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X-Brace Axial Forces

X-Braces are effective framing components for H-Frames. Brace axial forces can be either tensile or compressive, and are a function of the specific structure configuration and the total imposed wind and gravity loads. Factors which affect X-Brace axial forces include pole spacing, structure height, pole species and class, the quantity and position of the braces, stiffness of the upper H-Frame truss, and class of soil.

Strength & Stiffness Considerations

Several limiting factors must be balanced when selecting the most economical X-Brace design for a given set of structure and loading parameters. Limits for these factors have been determined for a wide variety of connection designs and wood sizes, using data developed by testing of components and full scale structures. Final brace selection is controlled by the most restrictive of these factors.

Axial Tension

End fittings must distribute the axial load into the wood member, controlling wood bearing stresses and fitting deformation.

Bearing on the Pole

Bearing stresses parallel or perpendicular to the grain in the wood pole are species sensitive, and must be limited under the fittings, washers, and bolts.

Compressive Buckling

The accepted industry method for determining the ultimate theoretical compressive strength of a wood X-Brace, either solid or laminated, is an adjusted Euler buckling equation. It is dependent on the slenderness ratio and material modulus of elasticity, and is affected by end fitting relative fixity and load eccentricity.

Vibration

Extremely slender compression members are subject to wind induced vibration. This can cause fatigue, and should be avoided.

The curved portions of the nomograph shown in X-Brace Member Axial Strength Limitations indicate relative X-Brace member compressive buckling strength for various wood sizes and pole spacing. Parameters selected for the curve plot include braces at 45 degrees, a conservative modulus of elasticity, a consistent adjustment factor for the Euler buckling equation, and a slenderness ratio maximum of 50. The curve position will vary with these parameters according to application.

The braces are also limited by the combination of axial Tension in the connection and bearing on the pole. The various X-Brace Series connection designs have been classed as Types A through G as shown below. The member load limit for each connection type is indicated by a horizontal line on the nomograph shown in X-Brace Member Axial Strength Limitations.

Type A Series 6680, 6685, 6696 7/8" Bolt 1/4" Fitting	Type A1 Series 6050, 6651 7/8" Bolt 3/8" Fitting	Type B Series 6695 1" Bolt 1/4" Fitting
Type C Series 41005 Double 7/8" Bolt 1/4" Fitting	Type D Series 675 Wrap Around	Type E Series 670, 671 Pinned
Type F Series 677 Heavy Pinned	Type G Series 678 Extra Heavy Pinned