Frames fail in cycles, not in kilos

Chairs rated for 150 kg static will not necessarily survive two years of 70 kg guests. Metal and wood do not break under one heavy load; they break after enough small ones, and the number that decides a frame's life is not kilos but cycles.

Published by JPChairs · Chair manufacturer in Foshan, China · 2026-10-23

Static strength is not service life

Every frame we build passes a static load test, and almost none of the chairs that come back under warranty failed one.

• Static test: 150 kg held 60 seconds on the seat and 75 kg on the back, and a sound frame does not move.
• Fatigue reality: a 1.2 mm tube that holds 150 kg once can crack at 60 kg repeated, after about 80,000 sittings.
• The mechanism: a crack starts at a stress riser, grows 0.1 to 1 mm per 10,000 cycles, and breaks in the last 5 per cent of its life.
• Endurance limit: for mild steel it sits near half the tensile strength, about 180 to 200 MPa; keep working stress under 90 MPa and life is effectively unlimited.
• Aluminium: it has no endurance limit, so it fails eventually at any load, and we design aluminium frames to 10 million cycles instead.
• What to ask for: a cycle count, not a kilo rating.

What the two standards actually cycle

Contract chairs are sold against EN and BIFMA, and the two are not interchangeable.

• EN 1728 seat front edge, 120 kg: 200,000 cycles for EN 16139 Level 1 severe use, 100,000 for Level 2 general, 50,000 for Level 3 domestic.
• EN 1728 back cycling: 50,000 cycles at 40 kg for Level 1 and 25,000 for Level 2.
• BIFMA X5.1 general use: 100,000 seat cycles at 104 kg, plus a 100 kg drop from 152 mm.
• EN 12520 domestic: 100,000 seat cycles at 110 kg; treat it as the floor for a restaurant, not the target.
• Our own cadence: one frame per 500 produced cycled on the rig, which costs 0.4 to 0.6 per cent of the order.
• Paperwork: a report without the clause number and the cycle count is not a test report.

Turning cycles into years

This is the arithmetic we use when a customer asks whether a frame will last ten years.

• Home or hotel room: 2 sittings a day over 300 days is 6,000 cycles a year and 60,000 in a decade.
• Restaurant at two turns: 12,000 a year, 120,000 in a decade, and Level 2 is not enough.
• Restaurant at four turns: 24,000 a year, 240,000 in a decade, which is why we push Level 1.
• Bar stools: guests drop into the seat, so a stool sees three to five times the peak load of a dining chair at the same count.
• Stacking: every stack and unstack is a cycle; 20 a week is 10,000 in a decade.
• Read it backwards: 100,000 cycles is eight years at two turns a day and four years at four.

The four places a crack starts

Fatigue returns are predictable, and the same four details appear in almost all of them.

• Weld toe: 65 to 70 per cent of our metal fatigue claims; a sharp as-welded toe triples local stress against a ground and blended one.
• Bend: a centreline radius under twice the tube diameter thins the outside wall by 18 to 25 per cent, and that is where the crack sits.
• Seat-to-leg bracket: the highest bending moment on the frame, and the reason we use 2.0 mm rather than 1.5 mm there.
• Bolt holes: a hole removes about 25 per cent of the section and concentrates stress two to three times at its edge.
• Stack contact point: repeated impact on the same 5 mm of tube, which is why nylon bumpers matter more than they look.
• Wood equivalent: short grain at a 90 degree mortise, where a 6 mm tenon shoulder beats a 4 mm one.

Four changes that roughly double the life

None of these is expensive on its own, and together they are the difference between a four year frame and a ten year one.

• 1.5 mm wall over 1.2 mm: 56 per cent more section modulus for 18 per cent more steel and about USD 0.90 a chair.
• Bend radius 2.5 times outside diameter over 1.5 times: costs nothing and roughly halves the thinning at the bend.
• Ground and blended welds: two to three times the life at the joint for 15 minutes of grinding.
• Gusset or continuous weld at the leg joint: takes it from 80,000 to 200,000 cycles for under USD 0.50.
• Total: USD 2.20 to 3.40 a chair, against USD 22 to 35 to replace a frame in year four.
• Weight penalty: 0.4 to 0.7 kg, which is 16 to 28 kg on a 40HC of 40 restaurant chairs.

What to put in the enquiry

Six lines, and the frame cannot be quietly thinned between the sample and the bulk run.

• Standard and level: EN 16139 Level 1 or BIFMA X5.1, with the clause and the cycle count, not the word tested.
• Tube: 25 x 1.5 mm minimum for a dining frame, 30 x 2.0 mm for a bar stool.
• Bend radius: 2.5 times outside diameter, stated on the drawing.
• Weld finish: welds at the seat joint ground smooth and blended.
• Evidence: one frame per 500 cycled to 100,000, with the report attached to the shipment.
• Duty: tell the factory the turns per day, because that number sets the level, not the venue type.

Metal frames built to these figures are listed on the metal frame chairs, and the cycling rig they are tested on is shown on the factory testing equipment.