John P. Judd and Dr. Fernando S. Fonseca, Civil and Environmental Engineering

OBJECTIVES

This paper summarizes the results of a research project to determine the response of Fiber Reinforced Polymer (FRP) panels fastened to wood framing, under two different loading schemes: monotonic and repeated cyclic loading. The results of the monotonic testing phase of the project were published and presented at the International Conference on Composites Engineering [2].

INTRODUCTION

Resistance to lateral forces, such as wind or earthquake loads, is provided in wood structures by horizontal and vertical diaphragms within the floor, roof, or walls. Diaphragms typically consist of wood 2’x4′ members fastened to plywood panels. Lateral forces are transferred from the diaphragm boundary, through plywood and wood, to supporting side walls and foundations. Although plywood is commonly used diaphragms, it may not be the optimum choice; plywood will limit the stiffness of the diaphragm, deteriorate and decay when subject to adverse moisture conditions, and increase the fire hazard. FRP panels may be a feasible alternative to plywood sheathing. The FRP panels may increase the diaphragm strength, while not susceptible to some problems inherent with plywood panels, such as deterioration and combustibility.

EXPERIMENTAL PROGRAM

The monotonic behavior of FRP panels fastened to wood framing was simulated using 76 small scale specimens tested to failure; each specimen consisted of 2″x4″x9″ Douglas Fir-Larch No.2 wood segments placed between two 6″x9″x3/8″ fiberglass panels. The top and bottom of the panel is attached to wood with 4 and 8 nails, respectively (to limit failure at the panel top). Three different nails (8d, 10d, and 16d) were used. The load and nail slip were recorded on both specimen sides. Each discontinuous load-slip curve was idealized, using an equation developed by Menegotto & Pinto, with smooth continuous curve [3]. The range of idealized load-slip curves for specimens using 8d nails is shown in Fig. 1. The peak cyclic response envelope was similarly idealized. An average load-slip curve for monotonic and cyclic loading is shown in Fig.2 and Fig. 3. The curves were terminated where the probability of failure exceeded 50%.

MONOTONIC RESPONSE

Monotonic failure of specimens with 8d nails consisted of nail withdrawal from the wood. As the nails were deformed, the clamping force between the panels and wood lowered. The result was a subsequent loss of stiffness and strength capacity. Using 10d or 16d common nails, the failure mode changed to splitting of the wood framing. Cross grain bending stresses, developed in the wood between the loading rod and the eccentric panel connections, caused the wood to split near the panel (where the nails were driven into the wood), or split in the middle, through the loading rod hole. Only one test (with 16d nails) had minor panel damage at the nail/panel-hole interface.

CYCLIC RESPONSE

Cyclic failure of specimens with 8d and 10d nails consisted of withdrawal, fatigue, and eventual fracture of the nails; repeated cyclic loads bent the nails back and forth until the nails broke. Specimens with 16d nails failed due to splitting of the wood framing. Approximately 90% of specimens with 16d nails exhibited visible panel damage, in the form of cracks propagating from the panel nail/panel-hole interface.

CONCLUSIONS

FRP panels provide a strong alternative for diaphragm use. The average monotonic load-slip curve for 8d common nails is compared, in Fig. 4, with a curve determined for plywood panels, using similar specimens and loading [1]. The fiberglass (FRP) connection holds approximately 150% more load and is twice as stiff in the elastic range, compared to conventional plywood.

In summary, fiberglass panel sheathing may be advantageous in diaphragms and shear walls, where: high strength and ductility demands are imposed on the diaphragm, such as in seismic areas; limited space is available for shear resistance; repair of plywood diaphragms is necessary; a fire hazard exists; and sheathing is exposed to adverse conditions [4].

REFERENCES

1. Dugan, K. “Nail Slip Curves,” Unpublished Report, Urbana, IL, 1995
2. Judd, J. and F. Fonseca. “Monotonic Response of Fiberglass Panels Fastened to Wood Framing,” ICCE/5, Las Vegas, NV, July 5-11, 1998.
3. Menegotto, M.. and P. Pinto. “Method of Analysis for Cyclically loaded Reinforced Concrete Plane Frames Including Changes in Geometry and Non-elastic Behavior of Elements Under Combined Normal Force and Bending,” IABSE Symposium, Lisbon, 1973.
4. Financial support for this project was made possible by a grant from the Office of Research and Creative Activities and the Department of Civil and Environmental Engineering. The FRP panels used in the project were donated by Strongwell.