Calculation basis and reference data

Weight starts on the containment and ends in the structure. Every check asks the same question at a different point on that path: can this part carry what is passing through it?

There are six checks on a plain bracket, plus two more that appear only when they apply. They are all the same shape: a demand computed from the load, against a capacity read from a catalogue or a standard, reported as a percentage. Nothing passes on a missing capacity — a value the tool does not hold reads NO DATA, never a silent pass.

The bearer's strength and stiffness, straight off the manufacturer's load table. One row per span, three ambient columns and — on the fire-rated sections — three more.

The Fmax column is a strength value: the total uniformly distributed load the section carries on a simply supported span at an allowable bending stress of σ = 175 N/mm². The L/200 and L/360 columns are deflection values — the load at which the span sags to that fraction of its length. Where the catalogue prints a dash, deflection does not govern at that span, or the section is off the end of its published deflection table.

Two hot-rolled channels are offered beside the Unistrut sections, each usable either way up. Their tables are the only ones in this document that nobody published — they are calculated, so the calculation is set out here in full.

Unistrut print load/span tables because the channel is their product. A 76 × 38 tapered flange channel or a 100 × 50 parallel flange channel is a structural section: no manufacturer publishes a bracket load table for one, and none will. You calculate it. That is not a weaker footing than a transcription — it is the same identity section 7 shows holding to 0.02 % down Unistrut's own printed column, run on a different Z.

The three formulae

σ = 165 N/mm² is 0.6 × the 275 N/mm² yield of S275. With the tool's default load factor of 1.3 that is 2.17 overall against yield, which sits alongside the 2.08 implied by Unistrut's own σ = 175. It is a choice, not a published value, and it is the one figure on this page most worth agreeing before you rely on a result.

Fire needs no test. BS 8519:2020 Annex E is normative, and it says the stresses in Table E.1 apply to unprotected drop rods and bearers made of mild steel. A plain steel bearer is a bearer. So the fire columns are the same formula at 9 / 9 / 6 N/mm² — the standard applied directly, rather than read across from a furnace test of a different arrangement the way section 8 has to do for the FR strut.

Which way up

Do not specify one for fire on its ambient strength

The 76 × 38 on edge is more than six times the fire-rated strut at ambient. At two hours that advantage is all but gone.

Three things a published strut table already answers

None of these can be calculated here — the tool does not know where you will drill, how the containment is fixed down, or where the load lands across the flange. So it says each of them on the sheet instead, and warns on the one case it can see.

• Holes. EN 1993-1-1 §6.2.5: a hole in the compression zone needs no allowance where a fastener fills it; one in the tension flange or the web tension zone may only be ignored where the net section still develops the gross yield. On a cantilever the top flange is the tension flange, so a rod hole over the support is not free there. And unlike Unistrut, whose published capacity has its slots already in it, a derived capacity is the gross section — drilling it is a deduction they never had to make.
• Lateral-torsional buckling, web vertical only. The top flange is in compression, and a compression flange free to move sideways buckles out of plane well before the section reaches σZ. The tables above assume it is restrained — containment bolted down at roughly 400 mm centres or closer, which is the normal arrangement. Where a bearer plainly has no such restraint (nothing on it, or its load hanging below on rods) the tool raises a warning; the cases in between are the engineer's. Laid flat there is no lateral-torsional buckling at all — bending is already about the weak axis, so there is no weaker axis to buckle into. That is the trade for a fifth of the capacity.
• Torsion. A channel's shear centre lies outside its web, so a vertical load applied in the plane of the web also twists it. Unistrut strut is symmetric about its loading plane and has no such term — and every beam check in this tool is a symmetric-beam model. Load through the shear centre, restrain the section against rotation, or check the torsion by hand. It matters most web-vertical with the load out on one flange, and least laid flat bearing on the web.

A drop rod is checked on the steel that is actually there — the root of the thread, not the bar it was cut from.

The last link, and the one most often assumed. Capacities are per fixing at the characteristic (unfactored) load — a published anchor rating already contains its factor of safety, so applying a load factor on top would count it twice.

Containment weight

Each run contributes its published weight per metre — the containment plus its cables — multiplied by the bracket spacing.

Load factors

Each check is a demand and a capacity. What follows is where each side of that comes from, and the arithmetic between them.

6.1 · A point load is not a spread load

The catalogue capacity is a uniformly distributed load. Real containment lands on the bracket through one fixing, so it is a point load — and a point load bends the channel harder. Each one is converted to the equivalent UDL that would produce the same bending moment:

6.2 · Between the rods, and past them

A channel is not one member. Between two rods it sags; where it runs on past the outer rod it hogs about that rod. Those are different actions with different arithmetic, so they are separate checks — and with three rods there are two bays, each checked at its own length.

6.3 · Where the moment capacity comes from

A cantilever needs a moment capacity, and the catalogue publishes loads. But an Fmax is the total UDL on a simply supported span, so the moment it implies is Fmax·L/8 — and that product is constant all the way down the column, because it is the section's own allowable moment:

6.4 · Strength and deflection are different questions

6.5 · How the load divides between the rods

Where the load sits decides what each rod carries. For two rods this is BS 8519's own method — Annex I, Formula I.1 — a moment balance about the first rod:

Annex I stops at two rods, and tells you to even out an uneven load or add a factor for it. Three or four rods are statically indeterminate, so the tool solves them with the three-moment (Clapeyron) equation instead — which returns exactly Annex I's answer when there are two.

6.6 · The rod itself

Straight out of BS 8519 Formula E.1. Where a bracket lifts a rod rather than pulling it — which a one-sided cantilever can do — the rod is reported as a positive magnitude with the word uplift and checked as a strut, never as a negative weight.

6.7 · Load conservation

Not an engineering check but a guard on the arithmetic: the sum of every element's own weight must equal the sum of what arrives at the structure. If they disagree, a load has been lost or double-counted somewhere in the roll-up, and the sheet says so rather than reporting a tidy answer.

BS 8519 exists because there is no point in a two-hour cable hanging from a bracket that fails in twenty minutes. Every element is checked twice: at ambient with the load factor, and again at its rating with the whole load still hanging.

The fire factor

Compare the fire columns in section 2 and 3 with the ambient strength column and the relationship is exact, on every span and both sections:

What was tested, and what was not

What carries a VERIFY mark, and why

A VERIFY value is one that must be confirmed against current manufacturer literature before a calculation is issued. Three kinds appear on this page:

• Indicative catalogue data — correct when transcribed, but editions change. Channel tables, anchor ratings, containment weights.
• The tool's own assumptions — values no document supplies: the ambient rod stress, the strut factors for an uplifted rod, the fraction of a channel nut's capacity assumed usable in fire.
• Read-across — a published figure applied to something it was not measured on. The cantilever in fire is the clearest case, and the sheet says so.

Every calculation counts how many VERIFY values it actually used and prints the number. A calculation with none is not possible; a calculation where you have checked each one is defensible.

This calculator is a design aid for cable-containment support engineering to BS 8519:2020, not a substitute for a structural engineer's assessment. The exported spreadsheet — with its working visible — is the record of calculation. Loading assumptions, fixing capacities and site conditions remain the responsibility of the design engineer.