In short: Under constant load, FRP keeps deflecting slowly over time (creep). Under a high sustained load it can eventually break (creep rupture). Design standards allow for this. ASCE/SEI 74 uses a time-effect factor of 0.4 for permanent load alone, compared with 1.0 for short wind or seismic loads. Long-term deflection should also be checked, not just the deflection on day one.
What is creep?
Creep is a slow increase in strain under a constant load. In glass FRP the resin is the part that creeps. The fibers carry most of the load and creep very little. Creep increases with stress, temperature and humidity.
What is creep rupture?
If a sustained stress is a large fraction of short-term strength, the part can fail after hours, months or years. That is why permanent loads, such as tank contents, roof dead load and equipment weight, are checked more strictly than short loads such as wind gusts.
How do design standards handle it?
| Load combination | λ |
|---|---|
| Permanent load only (1.4D) | 0.4 |
| Combinations with live load (typical occupancy) | 0.8 |
| Combinations with wind or earthquake | 1.0 |
The design check is Ru ≤ λ φ Rn. Europe's CEN/TS 19101 and the Italian CNR-DT 205 guide use similar reductions for long-term effects. Tank and pipe standards cover long-term behavior through their own design factors and long-term tests (for example, the hydrostatic design basis for pipe under ASTM D2992).
How do you allow for long-term deflection?
- Use creep factors: check deflection under sustained load with the long-term modulus or the creep coefficient from the design standard or the manufacturer.
- Keep sustained stress low: by choosing deeper sections or shorter spans.
- Watch temperature: creep speeds up near the resin's temperature limit.
- Grating and walkways: load tables usually give short-term deflection. Allow extra for permanent equipment loads. See FRP grating span and load tables.
Where creep matters most in practice
Creep checklist by application
| Application | Long-term load | What to check |
|---|---|---|
| Platforms holding fixed equipment | Pump skids, tanks or panels resting on grating | Use the long-term (creep) deflection, not the short-term table value |
| Cable tray | Full cable load for the life of the tray | Long-term deflection between supports; temperature factor |
| Tank roofs and covers | Self-weight, snow and fixed equipment | Long-term sag and ponding of water |
| Pultruded beams and stringers | Permanent dead load plus storage loads | Design manual creep factors from the profile maker |
Temperature, moisture and creep
Creep rises with temperature. A grating that is fine in a cool building may sag more on a hot roof or near hot process lines. Moisture and some chemicals also reduce stiffness over time. When you send load data, include the service temperature and the environment. See FRP temperature limits.
Practical ways to control creep
- Shorten the span. Deflection grows quickly with span, so an extra support is often the cheapest fix.
- Choose a deeper section. Stiffness rises sharply with depth.
- Keep permanent loads low. Do not store heavy items on walkways designed for foot traffic.
- Use pultruded profiles for sustained loads. Higher glass content along the length gives better stiffness.
- Inspect. Measure sag on long-term loaded items at planned intervals; see FRP inspection and maintenance.
Ask the supplier which creep or long-term factors are used in its load tables. Many standard tables show short-term behavior only. If your load is permanent, ask for a long-term check, and if the supplier cannot give one, use a more conservative span.
For design rules, see FRP structural design basics and FRP design manuals and load tables.
When you send load data, split it into permanent loads (equipment, fixed storage) and short-term loads (people, maintenance). That lets the supplier apply long-term factors only where they belong.
How FIBROVX can help
Send us your loads, spans and temperatures, and we will help you pick profiles or grating with margin for long-term loads. Request a quote. CIF by default, FOB on request.
Common mistakes
- Short-term strength for dead load: apply the time-effect factor.
- Day-one deflection only: long-term deflection can be noticeably larger.
- Ignoring heat: creep rises with temperature.
- Heavy equipment on light grating: use a long-term check.
Key terms
- Creep rupture: failure after a period under sustained load.
- Time-effect factor (λ): a reduction factor for load duration in LRFD.
- Sustained load: a load that stays for a long time, such as dead load.
FAQ
Does FRP creep more than steel? At room temperature, steel creeps very little. FRP does creep, so it needs long-term checks.
What time-effect factor does ASCE/SEI 74 use for dead load? 0.4 for the permanent-load-only combination (1.4D).
Do grating load tables include creep? Usually not. They give short-term deflection, so allow extra for permanent loads.
Does creep matter for tanks? Yes. Tank and pipe standards address it through their design factors and long-term test bases.
Related guides
- FRP structural design basics
- FRP design manual and load tables
- EN 13706 explained
- FRP structural shapes
- Request a quote
Sources
- ASCE/SEI 74-23, LRFD for Pultruded FRP Structures: https://ascelibrary.org/doi/book/10.1061/9780784415771
- ACMA, Pre-Standard for LRFD of Pultruded FRP Structures (2010): https://www.acmanet.org
- CNR-DT 205/2007, Guide for Structures made of FRP Pultruded Elements: https://www.cnr.it/en/node/2641