PEB Anchor Bolts and Foundations Set the Programme

Fast steel, slow concrete

The usual case for a pre-engineered building is speed. The frame is designed and fabricated in a factory while the site is prepared, then bolted together rather than cast. That is true, and it is why owners and CFOs are often surprised when a PEB project slips at the very point the steel arrives.

The reason is simple. Steel can only be erected on foundations whose anchor bolts match the frame. The bolts are cast into concrete weeks earlier, usually by a different contractor, from a drawing issued by the PEB supplier. If that drawing changed after the pour, or the bolts moved during it, the base plates will not land. The crane and erection crew wait, the steel sits in the yard, and the fix is engineered rework in concrete that has already cured.

The common misconception is that foundations are "the civil part" and steel is "the PEB part", meeting on site as two separate scopes. In reality the anchor bolt is where the two scopes become one, and the programme is decided there.

Who owns each part of the interface

Most of the delay at this interface comes from responsibilities that nobody wrote down. A typical split looks like this, though every contract should state its own.

The point for a buyer is not which column each item sits in. It is that every row has a named owner before the first pour. A PEB buyer's guide is a good place to see where this sits in the wider procurement.

Freeze the anchor bolt plan before casting

The anchor bolt plan follows the frame design. When the frame changes, the bolts can change with it. Common late changes that move bolts or reactions:

• Bay spacing or building length adjusted to suit the site
• A crane or monorail added or upgraded
• A mezzanine introduced or relocated
• Framed openings for doors, docks or louvres added to end walls and side walls
• An extension planned, needing a different end frame

Each of these is a reasonable decision, and each is expensive once bolts are cast. The discipline is straightforward: bolts are cast only to an anchor bolt plan issued for construction, under revision control, with the civil contractor confirming in writing which revision they are setting to. A drawing marked preliminary or for approval is for the foundation design, not for the pour.

The wider choice between steel and concrete frames, and how each handles changes during design, is covered in our PEB versus RCC industrial building comparison.

Templates, levels and grout pockets

The physical work of setting bolts is not complicated, but it is unforgiving.

Templates. A rigid template, drilled to the anchor bolt plan, holds each bolt group in its correct pattern while concrete is placed. It must be fixed so it cannot move or rotate, and set out from the grid lines, not from the previous foundation.

Projection and threads. Each bolt must project far enough above the concrete for levelling nuts or packing, the base plate, washers and the final nut, as the drawing requires. Threads are protected during the pour; concrete on a thread is removed before it hardens, not with a grinder later.

Levels and grout gap. The top of the pedestal is usually cast slightly below the underside of the base plate, leaving a gap that is grouted after the frame is aligned. The level and gap come from the PEB supplier's drawings.

Tolerance across the building. Accuracy matters within each bolt group and across the whole grid, because small errors repeated along a long building stop the last frames from fitting. Permissible deviations for erected steelwork are set out in IS 12843, and the general idea of tolerances is explained under construction tolerances. The tolerances that apply to a project are those stated on its drawings and specification.

How the frame then goes up is covered on our PEB erection and site works page.

What a mis-set bolt actually costs

Once concrete has cured, every remedy is engineering work:

• Enlarging or slotting holes in the base plate
• Fabricating new or adapted base plates
• Installing post-fixed anchors in place of cast-in bolts
• Cutting bolts and re-embedding new ones
• Breaking out and recasting the pedestal

Anchor bolts carry tension and shear from the frame, including uplift from wind under IS 875 (Part 3), so none of these is a site decision. Each needs the structural engineer's design and approval under IS 800:2007 and IS 456:2000, which takes time. Meanwhile erection of that frame waits, and because frames go up in sequence, so may the frames after it.

The commercial cost often exceeds the physical one: an idle crane, a crew that cannot be redeployed, steel stored and double-handled, and an argument over who pays. That argument is short when the interface was written down and long when it was not.

Plan the foundations backwards from steel delivery

The programme that works starts from the date the erector needs the first foundations, then works backwards. Before that date the foundations must be cast, cured to the engineer's requirement, surveyed and accepted. Before they can be cast, the anchor bolt plan must be issued for construction. Before that, the frame design must be stable enough that the bolts will not move.

Laid out this way, two things become visible. First, the design freeze for the frame is a foundation milestone, not only a steel milestone, so late changes to bays, cranes or mezzanines have a cost that the programme shows immediately. Second, foundations can be cast and accepted in the same order the frames will be erected, so erection can start at one end while the civil works finish at the other. That overlap is where much of the PEB time advantage comes from, and it only exists if the first foundations are right.

Common mistakes

• Casting to a preliminary anchor bolt plan while the frame design is still changing.
• Leaving bolt supply and setting undefined between the civil and PEB contracts.
• Setting out each foundation from the last one instead of from the grid, so errors accumulate.
• Templates not fixed firmly, so bolts move when concrete is vibrated.
• No survey after the pour, leaving errors to be found by the erector.
• Assuming foundations are ready by date rather than by test results.
• Grouting before alignment, locking a frame in the wrong position.

What to ask before the first foundation is cast

• Is the anchor bolt plan issued for construction, and which revision is the civil contractor using?
• Are any frame changes still possible, and what would they move?
• Who supplies the bolts and templates, and who sets and surveys them?
• What tolerances apply, and where are they written?
• What concrete strength must be reached before erection, and how will it be confirmed?
• Who accepts the foundations on behalf of the erector, and on what record?
• Who designs and approves any remedy if a bolt is out?

Standards referenced

Steel design and construction under IS 800:2007; concrete under IS 456:2000; wind loads under IS 875 (Part 3); foundation bolts under IS 5624; and tolerances for erection of steel structures under IS 12843. Bolt sizes, grades, embedment, edge distances, tolerances, curing requirements and any remedy for a mis-set bolt must be established by the project's structural engineer and the PEB supplier's design.