How Spacing and Anchorage Decide What a Support Carries

Why this matters

A support point has two completely independent limits and only one of them is on your side of the ceiling. Spacing decides how much of the run each point is asked to carry. The base material decides how much any anchor there is able to give back. Whichever number is smaller is the one that governs, and the failure you get when you ignore that is not a bent clamp, it is a hanger rod hanging out of a hole with a cone of concrete still attached to it.

The reason this catches good installers is that everything visible belongs to the span side. Spacing, rod, clamp, strut, the hardware you touch. The anchorage side is invisible after the first ten seconds of the drill and is set by things you did not choose: what the slab actually is, whether it is cracked, how deep the hole really went, how close the edge is.

This is a worksheet you fill in for one run, and it has two halves that have to be compared before either is accepted.

Before you drill anything overhead

Drilling concrete or masonry releases respirable crystalline silica, which is an inhalation hazard and needs an inhalation control, not a glove. Use a drill with an integrated dust-collection shroud and a filtered vacuum meeting the specified requirements, under 29 CFR 1926.1153 for construction work (its Table 1 covers handheld drills with dust collection and states the respiratory requirement that goes with them) or 29 CFR 1910.1053 for general industry, with any respirator worn under a program meeting 29 CFR 1910.134. Add eye protection and a hard hat, because overhead drilling puts spoil and occasionally a fragment directly above your face.

Scan before you drill. Cutting a reinforcing bar, a post-tensioning tendon or an embedded conduit is not a nuisance, it is a structural or electrical event: a severed tendon releases a large amount of stored energy along the length of the slab, so stop at first contact with unknown steel and get the drawings or the engineer of record rather than pushing through. If the run being supported already exists and is loaded, rig temporary support under it before you disturb the point you are working on.

The span side of the worksheet

Fill these in, in this order, because each one needs the one above it.

• Filled and finished weight per foot. Pipe or conduit, plus contents, plus insulation, plus jacketing. Take it from the pipe manufacturer's tables and the insulation data sheet, not from memory.
• The spacing the adopted code or the specification allows for that material, that size and that service, in the edition your jurisdiction adopted. This is a maximum, not a target.
• Tributary length per point, which for a uniform run on even spacing is one bay, half on each side.
• Concentrated loads and where they land. A valve, an actuator, a strainer, a fitting assembly, a piece of in-line equipment. These do not distribute; they split between the two adjacent points in inverse proportion to their distance from each.
• Load cases other than normal service. A line that runs empty in service and gets filled with water for a pressure test is carrying a different load on test day than any other day, and for thin-wall pipe in the small sizes the water alone can rival the pipe's own weight.

The anchorage side of the worksheet

• What the base material actually is, confirmed rather than assumed: normal-weight concrete, lightweight concrete, hollow-core, a composite deck with flutes, grout-filled block, hollow block, wood, or steel.
• Which anchor is listed for it, with its evaluation report and the manufacturer's printed installation instructions, both of which govern.
• Cracked or uncracked concrete. Concrete in the tension zone of a slab is assumed cracked for anchor design, and published capacities in cracked concrete are materially lower than in uncracked, commonly on the order of a third to a half lower depending on the anchor. That factor lives in the evaluation report and nowhere else.
• Achieved embedment, edge distance and spacing to the nearest other anchor. All three reduce capacity, and the first one is the one that changes silently in the field.
• The safety factor, because a published capacity is not an allowable load. A service-level allowable comes from dividing the characteristic capacity by the factor stated in the evaluation report, commonly four or greater.

Anchoring to concrete is governed by ACI 318 Chapter 17, which binds you through the building code your authority having jurisdiction adopted rather than on its own, and adhesive anchors in sustained tension overhead carry additional requirements there including installer qualification.

The worksheet, filled in

A horizontal run under an existing concrete deck. Take the filled and insulated weight as 5.0 pounds per foot, which is the kind of figure a 2-inch steel line with water and insulation produces, and confirm the real number from the tables rather than carrying mine.

Span side. Spacing from the adopted code table for this material and size comes out at 10 feet. Uniform tributary load per point is 5.0 pounds per foot times 10 feet, which is 50 pounds.

There is a valve assembly weighing 60 pounds sitting 2 feet from hanger 4, in the 10-foot bay between hangers 4 and 5. It splits by distance: hanger 4 takes 60 times 8 over 10, which is 48 pounds, and hanger 5 takes 60 times 2 over 10, which is 12 pounds.

So hanger 4 is asked for 50 plus 48, which is 98 pounds. Every other hanger on the run is asked for 50, and hanger 5 is asked for 62. One point on this run carries nearly twice what its neighbours carry, and nothing about the installation looks different there.

Anchorage side. The anchor selected is a mechanical expansion anchor listed for cracked concrete. Working through the evaluation report for this deck, at the planned embedment, in cracked concrete, away from edges, with the report's safety factor applied, the allowable comes out at 120 pounds per anchor. Against 98 pounds, one anchor per point clears with room, and the installer stops thinking about it.

What actually happened at hanger 4. Drilling that hole, the tech hit steel at about two thirds of the planned depth, backed off, and set the anchor at the depth the hole allowed rather than re-locating.

Where concrete breakout governs a single anchor away from edges, the breakout model in ACI 318 Chapter 17 scales strength with effective embedment to the 1.5 power. Two thirds of the embedment raised to the 1.5 power is about 0.54, so the allowable at that point is roughly 120 times 0.54, which is about 65 pounds. That scaling is a breakout relationship: where the anchor is governed by steel strength or by pullout instead of breakout, it does not apply, so the report's own tables for the shallower embedment are the authority and this arithmetic is the estimate you use to decide whether to go look them up.

Hanger 4 is asked for 98 pounds and its anchor can give about 65. The governing side flipped, at one point on the run, because of a decision made in about four seconds with a drill in hand.

Fixing the wrong side, then the right one

The instinct is to reduce spacing, so work it through and watch it fail.

Add a hanger directly under the valve. That new point picks up the valve's full 60 pounds plus its own tributary, which is half of the 2-foot side and half of the 8-foot side, so 1 plus 4, which is 5 feet, at 5.0 pounds per foot, so 25 pounds. The new point carries 85 pounds. Hanger 4 now has half of its 10-foot left bay plus half of the 2-foot right bay, so 5 plus 1, which is 6 feet, or 30 pounds.

Hanger 4 is fixed, comfortably. And the new point at 85 pounds still exceeds the 65 pounds a short-embedment anchor in that deck can give. Reducing spacing did what reducing spacing does, and it was never going to be enough, because the deficiency was never on the span side.

The real fix is on the anchorage side and there are three legitimate versions of it: re-drill at a clear location and get full embedment, use two anchors at the point with the spacing reduction the report requires applied to both, or change to an anchor type whose capacity at the achievable embedment covers the load. Which one you pick is a function of what the deck will let you do, and all three are decisions the evaluation report makes with you rather than for you.

The illegitimate version, and it is common, is to install the short anchor and add a second hanger nearby "to share the load". Two points do not share unless the run is stiff enough to redistribute, and a run that is stiff enough to redistribute is also stiff enough to keep loading the short anchor first.

What a torque wrench on an anchor does and does not prove

Torque-controlled expansion anchors have an installation torque in their instructions, and hitting it is required. It is not a capacity test. The torque sets the expansion; it tells you the anchor engaged the hole. It cannot tell you the hole was deep enough, that the concrete around it is sound, that the edge distance is adequate, or that the anchor is even the right one for the base material. An anchor set into a metal deck flute at correct torque reads exactly like an anchor set into 8 inches of sound concrete at correct torque.

If a point genuinely needs proof, that is a proof load applied by the method the specification or the engineer names, at the load and duration they name, and it is a different activity from setting the anchor.

The check that costs nothing and catches the most: record the achieved embedment at each point as you drill, not the planned one. That single column is what would have caught hanger 4 on the day, at the only moment it was still cheap to move the hole.

References

• ACI 318 Chapter 17, anchoring to concrete, which binds through the building code your authority having jurisdiction adopted in the edition it adopted, including the additional requirements for adhesive anchors in sustained tension.
• The anchor's evaluation report and the manufacturer's printed installation instructions, which own the published capacities, the cracked-concrete factors, the embedment, edge-distance and spacing reductions, the safety factor and the installation torque.
• 29 CFR 1926.1153 (construction) and 29 CFR 1910.1053 (general industry) for respirable crystalline silica controls when drilling, with respirator use under 29 CFR 1910.134.
• The mechanical and plumbing codes adopted by your authority having jurisdiction, which own maximum support spacing by material and size.
• See related: What a Support Has to Do Besides Hold Weight; How an Anchor Transfers Load Into Concrete; Why an Anchor Fails and Which Failure You Get.