Arches typically are made of glued-laminated timber and may be two-hinged, with hinges at each base, or three-hinged with a hinge at the crown. Figure 10.18 illustrates typical forms of arches.
Tudor arches are the most commonly used type of timber arch. They are gabled rigid frames with curved haunches. The half spans on each side of the crown usually are one piece of glued-laminated timber. This type of arch is frequently used in church and commercial construction.
A-frame arches are generally used where steep pitches are required. They may spring from grade, or concrete abutments, or other suitably designed supports.
Radial arches are often used where long clear spans are required. They have been employed for clear spans up to 300 ft.
Gothic, parabolic, and three-centered arches are often selected for architectural and aesthetic considerations.
Timber arches may be tied or buttressed. If an arch is tied, the tie rods, which resist the horizontal thrust, may be above the ceiling or below grade, and simple connections may be used where the arch is supported on masonry walls, concrete piers, or columns (Fig. 10.19)
Arches are economical because of the ease of fabricating them and simplicity of field erection. Field splice joints are minimized; generally there is only one simple connection, at the crown (Fig. 10.20). Except for extremely long spans, they are shipped in only two pieces. Erected, they need not be concealed by false ceilings, as may be necessary with trusses. Inasmuch as arches have large cross sections, they are classified as heavy-timber construction.
Changes in moisture content may be of great importance in three-hinged arches that become horizontal, or nearly so, at the crest of the roof. Shrinkage, increasing the relative end rotations may cause a depression at the crest and create drainage problems. For such arches, therefore, consideration must be given to moisture content of the member at the time of fabrication and in service, and to the change in end angles that results from change in moisture content and shrinkage across the grain.
A long-span arch may require a splice or moment connection to segment the arch to facilitate transportation to the job site. Figure 10.21 shows typical moment connections for wood arches. Moment connections should be located at the point of minimum moment when possible.
(See K. F. Faherty and T. G. Williamson, Wood Engineering and Construction Handbook, McGraw-Hill Publishing Company, New York, for additional information.)
