The rack is only as grounded as its bolts.
Direct answer: Anchors transfer uplift and shear into the slab; floor joints interrupt that path. Racking base plates should not straddle a joint, and where a joint cannot be avoided the base needs a spreader or a relocated anchor pattern agreed with the floor designer.
Why joints matter
Concrete slabs expand and contract through control joints; placing an upright leg directly over a joint concentrates stress on a moving line. Where the layout allows, shift footplates clear of joints or follow the supplier's guidance for joint-mounted bases.
Epoxy anchors and mechanical anchors behave differently in cracked versus uncracked concrete. Confirm the slab's condition before choosing hardware.
Anchor type by slab condition
Choosing hardware is a function of concrete condition, not of habit. This is the decision table we walk through with installers before a baseplate drawing is released.
| Slab condition | Suitable anchor type | What to verify before drilling |
|---|---|---|
| Uncracked, ≥150 mm reinforced slab | Mechanical wedge anchor | Edge distance and minimum spacing per anchor manufacturer |
| Cracked or uncertain slab | Chemical (epoxy) anchor rated for cracked concrete | Curing temperature, hole cleaning method, cure time |
| Slab over void or suspended deck | Through-bolt or engineered fixing | Structural engineer's approval and deck capacity |
| Heavily worn or spalled surface | Relocation of the leg, then standard anchor | Whether an alternate footplate position is available |
| Floor with under-slab heating or services | Shallow chemical anchor or cast-in fixings | Scanning results before any drilling |
What a plan detail should show
Every upright footplate on the drawing should carry four pieces of information: anchor type, anchor diameter and embedment, the number of anchors per plate, and the required edge distance. Where a plate lands near a control joint, the drawing should state the joint treatment rather than leave it to the installer.
Inspection follows the same logic. A rack safety inspection records whether anchors are present, whether any are loose or proud of the plate, and whether the slab around the plate shows cracking — three checks that catch most of the failures we see in the field.
FAQ
Do all uprights need two bolts per footplate?
Follow the rack manufacturer's drawings. Most front-loaded configurations require two anchors per baseplate to resist tipping during loading.
Can I reuse anchor holes after a relocation?
Re-drilling near old holes weakens the slab edge. Plan new hole positions with spacing rules in mind and fill abandoned holes properly.
Who is responsible for verifying slab capacity?
The building owner usually provides slab data; the racking supplier confirms whether the configuration fits it. Never assume a standard slab suits every bay.
All figures above refer to standard product data sheets; final confirmation is subject to written quotation.