This is the sixth column derived from my book Composites for Construction —— Structural Design with FRP Materials (see endnote). In this final installment, I will discuss the design basis for connecting pultruded FRP profiles with mechanical fasteners. The fasteners themselves can be metallic or FRP. Adhesively bonded joints are not discussed — while adhesives are important in all types of composite parts, their use in the construction of pultruded structures is limited, and they are seldom relied on to transfer all design loads between components in truss and frame structures.

What I discuss here is an allowable stress design (ASD) method; at the present time, a load and resistance factor design (LRFD) procedure canÂ’t be presented with confidence. The state of the practice of pultruded connection design isnÂ’’t as well developed as that for pultruded members themselves, and safety factors are larger — a factor of 4.0 is used for the ASD procedure.

Joining Light and Heavy Structures

The connection of individual pultruded parts is relatively easy and is generally familiar to construction workers who are skilled in steel and wood framing. “Light” truss structures or stick-frame systems typically consist of pinned truss connections with the line of action of the axial force in the truss members meeting at a point. Load transfer in the connection is by in-plane forces parallel to the member axes through either single or double overlapping members, known as lap joints. The connections are made with mechanical fasteners that join tubular or channel sections in single or double shear planar configurations, passing through all members in the connection region. Inserts can be used to improve the local bearing strength of individual, highly loaded members.

In contrast, a heavy braced frame system resembles typical steel frame beam-to-column connections, using shear connections, and it is assumed that no moment is transferred between the members. Lateral forces are resisted with diagonal bracing members (X, knee or K bracing). These members carry bending moments, shear forces and axial forces and, therefore, the connections are subjected to multiaxial stresses. Unlike light truss or stick-frame connections, the heavy frame systems feature additional parts, such as single-leg angles, channels, gusset plates, fillers, etc., to help transfer the greater forces. Connections of this type mimic Type 2 steel connections, as they are known in the American Institute of Steel Construction (AISC) ASD, or “simple connections” in AISC LRFD.

In addition to the conventional light truss or heavy frame connections described above, many pultruders produce custom connections for their parts. These, however, can be used only as part of the overall structural system and are not “designed” per se by the structural engineer. An example might be the snap-together fasteners for a composite transmission tower originally envisioned by Brandt Goldsworthy and Clem Hiel.

The fasteners used with pultruded components are generally galvanized or stainless steel. For corrosive environments or where electromagnetic transparency is an issue, manufacturers also offer composite nuts, bolts and threaded rods, generally compression molded. They are typically more expensive and larger in diameter than metal fasteners, requiring larger bolt hole clearances. While composite manufacturers typically provide guidance on minimum hole spacing and clearances for bolts used to join pultruded profiles and also publish tables for allowable bearing loads for different diameter fasteners, no specific design procedures are provided for these connections in manufacturersÂ’ guides. Under no circumstances should steel nuts be used with FRP bolts or vice versa.

Making Effective Connections

Many researchers contend that steel-like connections are not appropriate for pultrusions at all, citing unique failures that never occur in steel, like delaminations and failure at web-flange junctions. While there is some ongoing research into the behavior of “semirigid” pultruded beam-to-column connections, at present, such designs can’t be implemented with confidence. For now, only simple framing should be used, with lateral resistance developed using bracing members or supplementary lateral load-resisting systems, with the assumption that the connection will not allow any transfer of moment.

The following are issues to consider when designing bolted connections in composites:

• The bolt holes cause stress concentrations and reduce the net section in the pultruded material, reducing the efficiency of the connection.

• The pultruded parts that will be joined are made of orthotropic materials, and, therefore, the orientation of the individual connectors is critical, unlike steel-bolted connections, where the base material is isotropic.

• As noted above, pultruded materials have low through-the-thickness stiffness and strength and can crush under high bolt torques as well as creep over time. If FRP bolts are used, they, too, will lose tension due to strain relaxation, and if small nuts are used without washers, the nut could crush or pull through the material when the connection is loaded. For this reason, only bearing connections can be used; slip-critical or friction connections are not possible.

• Designers should be aware that the ranges of FRP fastener sizes and pultruded angle thicknesses are somewhat limited, so the types of connection that can be made are, likewise, limited.

• Unlike with steel, holes cannot be punched in pultruded material. They must be drilled, preferably with a diamond-tipped bit.

To determine the stresses in in-plane single- and multibolt lap joints and shear connections, elementary one-dimensional, mechanics-based equations are used, based on assumptions of linear elastic material behavior. But these equations are used only as a guide to the strength of the connection because at the ultimate load, the connection behavior is not linear and large deformations can significantly alter the stress distributions, changing the failure modes.

The average bearing stress at the hole in the base pultruded material given as

where P_b is the load transferred at an individual bolt location, d_b is the bolt diameter, and t_pl is the thickness of the base pultruded material.

The net-tension design stress at the hole location is given as where P_t is the

tensile load transferred by the entire lap joint, consisting of a number of columns and bolts, and the net area A_net is taken as

where n is the number of bolts in a row, W is the width of the plate at the critical section, d_h is the hole diameter and t_pl is the thickness of the base material. This assumes that the critical section will be perpendicular to the longitudinal direction of the material through the row of holes. Consult my book for additional equations for shear-out stress in the base material, shear stress on a bolt and stresses in out-of-plane shear connections. Critical connection limit states also are discussed. Note that the longitudinal or the transverse bearing strength of the pultruded material is considered the critical strength at failure, and it is based on testing, generally reported by the pultrusion manufacturers. If bearing strength is not available from data, it can be approximated as the material compressive strength to permit approximate calculations. Whether the longitudinal or the transverse bearing strength is used depends on the direction of the member relative to the load, as shown in the figure at the top of this page (where V is the shear stress). For example, consider the design of a simple shear web double clip-angle connection for a beam-to-column connection for a floor system. The beam is a wide-flange W 10 x 10×1/2 profile and the column is a W 8 x 8×3/8 profile, designed using FRP bolts and nuts. First, the design loads and ASD factors are determined for a simply supported beam and the maximum shear force at the end of the beam. A safety factor of 4 is used for all strengths in the connection parts. The allowable bearing stress in the web is taken as 4,000 psi. With two bolts, the maximum bearing stress in the angle legs, including the effects of shear force eccentricity, is 3,590 psi, while the bearing stress on the top bolts of the angles (assuming only the horizontal component due to the eccentricity) is 2,976 psi. After determining the critical stress and factored critical stresses and checking the ultimate strength of the trial connection, the connection design with two bolts is acceptable but with a recommendation that the web be strengthened by bonding 0.125-inch or 0.25-inch thick plates of pultruded material, with their longitudinal directions perpendicular to the web.

The key elements of the design are the bearing stresses in the web and the local bending and shear stresses in the angles. And the conservative safety factor of 4 reflects the uncertainty in the calculations and material properties when complex multiaxial stress states exist in pultruded parts. Full-scale tests of actual connections are recommended for nonstandard situations and to ensure efficient connection design.

Excerpted with permission from John Wiley & Sons Inc., New York, N.Y. Composites for Construction — Structural Design with FRP Materials is available for $135 (USD) at www.wiley.com, www.bn.com, www.amazon.com or www.borders.com.