Testing standard
Standard Test Methods of Static Tests of Lumber in Structural Sizes
Written and technically reviewed by Dak System Inc. engineeringLast reviewed
ASTM D198 covers static testing of lumber and wood-based products at their actual structural size — sawn lumber, glued laminated timber, composite lumber and prefabricated I-joists. It carries several procedures: flexure, compression, tension and torsion.
A member at its actual structural size is measured for section and moisture content, its significant knots and grain deviations noted against the span, and then loaded under whichever procedure applies. In flexure it is supported and loaded through bearing plates wide enough that the wood is not crushed at the contacts, with deflection measured on the member itself, at a rate chosen so failure falls in the time window the method specifies. Modulus of rupture and modulus of elasticity are calculated on the measured section, and the failure location is recorded against the mapped defects.
How a real beam behaves, as distinct from how its species behaves. That distinction is the reason the method exists. A small clear specimen tested to ASTM D143 has no knots, no grain deviation and no finger joints, so it characterises the wood; a structural member has all of them and fails at whichever is worst. It follows that a larger member tests weaker than a smaller one of the same grade — more highly stressed volume means more opportunity for a serious defect — which is a statistical effect and the reason small-clear results cannot simply be scaled up into design values.
A structural member fails where its worst defect is. Testing a small defect-free sample measures the species, not the beam.
Wood is not a uniform material and this method does not pretend it is. The scatter between nominally identical members is real, which is why structural timber is specified statistically rather than by a single value.
MOR = M c / I, evaluated at the maximum moment
On the measured section, not the nominal one. A 50 × 150 is not 50 by 150.
From the slope of the load-deflection curve over the elastic range
Requires deflection measured on the member. Crosshead travel includes support settlement and local crushing under the bearings.
Strength falls as member size rises, for the same species and grade
A bigger member contains more opportunities for a serious defect. This is why full-size testing exists and why small-clear results cannot simply be scaled.
Four-point bending gives a constant moment between the inner load points, which is what lets a knot or a grain deviation anywhere in that zone be the thing that fails rather than whatever sits under a single nose.
Specifications
Three-point where the method or the product standard calls for it.
SpecificationsConsiderable force and considerable span. A structural joist or glulam beam commonly needs tens to hundreds of kilonewtons and a test bed metres long, with bearing plates sized so the supports react against the member rather than indenting it — wood being weak perpendicular to the grain, a narrow support simply presses into it. Deflection has to be measured on the member: support settlement and local crushing under the bearings are a large share of crosshead travel over a long span.
Taking modulus of elasticity from the crosshead, which folds support settlement and bearing indentation into the timber's stiffness. Calculating stresses on nominal rather than measured dimensions. Omitting moisture content, without which two sets of results cannot be compared. Bearing plates too narrow, so the member fails locally at a load that says nothing about its capacity. And recording a failure load without noting the knot or grain feature it started from, which is the single most useful observation the test offers and costs nothing to make. Averaging short and long compression specimens together belongs here too: one crushes and the other buckles, so the mean describes neither regime and hides which one the member was actually in.
| ASTM D198 | ASTM D143 | |
|---|---|---|
| Specimen | The member at structural size | Small, clear, defect-free |
| Answers | How this beam behaves | How this species behaves |
| Defects | Included, and usually decisive | Deliberately excluded |
| Used for | Grading, design values, product qualification | Species properties and research |
Both are needed and neither substitutes for the other. Small clear values characterise a species; full-size values are what a design is built on, because a real beam fails at its worst knot.
It is the ASTM family of static test methods for lumber and wood-based products at their actual structural size — sawn lumber, glued laminated timber, composite lumber and prefabricated I-joists. It covers flexure, compression, tension and torsion procedures. The current designation is ASTM D198-22.
Because a structural member fails where its worst defect is, and a small clear specimen has none by definition. ASTM D143 measures the species; this measures the beam. The two are both useful and neither converts into the other — knots, grain deviation and finger joints are precisely what governs a real member, and they only exist at full size.
Because it contains more opportunities for a serious defect. A longer, deeper beam presents more highly stressed volume in which a knot of consequence might sit, so the chance that one does rises with size. It is a statistical effect rather than a material one, and it is why full-size results cannot be extrapolated from small-clear data by simple scaling.
Because a large timber span settles into its supports and crushes locally under the bearing plates, and both of those movements appear in crosshead travel. On a member metres long the sum is a substantial share of the total, so a modulus of elasticity taken from the crosshead reports the test set-up as much as the timber.
Because wood is weak perpendicular to the grain. A support or a loading nose that is too narrow indents the member instead of reacting against it, so the member fails locally in bearing at a load that says nothing about its bending capacity. Sizing the plates so the wood is not crushed is what keeps the failure where it belongs.
Because timber strength and stiffness move substantially with it, and a member tested green and one tested at service moisture are not comparable. Moisture content is recorded per member rather than assumed for the batch, since it varies across a stack and across a section.
No, and the method separates them. A short specimen crushes, which is a material strength. A long one buckles, which is a stability problem governed by slenderness and end conditions. Averaging the two describes neither, and the failure that matters in a structure depends entirely on which regime the member is in.
Dak verifies against whichever standard the method names, and where a class applies our frames sit a class tighter than it asks.
| The method asks for | Dak supplies | |
|---|---|---|
| Capacity | High — a structural-size joist or glulam beam commonly needs tens to hundreds of kilonewtons | Load cells from 1 kg to 60 ton on the Series 7200, and 0.5 to 100 kN on the Series 9000 |
| Force accuracy | ASTM E4 over the working range | Verified to ASTM E4, and to ISO 7500-1 Class 0.5 |
| Strain measurement | An extensometer of the class the method specifies | Certified to ISO 9513 Class 1 and ASTM E83 — non-contact video, clip-on and high-elongation |
| Gripping | Long-span bend fixtures with bearing plates, or compression platens and tension grips for the other procedures | Our bend fixtures, built to the specimen |
| Environment | 23 ± 2 °C standard laboratory atmosphere | 3009 series chambers, −150 °C to +400 °C — temperature only |
This page describes the method as practised. The governing text is the current edition from the issuing body. Tell us what you are testing and we will answer with the machine, the fixture and a quotation.
