out a triangular/circular arch that doe not require lintel, and I would have given you 5 stars on that:) Actually I think it is doable (in my wild dream), what do you think?

RE: Increasing headroom to existing PT beams to parking structure

It's possible... but it was a 10 or 12 storey building at the lower level... I wanted to have the top flange as a tension tie... was for a 6' wide door opening going from the street to the basement.

Dik

RE: Increasing headroom to existing PT beams to parking structure

(OP)
retired13 - thank you.


Quote (rapt)

Good to see it all worked so well in the end. What was the final decision on radius for the deviators?

Thanks. We had anticipated push-back from the peer-review - which we did get. I researched the restraining devices used in precast pre-tensioning applications for harped single strand where less than 2" dia pins/rollers are used with larger angle changes and no reported issues since their first use dating back to the late 1950's - see image below. however, our case was a bit different as we had 7 strands in a bundle over 3 layers (3+3+1), not single strands.



I also checked on some of the other eternal PT projects I have done and researched other external PT projects done in the US, where small dia pipe sections were used as external deviators. For example: one of Bondy's projects from a few decades ago:



When we considered the embedded existing tendons and their 'existing curvature' the re-profiled tendons had an effective radius/arc considerably larger than the point-contact of the pipe diameter. We decided upon a 3-½" OD pipe (0.6" wall and grout filled) and monitored the tendons at the deviators as we hydraulically re-profiled the tendons and there were no issues - i.e. no sheathing splitting, no wires crushing/bulging, no strand flattening.


Quote (dik)

Steel's much easier...

What can I say - horses for courses. Steel was not even in the stable...


Tomfh - Thank you. I will add it was a very stressful project (pun intended) - lots of 'moving' parts where much could go wrong real quick. Fortunately it did not.

RE: Increasing headroom to existing PT beams to parking structure

Nice effort.

I'm curious what lead to the 4" issue in the first place?

RE: Increasing headroom to existing PT beams to parking structure

What sort of crew performs heavy remedial work like that?

RE: Increasing headroom to existing PT beams to parking structure

I know iron worker are pretty good on heavy, difficult works. Don't know this project though.

RE: Increasing headroom to existing PT beams to parking structure

(OP)

Quote (Agent666)

Nice effort.

I'm curious what lead to the 4" issue in the first place?

Thanks. With all the technology used today for design and construction - BIM/REVIT and robotic total stations - it is only as good as the input data. Evidently, as a design-build project by the general contractor, they looked at numerous existing parking structures in the area and many have the 'head-ache bar' sign stating 6'8" MIN, so that is what they used on the first level. It was later 'discovered' that the local building code requires 7'0". So whilst they can build to a precision of 1/8" they screwed up by 4" due to data input. Costly mistake.


Quote (Tomfh)

What sort of crew performs heavy remedial work like that?

We are a specialized structural sub-contractor with design-build capabilities. We used a 5-man screw to do the 6 beams. Crew were masons - which caused a issue with the laborers union because they said that concrete demo/chipping was a laborers scope. We argued that it was preparatory work for concrete construction and highly specialized, so part of masons. By the time the unions argued among themselves about mason vs laborer responsibilities we had finished the project.

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