Mechanical Properties and Load Capacity

Which strength are we talking about?

"Strength" is not a single number; it is discussed under different headings depending on how the load sits on the profile. When choosing a section you look at each of them separately.

• Tensile strength: resistance to breaking when pulled along the profile. Because most of the fiber in pultrusion runs longitudinally, this is the direction in which GRP is strongest.
• Flexural strength: the load capacity when it works as a beam. Here section geometry matters as much as the material: capacity grows fast as height and wall thickness increase.
• Compressive strength: resistance to crushing under axial load, as in a column or post. In thin-walled sections buckling can arrive before crushing.
• Shear: critical at supports and connection points; it usually causes trouble at the ends of a profile rather than in the middle.
• Modulus of elasticity (E): sets how much the material will flex. Even when strength is adequate, a low modulus means more deflection. In GRP design this is usually the governing value.

Direction dependence: GRP is not the same in every direction

Steel behaves much the same whichever way you load it. GRP depends on fiber direction: very strong along the length, weaker across it. That is not a defect but design data — the profile is positioned so the strong fiber direction lines up with the load direction.

The practical consequence is that cutting, drilling and connection details cannot be done at random. Edge distance for a bolt hole, the use of washers and the tightening torque matter as much as the section itself.

Strength-to-weight ratio

This is the strongest card GRP holds. Its density is well below that of steel, while its tensile strength along the length is in a range comparable with structural steel. In other words, the same job can be done with a far lighter member.

• Freight and stock costs drop.
• Installation is often done without a crane, with a smaller crew.
• The load added to an existing structure drops, which is decisive in refurbishment work.
• With corrosion maintenance gone, lifetime cost drops.

What to look at when designing for load

In GRP the critical question is usually not "will it break" but "how far will it sag". Because the modulus is relatively low, the design is usually governed by the deflection limit rather than by strength.

• Span (L): deflection grows very fast with span. Adding one more support to shorten the span is often both more effective and more economical than increasing the section.
• Deflection limit: the allowable deflection is defined as a fraction of the span. That fraction changes with the intended use and the specification — a platform people walk on and an out-of-sight member are not held to the same limit.
• Support conditions: simply supported, fixed or continuous. The same section behaves noticeably differently when the support condition changes.
• Load type: distributed or point, static or moving, is there impact? Point and impact loads may call for local reinforcement.
• Section orientation: is the profile working about its strong axis or its weak one? In bending, section height is what decides.
• Local effects: local crushing at supports and load points, lateral buckling in thin-walled sections — these can become the limit before overall strength does.
• Connections: the area around a bolt hole is sensitive to crushing; edge distance, washer diameter and tightening torque are part of the design.
• Temperature: as temperature rises the stiffness of the resin drops, so load capacity falls in hot environments. Resin selection comes in here — resin systems.
• Long-term load: under sustained load deflection can increase somewhat over time; where permanent load dominates, this is allowed for from the start.
• Safety margin: set according to the intended use and the specification; kept wider on critical structures.