8.7—TESTING C8.7 Full-scale structural testing shall be used to verify the

strength and deflection of FRP members.

The bending test criteria for FRP poles is summarized in Article 8.7.1. Other tests that may be required are given in Article 8.7.2.

Because FRP poles are usually round tubular tapered members whose performance is dependent upon the composition of the material and the manufacturing procedure, testing is required to determine the bending and torsional strength, as well as the weathering resistance of FRP poles.

Cracking and early failure can occur at handholes during bending of poles, and early pole attachment failures can occur at cast shoe bases (i.e., pole-to-tube junction). These items can also be checked through testing.

8.7.1—Bending Strength of FRP Poles C8.7.1 The bending strength of FRP poles shall be determined

in accordance to ASTM D 4923 procedure, except for the following requirements:

• The pole shall be capable of sustaining an equivalent point load Pe applied 0.3 m (1 ft) from the top with a maximum deflection at the top no greater than 15 percent of the pole height above ground. The equivalent point load shall be computed as:

(

)

e

P M

= h

− (N) (8-1)

(

)

e

P M

= h

− (lb)

• The pole shall be capable of sustaining a maximum point load of 2.0Pe applied 0.3 m (1 ft) from the top before failure.

• For poles with mast arms, the slope at the top of the pole resulting from the dead load moment of the arm and luminaire shall not exceed 30 mm/m (0.35 in./ft).

A safety factor of 2.0 against failure in bending is specified for the test. The safety factor is greater than the 1.5 value specified by the ASTM standard in order to account for the variability in mechanical properties of FRP.

8.7.2—Other Tests

The following additional tests shall be performed if required by the Owner:

a. Torsional strength per ASTM D 4923, b. Fatigue strength per ASTM D 4923, c. Weathering resistance per ASTM G 53, d. Adhesion of coatings,

e. Color change from UV exposure, and f. Fatigue strength of connections.

SECTION 8: FIBER-REINFORCED COMPOSITES DESIGN 8-7

8.8—ALLOWABLE STRESSES C8.8

Allowable stresses may be computed by provisions of Articles 8.8.2 through 8.8.7 if the design calculations provided are verified by documented test results on similar structures.

Allowable stress equations presented in Article 8.8 are intended for normal loading conditions and are obtained from various sources (Johnson, 1985; British Standards Institute, 1994). If the applied load is long term or cyclic, or if elevated temperatures and exposure to aggressive environments are expected, reductions in the allowable stress should be considered.

Allowable stress equations may also be obtained from the Manufacturer provided that adequate supporting documentation including test data is made available.

8.8.1—Determination of Mechanical Properties of FRP C8.8.1 The strength of FRP laminates shall be determined by

testing using flat sheet samples in accordance with the ASTM standards listed in Table 8-2. Samples shall be manufactured in the same manner as that proposed for the structural member.

For structural members where the fiber orientation changes along the member, sheet samples shall be taken at locations of critical stresses.

Because the mechanical properties of the FRP material could vary significantly depending on the particular composition and the manufacturing process, the test samples must be representative of the actual conditions in the final product.

The proposed safety factors shown in Table 8-2 are minimum values based on common industry practice. Other values may be used when agreed on by the Owner and the Manufacturer.

Table 8-2—Standard Tests for Determining the Mechanical Properties of FRP

Property Standard Test

Minimum Safety

Poisson’s ratio in the longitudinal direction, υ12 ASTM D 3039 —

8.8.2—Allowable Bending Stress for Tubular Sections C8.8.2 The allowable bending stress for tubular sections may be

calculated as follows:

• For round tubular sections:

1

For thin-walled FRP sections, local buckling is a major parameter that controls the strength of the member in bending. The allowable bending stress is defined as a function of the critical buckling stress of the section.

Equations to obtain the critical buckling stress are based on the plate theory for orthotropic elements, and they are expressed in terms of the aspect ratio b/t of the plate or the aspect ratio D/t of the cylinder. For polygonal sections, the critical buckling stress is determined for a long plate with simply supported long edges. Because there is some edge restraint at the intersection between sides of the polygon, the assumption of simply supported long edges leads to conservative values for the critical buckling stress.

• For polygonal sections (hexdecagonal, dodecagonal, octagonal, and square tubular sections):

1

According to Johnson (1985), it has been shown that the critical compressive stress caused by bending is 30 percent higher than the critical compressive stress caused by axial compressive loads for round tubular sections. Therefore, the critical buckling stress for a round tubular member under bending Eq. 8-2 is taken as 1.3 times the critical buckling stress for a round tubular member under axial compression Eq. 8-8.

Eqs. 8-2 and 8-3 may be used for planar isotropic materials by setting E1 = E2 in the equations for K1 and μ.

8.8.3—Allowable Bending Stress for W and I Sections C8.8.3 The allowable bending stress for W and I sections may

be calculated as follows:

• For laterally supported W and I shapes:

2

Allowable bending stresses for W and I shapes are provided for isotropic materials. For thin-walled FRP sections, local buckling is the parameter controlling the strength of the member in bending; therefore, the critical bending stress is defined as the critical buckling stress of the section. The equations to obtain the critical buckling stress are based on the plate theory for isotropic elements, and they are expressed in terms of the aspect ratio (b/t) of the plate.

Barbero and Raftoyiannis (1993) has proposed a general equation for the allowable bending stress of pultruded W and I beams based on the equations developed for the allowable bending stresses of wood members. A main assumption in those equations is that pultruded W and I shapes are members with unidirectional fibers similar to wood. Because only a very limited number of tests have been performed to validate those equations, they have not been included in these Specifications.

SECTION 8: FIBER-REINFORCED COMPOSITES DESIGN 8-9

8.8.4—Allowable Compression Stress—Flexural Buckling

C8.8.4

The allowable compression stress considering flexural buckling may be calculated as follows:

• For:

Compression design of FRP members is generally controlled by buckling. Flexural buckling and local buckling should be considered. Equations for the allowable compression stress for flexural buckling are taken from the Structural Plastics Design Manual, and they are applicable to isotropic and orthotropic materials.

8.8.5—Allowable Compression Stress—Local Buckling C8.8.5 The allowable compressive stress considering local

buckling may be calculated as follows:

• For round tubular sections:

1 1

• For polygonal sections (hexdecagonal, dodecagonal, octagonal, and square tubular sections):

1 1

where K1 and μ are determined according to Article 8.8.2.

Compression design of FRP hollow tubes is usually controlled by local buckling of the wall, except for unusual combinations of very long members with large axial loads.

Equations provided for the allowable compressive stress for short-column action are developed for orthotropic materials, but they may be used for planar isotropic materials by setting E1 = E2 in the equations for K1 and μ.

In practice, the critical buckling stress of round tubular sections is about 50 percent of the theoretical critical buckling stress as a result of geometrical and material imperfections. To account for this difference between the theoretical and the actual buckling stress, the theoretical buckling stress in Eq. 8-8 has been multiplied by a factor of 0.50.

8.8.6—Allowable Tension Stress C8.8.6 The allowable tension stress on the net cross-sectional

area may be calculated as follows:

tu t

F F

= n (8-10)

The net cross-sectional area shall be calculated as the remaining area after discounting holes or other discontinuities in the member.

Bolt holes and handholes produce abrupt reductions in the cross-sectional area of the member that generate stress concentrations. The Designer should account for the stress concentrations that may reduce the capacity of the member under tension. Information for computing the reduction in capacity due to stress concentrations at discontinuities in tension members is given in the Structural Plastics Design Manual.

8.8.7—Allowable Shear Stress C8.8.7

The allowable shear stress for tubular members under transverse loads or torsion may be calculated as follows:

(

)

where μ is determined according to Article 8.8.2.

The allowable shear stress equation is provided for orthotropic materials, but may be used for planar isotropic materials by setting E1 = E2 in the equation for μ. Equations to compute maximum shear stresses due to transverse loads and torsion are provided in Appendix B “Design Aids,” to determine the computed shear stress, fv.