Precast Connections And Site Tolerances

Precast concrete is manufactured under controlled conditions to tight tolerances. It is then installed into a structure built on site, where the supporting elements have their own, looser tolerances.

The connection is where those two worlds meet, and a connection detailed without an honest allowance for that difference is a connection that gets modified on site.

The tolerance stack

The problem is cumulative, and each contributor is reasonable on its own.

Manufacturing tolerance on the panel dimensions and the position of cast-in items.

Tolerance on the supporting structure, meaning the position and level of the slab, footing or corbel the panel sits on. This is the largest contributor and it is set by [tolerances in concrete construction](/blog/tolerances-in-concrete-construction).

Setting out tolerance on site.

Erection tolerance, meaning how accurately the panel can be positioned and held.

Movement, because the structure deflects and shortens over time, which is the [shrinkage and creep](/blog/shrinkage-and-creep-in-concrete-structures) question again.

Added without thought, those can exceed the gap the detail provided. The design has to accommodate the realistic sum, not the best case.

What a good connection allows

Adjustment in three directions, so the panel can be positioned rather than forced. Slotted plates, shims, oversized holes with washers, and grout gaps are all ways of buying adjustment.

A grout or packing zone that can vary within a stated range, with a minimum and a maximum both specified. A detail that says "grout gap 20mm" without a tolerance is a detail that will be wrong.

Shim access, so the level can be set before the load is transferred.

A defined load path once set, because an adjustable connection has to become a determinate one. Bearing on shims is temporary; the permanent condition is usually bearing on grout, and the transition needs to be deliberate.

Fire and durability protection of the connection components, which is frequently the item forgotten once the geometry is solved.

Accommodation of movement where it is needed, meaning connections that are deliberately free in one direction so the panel is not restrained against a movement the structure will make anyway. Restraining a panel against the frame's shortening cracks one of them.

The failure patterns

Cast-in item in the wrong place, so the bolt does not line up. Resolved on site by enlarging the hole, which may remove the edge distance the anchor needed.

Edge distance lost, because the connection ended up closer to the panel edge than the detail assumed. Concrete anchors depend heavily on edge distance and a small shift can be a large capacity reduction.

A gap larger than the detail allowed, packed with whatever was available rather than the specified grout.

Welding to a cast-in plate without checking the plate anchorage, which is a common site fix and changes the load path.

Shims left in where the design assumed full grout bearing, leaving a point load.

Grout not placed properly, so the bearing area is a fraction of what was drawn. Voids in a bearing grout are invisible after the fact and they concentrate the load.

A panel forced into position, which locks in stress that nobody calculated.

The programme consequence

Every one of those takes time while the panel is still on temporary bracing, which is exactly the condition that [temporary bracing of precast panels](/blog/temporary-bracing-of-precast-panels) exists to manage. The longer the connection takes, the longer the structure is in its most vulnerable state.

Which is the practical argument for resolving tolerance at the drawing stage: it is not about neatness, it is about how long the panels stand on props.

Getting it right before fabrication

Survey the supporting structure before the panels are cast, not after. If the slab edge is out, the panels can be cast to suit. Once they are cast, the structure has to be modified instead.

Shop drawings checked against the structural drawings, including the position of every cast-in item, in the same way as [steel connection design and shop drawings](/blog/steel-connection-design-and-shop-drawings).

A stated tolerance on the drawings, for the panel and for the supporting structure, so both parties know what they are working to.

Trial assembly or a first-panel hold point, so a systematic error is found on panel one rather than panel forty.

One party owning the interface, because a tolerance problem between a precaster and a concretor with no single responsible party becomes a variation claim rather than a solution. That is the same boundary question as [who is responsible when a design changes on site](/blog/who-is-responsible-when-a-design-changes-on-site).

ACSES provides structural engineering and shop drawing review across Sydney. Talk to us about a project.