Mathew Fielding and Dr. Fernando Fonseca, Civil Engineering
Three light gauge aluminum connections were designed, constructed and tested to determine the possibility of resisting an applied moment in timber beams and columns. A timber column 3 inches by 5.5 inches was axially compressed with 1000 lb. while the attached beam (3 inches by 3.5 inches) was loaded to determine the moment resisting capacity of the system. Fracture of the connection occurred at an average of 1550 ft-lb; this corresponded to a deflection of .4 inches. Actual failure of the joint occurred at an average of 1950 ft-lb. with an average deflection of .58 inches.
Construction
Each aluminum connector was nailed in the same manner to Douglas Fir-Larch members. The beams were constructed of two nominal 2X4s which were nailed together. Each column was made of two nominal 2X6s that were also nailed together. The system resembled a “T” turned on its side. Each beam was a continuous member that was “sandwiched” between two columns.
Testing
An axial force was placed on the column by means of a hydraulic press. A hydraulic jack was then used to apply force at the free end of the beam in order to create a moment. The applied force was measured by placing a load cell between the jack and the beam. A moment was then measured by loading the beam at a uniform rate a distance of 16.75 inches from the joint.
Results
Figure 1 shows the results of the 3 trials. The applied moment is plotted against the deflection of the end of the cantilevered beam. As seen in the plot, the behavior of each system was fairly uniform. Failure of each joint occurred as cracks began at the intersection of the beam and column (an area of stress concentration) and propagated along the side of the connection following the line of nails used to attach the connection to the column.
Conclusions and Significance
Substantial moment resisting capacity was seen in this connection. The strength of the connection comes from the fact that the axial compressive forces applied to the column actually aid in the ability of the connection to resist a moment applied to the beam. Further tests are required to determine more precisely how axial compressive forces applied to the column influence the ability of the connection to resist applied moments.
Applications of this connection can be utilized in timber frames to aid in the resistance of lateral forces. In particular, increased open space in shear walls (i.e. doors and windows) is possible by using this connection to resist wind and seismic loads.