Coring a Post-Tensioned Band Beam
During rough-in on a 10-story cast-in-place office building, the mechanical subcontractor submits a field request to core three openings through a 16 in (400 mm) post-tensioned band beam: two 6 in (150 mm) holes for condensate drains at the quarter points and one 8 in (200 mm) hole for the fire-protection main at midspan. The beam is 24 in wide with unbonded tendons, and the structural general notes state that no cutting, notching, or coring is permitted without the engineer of record's written approval. A ground-penetrating radar scan shows the lowest tendon about 3.5 in above the soffit at midspan and about 7 in at the quarter points; the drafter picked the midspan location because it was clear of the web reinforcement shown on the framing plan. The mechanical superintendent has 40 people idle in the corridor and asks you to approve the two drain cores "since they're small" and to route the fire main below the beam instead. The beam depth governs the corridor ceiling at 9 ft 6 in.
1. Evaluate the coring request on structural grounds. Why is the midspan location the worst choice, and what happens if a tendon is cut?
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The drafter solved the wrong problem. The framing plan shows mild reinforcement; it does not show the tendon profile, which is what governs here. In a post-tensioned band beam the tendons are draped: high over the supports to resist negative moment, low at midspan to resist positive moment. The GPR result confirms it - 3.5 in of cover at midspan versus 7 in at the quarter points. The location chosen because it "looked clear" is the location where the tendon is closest to the drill.
Why a midspan soffit core is the worst case:
- It is the zone of maximum tendon density and maximum reliance on the tendon's eccentricity for the beam's flexural capacity.
- It is where the prestressing force is being delivered into the concrete over the shortest transfer length; interrupting that region damages the anchorage of the force itself, not just a hole in a section.
- A 24 in wide web gives very little room to miss tendons with an 8 in barrel.
If a tendon is cut: the stored force releases instantly. A single 0.6 in strand stressed to roughly 70 percent of its 270 ksi ultimate strength carries on the order of 30 kips; a tendon of several strands releases hundreds of thousands of pounds in milliseconds. The tendon can whip out of the beam end, the beam loses capacity and may deflect and crack, the fire main or workers in the corridor are exposed, and the repair requires the engineer of record and a specialty post-tensioning contractor to re-stress or add supplementary reinforcement. There is also a personal-injury exposure that no schedule recovery justifies.
Conclusion: reject the midspan core outright, and treat the quarter-point cores as unapproved pending the engineer's review - smaller holes are not automatically safe holes.
2. State the process that must be completed before any core is drilled, and evaluate the fallback of routing the fire main below the beam.
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Required process, in order:
- Written request to the engineer of record through the architect (an RFI, not a field request to the GC), attaching the GPR scan, proposed sizes and locations, and a section cut through the beam at each location.
- The engineer's evaluation of the effect on flexural capacity, shear, and the tendon profile, and a written approval with a location drawing signed and sealed by the EOR. Nothing proceeds on a verbal yes.
- A second scan at the exact approved spot, plus a pothole or soft-dig with a non-destructive method to verify no tendon or conduit lies within the core barrel plus clearance.
- Drill with a rig anchored against kickback, with the post-tensioning specialty contractor on site and a documented procedure for what to do if a tendon is contacted.
- Record the as-built location of every penetration on the record drawings.
The below-beam route for the 8 in main: this is very likely the right answer, but it is not a free pass. Check it against:
- Corridor ceiling height: 9 ft 6 in clear, less an 8 in main, insulation, hanger length, and hanger rod drop. It may fit, but the ceiling plenum is usually shared with ductwork and sprinkler branch lines, so it needs a coordination drawing, not a field decision.
- Slope and drainage for condensate - the drains may not tolerate the same reroute as the fire main.
- Sprinkler head locations and coverage once the main moves.
Handling the pressure: separate the two issues. Release the below-beam route today so the 40 idle people can proceed, and keep the core request in the EOR's queue. Giving the superintendent a workable answer today is what buys the time to do the structural question properly.
3. A week later, the EOR approves one 6 in drain core at a quarter point but not the fire main. Write the instruction you would issue to the mechanical subcontractor.
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Instruction content:
- One 6 in core approved at the location shown on the EOR's sketch, with dimensions referenced from the column line and the soffit, not from a field measurement.
- The 8 in fire main core is denied. Route the main below the beam per the coordinated section to be issued by [date].
- The second 6 in drain core is not approved at the submitted location; resubmit with a revised location for the EOR's review.
Conditions placed on the approved core:
- GPR re-scan and soft-dig at the approved point immediately before drilling, witnessed by the superintendent.
- No drilling until the scan is reviewed and the point is marked.
- Rig anchored to prevent kickback; post-tensioning specialty contractor on standby during the operation.
- Stop immediately and notify the superintendent if the bit contacts steel at an unexpected depth or if water or grout appears.
- Report the as-drilled location for the record drawings and photograph the penetration before the sleeve is installed.
Why write it this way: a partial approval is the most dangerous outcome, because a crew hears "approved" and finishes the set. The instruction must state explicitly what is approved, what is denied, and that the approved core does not authorize any other penetration in that beam or in any similar beam elsewhere on the floor. Copy the architect and the EOR so the record shows a controlled approval, and note in the daily report that the mechanical crew was given a released alternative for the fire main so the denial does not read as a general stop.