Masonry Magazine August 1975 Page. 13
Technical Notes on Brick Construction
Brick Institute of America 1750 Old Meadow Road, Mclean, Virginia 22101
31A
OCT.
1967
STRUCTURAL DESIGN OF BRICK MASONRY ARCHES
INTRODUCTION
The railway bridge at Maidenhead, England, constructed in 1838, is a brick arch with a span of 128 ft and a rise of 24.3 ft. This arch was designed by engineer Marc Brunel, who is also credited with being the first to use reinforced brick masonry. A similar brick arch railway bridge was constructed on North Avenue, Baltimore, Maryland, in 1895. It has a span of 180 ft and a rise of 26 ft. These outstanding examples are cited only to illustrate the structural capabilities of the brick arch-capabilities on which designers may rely when architectural or structural considerations suggest their use in modern design.
This issue of Technical Notes covers the structural design of major and minor brick masonry arches.
Minor arches are those whose spans do not exceed 6 ft and with maximum rise-to-span ratios of 0.15. Coefficients are given from which the horizontal thrust of such arches may be determined. Equations are presented for obtaining compressive stresses developed in the masonry and for determining stability against sliding.
Derivation of thrust coefficients and equations are based on the hypothesis of least crown thrust, as described in Technical Notes, No. 31, and the following assumptions have been made:
1. The thrust at the crown is horizontal and passes through the upper 1 point of the arch.
2. The reaction passes through the lower 1½ point of the arch at the skewback.
3. The equilibrium polygon lies completely within the middle 1 of the arch.
Figure 1 illustrates jack and segmental arches.
Major arches are those with spans in excess of 6 ft or rise-to-span ratios greater than 0.15. In this issue of Technical Notes an example is given of major arch design based on the equations for redundant moments and forces presented in the publication, "Frames and Arches". The method of analysis presented in this book is substantially shorter than others in current use.
"Frames and Arches," by Valerian Leontovich, MeGraw Hill Book Co., 1959.
MINOR ARCH LOADING
The loads falling upon a minor arch may consist of live loads and dead loads from floors, roofs, walls and other structural members. These are applied as point loads or as uniform loads fully or partially distributed. A method of determining imposed loads on a member spanning small openings is described in Technical Notes, No. 17H. A brief resume of that explanation is given here.
The dead load of a wall above an arch may be assumed to be the weight of wall contained within a triangle immediately above the opening. The sides of this triangle are at 45-deg angles to the base. Therefore, its height is 1% of the span. Such triangular loading may be assumed to be equivalent to a uniformly distributed load of 11 times the triangular load.
Superimposed uniform loads above this triangle may be carried by arching action of the masonry wall itself. Uniform live and dead loads occurring below the apex of the triangle are ap-
SKEWBACK
AXIS
RISE
SOFFIT
SPRING LINE
SPAN
SEGMENTAL ARCH
SKEWBACK
K
CAMBER-
SOFFIT
SPRING LINE
SPAN
JACK ARCH
FIG. 1