Brick Masonry Arches Introduction - Applied Architecture Structures | ARCH 631, Study notes of Architecture

Download Brick Masonry Arches Introduction - Applied Architecture Structures | ARCH 631 and more Study notes Architecture in PDF only on Docsity! INTRODUCTION In the latter part of the 19th century, an arch was dis- covered in the ruins of Babylonia. Archeologists esti- mate that the arch was constructed about the year 1400 B.C. Built of well-baked, cigar-shaped brick and laid with clay mortar, this arch is probably the oldest known to man. The Chinese, Egyptians and others also made use of the arch before the Christian era. Later, more elaborate arches, vaults and domes with complicated forms and intersections were constructed by Roman builders during the Middle Ages. The brick arch is the consummate example of form following function. Its aesthetic appeal lies in the vari- ety of forms which can be used to express unity, bal- ance, proportion, scale and character. Its structural ad- vantage results from the fact that under uniform load, the induced stresses are principally compressive. Be- cause brick masonry has greater resistance to compres- sion than tension, the masonry arch is frequently the most efficient structural element to span openings. This Technical Notes addresses the detailing and con- struction of brick masonry arches. The common types of brick masonry arches are presented, along with prop- er arch terminology. Methods of selecting the type and configuration of brick masonry arches most appropriate for the application are discussed. Proper material selec- tion and construction methods are recommended. Other Technical Notes in this series discuss the structural de- sign of brick masonry arches and lintels. ARCH TYPES AND TERMINOLOGY Many arch forms have been developed during the centuries of use, ranging from the jack arch through the circular, elliptical and parabolic to the Gothic arch. Fig- BRICK MASONRY ARCHES INTRODUCTION A b s t r a c t : The masonry arch is one of the oldest structural elements. Brick masonry arches have been used for hundreds of years. This Technical Notes is an introduction to brick masonry arches. Many of the different types of brick masonry arches are discussed and a glossary of arch terms is provided. Material selection, proper construction methods, detailing and arch construction recommendations are discussed to ensure proper structural support, durability and weather resistance of the brick masonry arch. Key Words: arch, brick, reinforced, unreinforced. Structural Brick Arches FIG. 1 January 1995 31 REVISEDTechnical Notes on Brick Construction Brick Industry Association 11490 Commerce Park Drive, Reston, Virginia 20191 ARCH 631 Note Set 6.1 F2008abn 2 a) JACK b) SEGMENTAL c) SEMICIRCULAR j) GOTHIC h)TUDOR i) TRIANGULAR g) VENETIAN d) BULLSEYE e) HORSESHOE f) MULTICENTERED Arch Types FIG. 2 ARCH 631 Note Set 6.1 F2008abn 5 Installation of flashing with other arch types, such as segmental and semicircular arches, can be more diff i- cult. This is because most rigid flashing materials are hard to bend around an arch with tight curvature. If the arch span is less than about 3 ft (0.9 m), one section of tray flashing can be placed in the first horizontal mortar joint above the keystone, as illustrated in Fig. 5b. For arch spans greater than 3 ft (0.9 m), flashing can be bent along the curve of the arch with overlapping sections, as illustrated in Fig. 4. Alternately, a combination of stepped and tray flashing can be used, as shown in Fig. 5c. To form a step, the end nearest the arch should be turned up to form an end dam, while the opposite end is laid flat. A minimum of No. 15 building paper or equivalent moisture resistant protection should be in- stalled on the exterior face of the backing over the full height of the arch and abutments. The building paper or equivalent should overlap the arch flashing. The design of a structural masonry arch should in- clude consideration of the effect of flashing on the strength of the arch. Flashing acts as a bond break. If flashing is installed above the arch, the loading on the arch will likely be increased, and the structural resis- tance of the arch will be reduced. Installation of flash- ing at the abutments will affect their structural resis- tance and should also be considered. Consult Technical Notes 31A for a more extensive discussion of arch loads and structural resistance of brick masonry arches. DETAILING CONSIDERATIONS The brick masonry arch should serve its structural purpose and also provide an attractive architectural ele- ment to complement its surrounding structure. Careful consideration should be given to the options available for the arch, soffit and skewback. Proper configuration of the abutments and location of expansion joints should be considered for any arch design. Arch Arches can be configured in a variety of arch depths, brick sizes and shapes and bonding patterns. The arch is normally composed of an odd number of units for aesthetic purposes. Some of the more common arch configurations are illustrated in Fig. 6. Arch voussoirs are typically laid in radial orientation and are most often of similar size and color to the surrounding brickwork. However, the arch can be formed with brick which are thinner or wider than the surrounding brickwork and of a different color for variation. Another variation is to project or recess rings of multiple-ring arches to pro- vide shadow lines or a label course. Brick masonry arches are constructed with two dif- ferent types of units. The first is tapered or wedge- shaped brick. These brick are tapered in the appropriate manner to obtain mortar joints of uniform thickness along the arch depth. The second is uncut, rectangular brick. When rectangular brick are used, the mortar joints are tapered to obtain the desired arch curvature. In some cases, a combination of these is used. For ex- ample, a slanted arch is formed with a tapered keystone and rectangular brick. This arch is similar to a jack WINDOW TRIM TRAY FLASHING TRAY FLASHING MASONRY BACKING END DAM WINDOW TRIM BUILDING PAPER OVERLAPPING FLASHING ARCH SPAN CENTER LINE WEEP HOLES AT REGULAR SPACING & AT ENDS WEEP HOLE AT END TYP. TRAY FLASHING STEP FLASHING TRAY FLASHING EXTERIOR SHEATHING & BUILDING PAPER WEEP HOLE AT EACH END OF TRAY FLASHING EXTERIOR SHEATING &BUILDING PAPER 4 IN. (1 0 0 m m ) MIN. Flashing Arches FIG. 5 END DAM BUILDING PAPER OVERLAPPING FLASHING INTERIOR SHEATING WEEP HOLE STUD AT JAMB a) FLASHING A JACK ARCH b) SHORT SPAN ARCHES c)LONG SPAN ARCHES ARCH 631 Note Set 6.1 F2008abn 6 arch, but can be more economical because it requires only one special-shaped brick. Selection of tapered or rectangular brick can be de- termined by the arch type, arch dimensions and by the appearance desired. Some arch types require more unique shapes and sizes of brick if uniform mortar joint thickness is desired. For example, the brick in a tradi- tional jack arch or elliptical arch are all different sizes and shapes from the abutment to the keystone. Con- versely, the voussoirs of a semicircular arch are all the same size and shape. Arch types with many diff e r e n t brick shapes and sizes should be special ordered from the brickmanufacturer rather than cut in the field. The arch span should also be considered when select- ing the arch brick. For short arch spans, use of tapered brick is recommended to avoid excessively wide mortar joints at the extrados. Larger span arches require less taper of the voussoirs and, consequently, can be formed with rectangular brick and tapered mortar joints. The thickness of mortar joints between arch brick should be a maximum of 3⁄4 in. (19 mm) and a minimum of 1⁄8 in. (3 mm). When using mortar joints thinner than 1⁄4 in. (6 mm), consideration should be given to the use of very uniform brick that meet the dimensional tolerance limits of ASTM C 216, Type FBX, or the use of gauged brick- work. Refer to Table 1 for determination of the mini- mum segmental and semicircular arch radii permitted for rectangular brick and tapered mortar joints. Ty p i- c a l l y, the use of tapered brick and uniform thickness mortar joints will be more aesthetically appealing. D e p t h. The arch depth will depend upon the size and orientation of the brick used to form the arch. Typ- ically, the arch depth is a multiple of the brick’s width. For structural arches, a minimum arch depth is deter- mined from the structural requirements. If the arch is supported by a lintel, any arch depth may be used. The depth of the arch should also be detailed based on the scale of the arch in relation to the scale of the building and surrounding brickwork. To provide proper visual balance and scale, the arch depth should increase with increasing arch span. Because aesthetics of an arch are subjective, there are no hard rules for this. H o w e v e r, the following rules-of-thumb will help pro- vide an arch with proper scale. For segmental and semicircular arches, the arch depth should equal or ex- ceed 1 in. (25 mm) for every foot (300 mm) of arch span or 4 in. (100 mm), whichever is greater. For jack arches, the arch depth should equal or exceed 4 in. (100 mm) plus 1 in. (25 mm) for every foot (300 mm) of arch span or 8 in. (200 mm), whichever is greater. For example, the minimum arch depth for an 8 ft (2.4 m) span should be 8 in. (200 mm) for segmental arches and 12 in. (300 mm) for jack arches. The depth of jack arches will also be a function of the coursing of the surrounding brick masonry. The springing and the extrados of the jack arch should coin- cide with horizontal mortar joints in the surrounding brick masonry. Typically, the depth of a jack arch will equal the height of 3, 4 or 5 courses of the surrounding brickwork, depending upon the course height. Typical Arch Configurations FIG. 6 1Based on 1⁄4 in. (6 mm) mortar joint width at the intrados and 1⁄2 in. (13 mm) mortar joint width at the extrados. If the mortar joint thickness at theextra dos is 3⁄4 in. (19 mm), divide minimum radius value by 2. 21 in. = 25.4 mm; 1 ft = 0.3 m Nominal Face Dimensions Minimum Permissible of Arch Brick, in. Radius of Arch (height by width) to Intrados, ft 4 x 22⁄3 3.3 8 x 22⁄3 6.7 12 x 22⁄3 10.0 16 x 22⁄3 13.3 4 x 31⁄5 4.0 8 x 31⁄5 8.0 12 x 31⁄5 12.0 16 x 31⁄5 16.0 4 x 4 5.2 8 x 4 10.3 12 x 4 15.5 16 x 4 20.7 TABLE 1 Minimum Radius for Uncut Arch Brick1,2 a) 8 IN. (200 mm) ARCH b) 12 IN. (300 mm) ARCH c) 16 IN. (400 mm) ARCH d) THREE COURSE JACK ARCH e) FOUR OR FIVE COURSE JACK ARCH ARCH 631 Note Set 6.1 F2008abn 7 should be placed along the arch span at a maximum spacing of 24 in. (600 mm) on center. Structural resistance of the arch should be evaluated at sections through the soffit, the exterior wall face and the interior wall face. Deeper soffits may require an in- crease in arch depth. If the arch is structural, connec- tion of the brick masonry forming the soffit to interior framing members with wall ties or connectors may not be required. Skewback For flat arches and arch types that have horizontal skewbacks, such as jack and semicircular arches, re- spectively, the most desirable spring line location is co- incident with a bed joint in the abutment. For other arch types, it is preferred to have the spring line pass about midway through a brick course in the abutment, as illustrated in Fig. 8, to avoid a thick mortar joint at the springing. The brick in the abutment at the spring- Keystone. The keystone may be a single brick, mul- tiple brick, stone, precast concrete or terra cotta. Avoid using a keystone which is much taller than the adjacent voussoirs. A rule-of-thumb is that the keystone should not extend above adjacent arch brick by more than one- third the arch depth. When a keystone is used that is larger than adjacent arch brick or formed with different material, one option is to use springers that match the keystone. The use of a large keystone has its basis in both pur- pose and visual effect. With most arch types, the likely location of the first crack when the arch fails is at the mortar joint nearest to the midspan of the arch. Use of a l a rge keystone at this point moves the first mortar joint further from the midspan and increases the resis- tance to cracking at this point. Aesthetically, a larg e keystone adds variation of scale and can introduce other masonry materials in the facade for additional color and texture. If the keystone is formed with more than one mason- ry unit, avoid placing the smaller unit at the bottom. Such units are more likely to slip when the arch settles under load. Also, it is preferred to have the arch crown (the top of the keystone) coincident with a horizontal mortar joint in the surrounding brickwork to give the arch a neater appearance. Soffit A brick masonry soffit is one attractive feature of a structural brick masonry arch. Many bonding patterns and arrangements can be used to form the arch soff i t . Deep soffits are common on building arcades or arched entranceways. In this case, it is common to form a U- shaped wall section, as illustrated in Fig. 7. The arches on either wall face should be bonded to the brick ma- sonry forming the soffit. Bonding pattern or metal ties should be used to tie the brick masonry forming the sof- fit together structurally and to tie the arches on either wall face to the soffit. If metal ties are used to bond the m a s o n r y, corrosion resistant box or Z metal wire ties Option For Intersecting Arches FIG. 9 Skewback Options FIG. 8 Structural Arch Soffit Option FIG. 7 OUTSIDE FACE OF WALL WALL TIE WEEP HOLE TRAY FLASHING AT ARCH CROWN BUILDING PAPER & EXTERIOR SHEATHING FRAMING TRIM INSIDE FACE OF WALL WALL TIE KEYSTONE BRICK MASONRY BEYOND KEYSTONE Z TIE @ 24 IN. (600 mm) MAX. ALONG ARCH RING OR AS REQUIRED a) JACK ARCH SKEWBACK b) SEMICIRCULAR ARCH SKEWBACK c) SEGMENTAL ARCH SKEWBACK REGULAR MORTAR JOINT THICKNESS CANT BRICK SPECIAL BRICKARCH BRICK ARCH 631 Note Set 6.1 F2008abn