The number behind every deflection table

When selecting grating, engineers look at the load first and the deflection second. The key material property in that deflection calculation is flexural modulus. North American projects usually require flexural data to ASTM D790; understanding its span ratios and correction rules is how you decide whether the reported modulus can go straight into a design.

What the standard is

• Current edition: ASTM D790-25, published 1 December 2025, superseding D790-17;
• Title: Standard Test Methods for Flexural Properties of Unreinforced and Reinforced Plastics and Electrical Insulating Materials;
• Method: simply supported beam under three-point loading, for rigid and semi-rigid materials;
• Four-point bending is covered separately by ASTM D6272; the method parallels ISO 178 but differs in technical detail.

Procedure A versus Procedure B

D790 defines the test condition by the ratio of support span to specimen depth: Procedure A is normally 16:1 and Procedure B is 32:1. A shorter span makes interlaminar shear more likely; a longer span lowers the bending stress and increases deflection. Strengths from the two procedures are not directly comparable, and the report must state which was used.

The familiar specimen is about 127 mm × 12.7 mm × 3.2 mm, but the standard allows adjustment for material thickness. When sampling a grating profile, the specimen thickness is often the product wall thickness, which makes it even more important to report the actual dimensions.

What it measures and how to read it

• Flexural strength — maximum outer-fibre stress before failure; if the specimen neither breaks nor yields within 5.0% outer-fibre strain, the standard says flexural strength cannot be reported;
• Flexural modulus — derived from the initial linear portion of the load–deflection curve;
• Deflection measurement — crosshead displacement or a deflectometer may be used, and the report must say which, because the results differ;
• Large-deflection correction — required at larger deflections, and the report should state whether it was applied.

What it means for FRP grating

Grating load tables are usually published for uniform load at a specified deflection limit (for example L/200 or 6 mm). A modulus measured by D790 Procedure A cannot simply be applied to data measured by Procedure B or GB/T 1449. Span ratio, specimen orientation and fibre content all change the modulus, and there is no universal conversion factor between standards.

For pultruded grating, longitudinal modulus drives the span calculation, while transverse modulus matters for load distribution and connection checks; testing only one direction does not cover the real load path.

Common pitfalls

• No procedure stated. “Tested to D790” does not identify the span ratio, so the result cannot be verified.
• Using strength as modulus. Deflection tables use modulus, not strength.
• Ignoring large-deflection correction. With thin sections or long spans, an uncorrected modulus reads high.
• Comparing directly with GB/T 1449. Both are three-point methods, but specimen and span ratio differ, so values are not interchangeable.

Edition history

D790-25 was published on 1 December 2025 and supersedes D790-17. Purchasing and testing contracts should cite “D790-25” explicitly so the laboratory does not use an older edition or choose the procedure on its own.

Where to find it

The official entry is in the Niteck FRP grating standards library. To match a span and deflection limit to a product, see load capacity; for product parameters and tolerances, see specifications, for checking against project conditions.

This article explains the method and does not replace the standard text; the current edition governs all test conditions and criteria.