What a short-term load table does not answer

FRP grating load tables usually give a short-term ultimate load and the matching deflection, answering “will it fail now?”. But grating stays on a platform for years, and deflection under sustained load grows with time.

That long-term deformation is creep. A short-term table cannot replace a creep assessment, and creep data cannot be used as a short-term ultimate load.

What creep is

Creep is the increase in strain with time under a constant load. The fibres in FRP are close to elastic, while the resin matrix is viscoelastic; a sustained load makes the resin deform slowly and redistributes stress to the fibres and interface.

Higher temperature, moisture and load level all accelerate creep. For grating under long-term load, creep may govern the design rather than short-term strength.

What ISO 899-1:2017 measures

ISO 899-1:2017 is a test method for tensile creep of plastics. A constant tensile load is applied and strain is measured at a specified temperature and time, giving a creep curve and creep modulus.

The standard applies to rigid and semi-rigid plastics and can compare resin systems, fibre directions and processes. The report should state stress level, temperature, humidity, loading time and specimen direction.

Creep curve, creep modulus and isochronous curve

A creep curve plots strain against time. Creep modulus is stress divided by strain at a given time and falls as loading continues; an isochronous curve plots stress against strain at selected times.

Design asks whether deflection at the end of the service life is still within the limit at the target load. That requires the creep modulus in a long-term deflection calculation, not just the initial elastic modulus.

Where creep comes from in FRP grating

In molded grating, resin-rich areas, fibre junctions and the interface are more prone to viscoelastic deformation. In pultruded grating the continuous fibres carry most of the load, but the resin matrix and interlaminar regions still creep.

Temperature is a major variable. Creep rate rises sharply near the resin glass transition temperature. Outdoor grating at summer temperatures may deflect more over time than room-temperature laboratory data suggests.

Effects of temperature, moisture and load level

Creep tests usually run under standard laboratory conditions, while real service may combine heat, moisture and chemical media. Moisture plasticizes the resin, lowering modulus and accelerating creep.

A report with only room-temperature data therefore does not represent every environment. For hot, wet or highly corrosive projects, ask for data under those conditions or use a more conservative design margin.

How to leave a long-term margin

Choose span and section from the short-term load table, then check deflection with the long-term creep modulus; if creep data is limited, reduce allowable stress or shorten the span. The long-term deflection limit comes from the project specification.

The site load calculator is for short-term selection and does not include long-term creep; state this in the design calculation. See FRP grating load capacity, for load tables and notes.

Four items to confirm in a report

• Stress level: how the test stress relates to the design working stress.
• Time and temperature: duration, temperature and whether humidity was controlled.
• Specimen direction: along the load-bearing bar or transverse, and whether molded and pultruded were tested separately.
• Measure: creep modulus, strain at a stated time, or creep rupture time.

Only with these four can creep data be used for design comparison. See the FRP grating standards library, then use the grating load calculator, for span and load checks.