Glulam Beam Deflection Checker

SERVICE load, not factored — deflection is a serviceability check and the load factors belong to the strength check instead. Which combination depends on which limit you are testing: dead plus live against the total-deflection limit, live alone against the live-load limit, and the second of those is usually what a floor is judged on because a permanent sag nobody walks past is less objectionable than a floor that moves. This field is NEWTONS PER METRE and does not follow the unit toggle — there is no dimension for a force per length, so it stays exactly as typed while the span and E beside it switch systems. A US line load is quoted in lbf per foot (plf), and BOTH halves of that unit convert: 1 lbf = 4.4482216152605 N and 1 international foot = 0.3048 m, so 1 lbf per foot = 14.5939 N/m. Multiply a plf figure by 14.5939 before entering it — the factor is not the 3.28084 the length toggle uses on the span, because the force converts as well as the length. Typing the plf number raw is neither refused nor flagged: this field spans 100 to 50,000 N/m, which is 6.85 to 3,426 plf, so any realistic line load lands inside the range and is simply accepted, and because deflection is linear in load the answer comes back at 6.85% of the truth. On the defaults here that turns 800 plf — 11,675 N/m, 35.8 mm, clearly past the 16.67 mm L/360 limit — into 2.45 mm, comfortably inside it.

Deflection goes as the FOURTH power of span, which makes this the most sensitive number on the page by a wide margin: getting the span 5% wrong moves the answer by 22%. Measure the clear distance the beam actually has to cross, and where the bearing is generous take it to the centres of bearing rather than to the face — a 200 mm bearing at each end is 200 mm of span nobody is entitled to ignore.

Use the deflection modulus for the layup, not the minimum modulus quoted alongside it — that one exists for stability calculations and using it here overstates deflection substantially. Glulam is also laid up with its stiffest laminations at the extreme fibres, so the published E belongs to the beam as manufactured and to a specific orientation: a member installed on its side is a different beam.

For a rectangular glulam this is b·d³/12, so depth dominates: a beam 10% deeper is a third stiffer, while one 10% wider is only a tenth. Manufacturers publish it per section, and the field wants ×10⁶ mm⁴ — a 130 × 450 section is 130 × 450³ / 12 = 987 × 10⁶ mm⁴, so it goes in as 987 rather than as the raw figure.

L/360 under live load and L/240 under total load are the ordinary floor pair, with roofs commonly at L/240 and brittle finishes tighter again. One thing this elastic check does not include: timber CREEPS under sustained load, and the codes handle it by multiplying the long-term part of the deflection — typically by 1.5 for dry service and 2.0 for green — before comparing against the total-load limit. A beam that passes the elastic check by a small margin may not pass once the dead load has been sitting on it for a year.