Bracing Layout vs. Slab Capacity on a Post-Tensioned Slab
You are planning erection of 48 tilt-up panels on a project where the interior slab-on-grade is post-tensioned. The bracing plan shows two diagonal braces per panel anchored to the slab with 3/4 in. wedge anchors at 45 degrees, each brace designed for a 4,000 lb tension load. The slab is 6 in. thick, 4,000 psi, with unbonded tendons at 4 in. on center in one direction and 5 ft on center in the other. The tendon layout plan shows a tendon passing within 6 in. of 14 of the brace anchor locations. The slab was stressed only four days ago. The general contractor wants erection to start Monday to hold the schedule.
1. What is the structural concern with anchoring braces where tendons pass nearby, and how do you resolve it before erection?
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Drilling a wedge anchor into or immediately adjacent to a stressed unbonded tendon risks severing or nicking the strand, which can cause a sudden loss of prestress, a violent strand failure, and loss of slab capacity. Even a near-miss can crack the concrete around the tendon and reduce the anchor's pullout capacity, since the anchor's cone of influence overlaps the tendon duct.
The 4,000 lb design tension per brace must be resisted by the slab in the direction of the brace, and a tendon-parallel anchor row concentrates that load along a line the slab was not detailed for.
Resolution:
- Do not drill until the post-tensioning supplier marks the tendon locations on the slab with a scanning or as-built layout, and the structural engineer reviews the anchor locations.
- Relocate brace feet away from tendons (typically a minimum clear distance established by the PT supplier and engineer), or switch to cast-in anchors placed before the slab pour on future phases.
- Get written engineer approval for any anchor within the PT exclusion zone, and consider a through-bolt or a larger bearing plate to spread load.
- Document the resolution and brief the drill crew on the marked zones.
2. The GC pushes to start Monday. What is your response, and what does the four-day-old stressed slab add to the concern?
Read the worked answer
The schedule pressure does not change the technical facts. My response: erection cannot begin until (1) tendon locations are marked and verified, (2) the engineer has approved the anchor layout, and (3) the slab has reached the strength assumed in the brace design.
The four-day-old stressed slab matters because:
- Concrete strength gain continues after stressing. The brace anchor design assumes a slab strength; if the design used 28-day strength and the slab is only four days old, pullout capacity may be lower than assumed.
- Creep and elastic shortening from the post-tensioning are still occurring, which can move the slab relative to the braces and change brace loads.
- Grout and anchorage at the stressing ends may not be fully complete.
Actions: obtain cylinder breaks or maturity data confirming the slab has reached the design strength for anchor pullout, confirm stressing is complete and grouted, and get the engineer's written concurrence. Communicate the delay in writing to the GC with the technical basis so the schedule impact is documented and not attributed to you.