Anchor Short-Edge Distance After Layout Shift
A contractor is installing M16 mechanical wedge anchors through a cast-in-place slab-on-grade to anchor a guardrail post base plate. The approved submittal specifies a minimum edge distance of 100 mm and a minimum spacing of 130 mm. During a site walk, the inspector finds that a conduit sleeve was moved by the electrical trade, forcing the post 40 mm toward a free slab edge. The as-built edge distance is now 62 mm to the anchor centerline. The slab edge is a control joint that was saw-cut after curing, and the anchor nearest the edge is 75 mm deep in 150 mm concrete. The contractor proposes to keep the anchors and just add a plate washer. The design engineer has not yet been contacted.
1. What should the inspector do with the proposed plate-washer fix, and what is the controlling technical concern?
Read the worked answer
A plate washer does not restore edge distance. The controlling concern is the concrete breakout cone at the free edge: with 62 mm to the centerline, the breakout surface is truncated by the edge, and the anchor's capacity against a tension/shear load toward the edge is governed by the reduced edge distance, not by the washer.
Steps:
- Reject the field fix as a capacity substitute. A washer may help bearing/plate bending but does not change edge distance or the breakout geometry.
- Flag the anchor as non-conforming to the approved 100 mm edge distance.
- Require the contractor to submit the as-built geometry (62 mm edge, 75 mm embedment, 150 mm slab thickness) to the design engineer for evaluation against the anchor manufacturer's published edge-distance and reduced-spacing tables.
- Do not accept until the engineer either approves a documented reduced-capacity design or directs relocation/reinforcement.
The inspector documents and stops; the engineer resolves capacity.
2. The control joint runs 20 mm from the anchor. How does this change the evaluation?
Read the worked answer
A saw-cut control joint near an anchor is a free edge for breakout purposes, and it may also create a crack plane through the anchor's load zone. The inspector should:
- Treat the joint as an additional edge/reduced-confinement condition and report its location relative to the anchor.
- Note whether the anchor sits on the near side (thin concrete between anchor and joint) - a 20 mm offset means very little concrete on that side.
- Require the engineer to consider both the slab free edge and the joint in the breakout assessment.
Do not evaluate capacity yourself; document geometry precisely and escalate. The inspector's job is to prevent concealment of a non-conforming condition.