How Drawing Review Caught a Tendon Conflict on a North Carolina School

A Spec That Allowed Coring Through a Post-Tensioned School Slab

The architect/engineer (A/E) and the owner's rep were checking the construction documents for a school in North Carolina. The set was the usual mix of architectural, structural, mechanical, electrical, and plumbing sheets, and both sides wanted the constructability problems out before the set went to the builder.

The plumbing spec allowed the 4-inch sanitary stack to be core drilled through the second-floor slab after the slab cured. The structural notes said the slab was post-tensioned and prohibited any coring or cutting within the tendon zones.

Separately, a sign for the administration office pointed left. Left ended at a dead end. It was one sign on one sheet, with nothing in the set that tied it to the floor plan.

Helonic Put the 4-Inch Stack on a Banded Tendon Run

The team uploaded the PDF set and Helonic read the plumbing riser diagram, the structural slab notes, and the post-tension layout together.

The finding that mattered most was a conflict over how the 4-inch sanitary stack gets through the second-floor slab. When the stack location was overlaid on the tendon layout, it landed directly on a banded tendon run, with no sleeve called out anywhere in the set.

This one is easy to miss because each sheet is correct on its own. The plumber follows the spec and the structural engineer follows the notes, and the conflict only shows up in the field after the pour.

At that point, coring risks striking a stressed tendon, which is a serious safety hazard and can compromise the slab. The fix then means stopping work, X-raying the slab, and re-routing the stack or redesigning the reinforcement. Found on paper, the fix is moving the stack a few inches or adding a cast-in sleeve to the pour package.

The team put the field version at about $60,000 and three weeks: stopping work on the floor, scanning the slab, engineering a re-route or a reinforcement change, re-piping the stack, and re-inspecting.

The sign was the second finding, and the one the team talked about most. Helonic read the administration sign, took the direction it pointed, and followed that direction on the floor plan. The sign pointed left. Left ended at a dead end, and following it did not lead to the office.

The finding was specific. It named the sign, the direction it pointed, and where that direction ended, and it pointed to the sheet, so a reviewer could confirm it at a glance.

It is a different kind of issue from a stack on a tendon. There is no code section and no structural calculation behind it. It is a detail where a sign and a floor plan disagree, and it is the kind of thing a reviewer working through a full set can read past, because each sheet looks fine alone.

The team told Helonic they had also tried other drawing review tools on this set, and that none of them found the sign.

See K-12 school construction drawing review, North Carolina plan review requirements, K-12 school construction compliance, conflicts between specs and drawings, spec analysis, and the New York office building review.

About $60,000 and 3 Weeks Kept Out of the Schedule

The owner's rep sent the conflict to the A/E as a coordination comment, with the stack location marked on the tendon layout. Both sides treated it as the high-value kind of catch: two sheets that are each right and together put a drill on a stressed tendon. The fix on paper was to move the stack a few inches or add a cast-in sleeve to the pour package.

That change cost a revised drawing. The same fix after the pour was estimated at about $60,000 and three weeks of delay to the work that follows the second-floor slab. On a school with a fixed opening date, the delay is the larger problem, and a struck tendon would have made it worse.

The team's reaction to the sign centered on a question: if the system finds a sign pointing the wrong way, what else is it finding? Of all the tools they tried, this was the finding that stood out. It showed how granular the checks are, and how detailed and descriptive the system is when it finds something.

The stack showed the system could catch a serious conflict between two sheets. The sign showed it was reading the small details as well, and that combination is what left the team impressed. The A/E corrected the sign direction on the plans.

A spec that allows coring and notes that forbid it is a drawing-to-spec conflict. Across 1,000+ drawing packages Helonic has reviewed, about 6 in 10 sets with specifications had at least one conflict between the drawings and the project manual. See what Helonic found across 1,000+ drawing reviews.

ACI 318 Post-Tensioned Slabs and the North Carolina State Building Code

North Carolina schools are permitted under the North Carolina State Building Code, which adopts the International Building Code with state amendments.

Post-tensioned slabs are designed under ACI 318, the concrete code the building code references, and built to Post-Tensioning Institute practice. The tendons carry stress for the life of the slab, which is why PT structural notes prohibit coring in tendon zones without scanning and the engineer's approval.

A specification that allows coring after cure is written for a conventionally reinforced slab. On a PT slab it conflicts with the structural notes, and the notes are the ones protecting the tendons.

Wayfinding signs are not code items the way exit signs are. The dead-end sign had no code section behind it, which is part of why it is easy to read past and why the team valued the catch.

“Both sheets were right. The spec said core it, the notes said never core a tendon zone, and the stack sat on a banded tendon. You only find that after the pour. And we tried other tools on this set. None of them found the sign pointing left into a dead end. If it can find that, what else can it find?”

North Carolina School Drawing Review: Questions and Answers