1298378161

Ewing [30] studied the effects of various stiffnesses of wood roof diaphragms on the response of masonry buildings. Little work has been done on combined loading; Frodin and Ross (31) tested two plywood sheathed panels subjected to racking plus uplift.

Although the seismic performance of wood. wood-based. and other sheathing materials is a functlon of dynamie fastener properties, little work has been done on joints subjected to cyclic and vibrational loads. The slip-damping of nailed joints was found to be an order of magnitude greater than the materia! damping [32]. Wilkinson [33] found joints to be considerably stiffer under vibrational load than static load. He implied that this result was related to ratę and duration of load and concluded that a 33% duration of load inerease in design stresses for earth-quake and wind is probably conservative.

Cyclic loading does not. however. appear to influence the strength of nailed joints. Mack [34] found the ultimate strength to be unaffected by repetitive loading. and Armstrong and Schuster [35] found that load-slip behavior was dependent on the highest load applied. It has been found the effect of direction of load on slip modulus is smali for plywood [36]. A study of the joint adequacy between timber rafters and ledger beams to masonry walls showed that ledgers failed when subjected to cross grain tension [37].

The response of an entire structure differs from that of its components. Natural frequencies and coeffi-cients of critical damping have been determjned in a few studies. Values of 5 to 9 Hz and 4% to 6% damping were found for one-story school buildings with plywood shear walls and glulam roof framing [38]. Medearis [39] surveyed 63 one-and-one-half and two-story residences of various ages in four States He found little difference due to age of eon-struction or geographic location. Natural frequencies from 4 to 18 Hz. corresponding to building heights of 40 to 10 feet (frequency is inverse to height). and average damping ratios of 5.2% were observed. Tests of a two-story residence revealed that the natural frequency was 9 Hz; average damping was 6% [40]. Average natural frequency and damping of 7 Hz and 4 6% were obtained from measurements of the dynamie response of 23 one- and two-story single family residences from surface blasts [41]. In-dividual shear wali natural frequency averaged 15 Hz.

Two studies have been concerned wlth the response of Japanese houses. Natural frequencies of a two-story house varied from 7 to 11.5 Hz [42]. Those for one-story houses were 2Va to 5 Hz; the values for two-story houses were 2 to 4 Hz [43].

DESIGN PHILOSOPHY

In comparison to other types of buildings. Iow-rise timber structures generally have higher natural fre-quencies (thus lower natural periods) and about average (5% to 10%) coefficient of critical damping. Because they are relatively rigid low-mass structures. their motion is nearly the same as that of the ground. and dynamie forces are very nearly equal to those associated with the ground accelerations applied to the structure as a rigid body; natural frequencies of higher modes of vibration are not important.

Current design is based on a static base shear equation that represents a linear approximation of a tripartite (acceleration, velocity. and displacement) response spectrum. Horizontal and vertical distributions of base shear imply that the building is vibrating later-ally in one direction and that it is symmetric. If nonsymmetry and corresponding torsion are present, coupled lateral-torsional analysis is suggested [44].

Kan and Chopra [45-48] analyzed coupled torsional and translational motion by simultaneous solution of the equations of motion. They found that coupling of the two motions occurs if there is a large eccen-tricity between the centers of mass and rigidity in nonsymmetrical buildings or if the natural lateral frequency is close to the natural torsional frequency in symmetric buildings. They also found that the maximum base shear in a torsionally coupled system is less than in the corresponding uncoupled system; however. the dynamie torque can considerably ex-ceed the static torque that is a product of horizontal shear times eccentricity between centers of mass and rigidity. Other studies of coupled lateral torsional motion for a one-story structure have been reported [49. 50).

A second technique [51-53] for studying the torsion problem is to treat the building as a cantilever mem-ber with open or closed cross section The result is vertical warping displacements and stresses in addi-tion to horizontal shear stresses. The technique has