Multicentred Arches Essay

Multilayered arch An arch having a shape composed of a series of circular arcs with different radii, giving an approximation to an ellipse. These arcs are symmetrically disposed about a vertical axis and occur in odd numbers. Construction known as an ‘arch ring’, made of truncated wedge-shaped visitors that by mutual pressure stay in place, set out in a curved form to span an opening and carry a superimposed load. Installation of flashing with other arch types, such as segmental and semicircular arches, can be more difficult.

This is because most rigid flashing materials are hard o 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. B. 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. C. 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 alluding paper or equivalent moisture resistant protection should be installed 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 include 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 resistance of the arch will be reduced.

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Installation of flashing at the abutments will affect their structural resistance and should also be considered. Consult Technical Notes AAA 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 element to complement its surrounding structure. Careful consideration should be given to the options available for the arch, soft and skewbald. 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 visitors 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 provide shadow lines or a label course. Tapered or wedge-shaped brick. These brick are tapered in the appropriate manner o 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 example, a slanted arch is formed with a tapered keystone and rectangular brick.

This arch is similar to a Jack arch, but can be more economical because it requires only one special-shaped brick. Selection of tapered or rectangular brick can be determined by the arch type, arch dimensions and by the appearance desired. Some arch types require more unique shapes and sizes of brick f uniform mortar Joint thickness is desired. For example, the brick in a traditional jack arch or elliptical arch are all different sizes and shapes from the abutment to the keystone. Conversely, the visitors of a semicircular arch are all the same size and shape.

Arch types with many different brick shapes and sizes should be special ordered from the brick manufacturer rather than cut in the field. The arch span should also be considered when selecting the arch brick. For short arch spans, use of tapered brick is recommended to avoid excessively wide mortar joints at the extracts. Larger span arches require less taper of the visitors 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 FIX, or the use of gauged brickwork. Refer to Table 1 for determination of the minimum segmental and semicircular arch radii permitted for rectangular brick and tapered mortar Joints. Typically, the use of tapered brick and uniform thickness mortar Joints will be more aesthetically appealing. Depth. The arch depth will depend upon the size and orientation of the brick used to form the arch.

Typically, the arch depth is a multiple of the brick’s width. For structural arches, a minimum arch depth is determined 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 lance 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. However, the following rules-of-thumb will help provide an arch with proper scale.

For segmental and semicircular arches, the arch depth should equal or exceed 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. 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 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. Keystone. The keystone may be a single brick, multiple brick, stone, precepts concrete or terra cotta. Avoid using a keystone which is much taller than the adjacent visitors. 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 Springer that match the keystone.

The use of a large keystone has its basis in both purpose 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 midshipman of the arch. Use of a large keystone at this point moves the first mortar Joint further from the midshipman and increases the resistance to cracking at this point. Aesthetically, a large keystone adds ration 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 masonry 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. Soft A brick masonry soft is one attractive feature of a structural brick masonry arch. Many bonding patterns and arrangements can be used to form the arch soft. Deep shifts are common on building arcades or arched entranceway. 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 masonry forming the soft. Bonding pattern or metal ties should be used to tie the brick masonry forming the soft together structurally and to tie the arches on either wall face to the soft. If metal ties are used to bond the masonry, corrosion resistant box or Z metal wire ties 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 soft, the exterior wall face and the interior wall face.

Deeper shifts may require an increase in arch depth. If the arch is structural, connection of the brick masonry forming the soft to interior framing members with wall ties or connectors may not be required. Skewbald For flat arches and arch types that have horizontal sawbucks, such as Jack and semicircular arches, respectively, the most desirable spring line location is coincident with a bed Joint in the abutment. For other arch types, it is preferred to have the bring 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 springing should be cut or be a special can’t-shaped brick. This allows vertical alignment with the brick beneath, producing more accurate alignment of the arch. Intersection of the arches may occur at the skewbald. Attention should be given to proper bonding of the arches for both visual appeal and structural bonding. Creation of a vertical line between arches should be avoided. Rather, special shape brick household be used to mesh the two arches properly. Abutments An arch abutment can be a column, wall or combination of wall and shelf angle.

Failure of an abutment occurs from excessive lateral movement of the abutment or exceeding the flexural, compressive or shear strength of the abutment. Lateral movement of the abutment is due to the horizontal thrust of the arch. Thrust develops in all arches and the thrust force is greater for flatter arches. The thrust should be resisted so that lateral movement of the abutment does not cause failure in the arch. If the abutment is formed by a combination of brickwork and a non- season structural member, rigidity of the non-masonry structural member and rigidity of the ties are very important.

Adjustable ties or single or double wire ties are recommended. Corrugated ties should not be used in this application because they do not provide adequate axial stiffness. Consult Technical Notes 31 A for further discussion of abutment and tie stiffness requirements. Lateral Bracing In addition to gravity loads, out-of-plane loads should be considered when designing a masonry arch. The arch should have adequate resistance to out-of-plane loads or lateral bracing should be provided. In veneer construction, lateral bracing is provided by the backing through the use of wall ties.

Arches which are not laterally braced may require increased masonry thickness or reinforcement to carry loads perpendicular to the arch plane in addition to vertical loads. Expansion Joints Thermal and moisture movements of brick masonry are controlled by the use of expansion Joints. Expansion Joints avoid cracking of the brickwork and also reduce the size of wall sections. Reduction of wall size has a very important effect upon the performance of structural brick masonry arches. The state of stress in a structural Rick arch and the surrounding masonry is very sensitive to the relative movements of the abutments.

If an inadequate number of expansion Joints are provided, the differential movement of abutments can cause cracking and downward displacement of brick in the masonry arch and surrounding masonry. Proper size and spacing of expansion Joints is discussed in Technical Notes AAA Revised. If the arch is structural, care should be taken not to affect the integrity of the arch by detailing expansion joints too close to the arch and its abutments. Vertical expansion Joints should not be placed in the masonry directly above a structural arch.

This region of masonry is in compression, so an expansion Joint will cause displacement when centering is removed and possible collapse of the arch and surrounding brickwork. In addition, vertical expansion Joints should not be placed in close proximity to the springing. The expansion Joint will reduce the effective width of the abutment and its ability to resist horizontal thrust from the arch. If the arch is non-structural, placement of expansion joints may be at the arch crown and also at a sufficient distance away from the arch crown is not preferred because it disrupts ones traditional view of the arch as a tutorial element.

Refer to Fig. 10 for suggested expansion Joint locations for structural and non-structural arches. Expansion Joints Near Arches FIG. Detailing of expansion Joints can be difficult with very long span arches or runs of multiple arches along an arcade. Structural analysis of the arch should consider the location of expansion Joints. For the particular case of multiple arches closely spaced, vertical expansion Joints should be detailed at a sufficient distance away from the end arches so that horizontal arch thrusts are adequately resisted by the abutments to avoid overturning of the abutments.

For long arcades, expansion Joints should also be placed along the centerline of abutments between arches when necessary. In this case, horizontal thrusts from adjacent arches will not be counteracting, so the effective abutment length should be halved and overturning of each half of the abutment should be checked. Refer to Technical Notes 31 A for further discussion of abutment design for adequate stiffness. MATERIAL SELECTION To provide a weather resistant barrier and maintain its structural resistance, the arch must be constructed with durable materials.

The strength of an arch depends upon he compressive strength and the flexural tensile strength of the masonry. Selection of brick and mortar should consider these properties. Brick Solid or hollow clay brick may be used to form the arch and the surrounding brickwork. Solid brick should comply with the requirements of ASTM C 216 Specification for Facing Brick. Hollow brick should comply with the requirements of ASTM C 652 Specification for Hollow Brick. Refer to Technical Notes 9 Series for a discussion of brick selection and classification.

The compressive strength of masonry is related to the compressive strength of the brick, the mortar type and the grout trench. For structural arches, brick should be selected with consideration of the required compressive strength of masonry. Typically, compressive strength of the brick masonry will not limit the design of the arch. Tapered visitors can be cut from rectangular units at the Job site or special ordered from the brick manufacturer. Before specifying manufactured special arch shapes, the designer should determine the availability of special shapes for the arch type and brick color and texture desired.

Many brick manufacturers produce tapered arch brick for the more common arch types as part of their regular stock of special shapes. Be sure to contact the manufacturer as early as possible if special shapes are needed. In many instances, production of the special shapes may require a color matching process and adequate lead time for the manufacturer. Mortar Mortar used to construct brick masonry arches should meet the requirements of ASTM C 270 Standard Specification for Masonry Mortar. Consult Technical Notes 8 Series for a discussion of mortar types and kinds for brick masonry.

For structural arches, the flexural tensile strength of the masonry should be considered when selecting the mortar. The flexural tensile strength of the masonry will affect the load assistance of the arch and the abutments. The proper performance of a brick masonry arch depends upon proper methods of construction and attention to workmanship. Layout of the arch prior to construction will help avoid poor spacing of visitors, which results in thicker mortar Joints and unsymmetrical arches. Some arch applications, such as barrel vaults and domes, can be entirely self-supporting, even during construction.

However, most applications of the masonry arch used today require proper shoring and bracing. Centering Both structural and non-structural arches should be properly supported throughout construction. Brick masonry arches are constructed with the aid of temporary shoring, termed centering, or permanent supports, such as a structural steel angle. Centering is used to carry the weight of a brick masonry arch and the loads being supported by the arch until the arch itself has gained sufficient strength. The term “centering” is used because the shoring is marked for proper positioning of the brick forming the arch.

Centering is typically provided by wood construction. An example of centering for an arch is shown in Fig. 11. Careful construction of the centering will ensure a more pleasing arch appearance and avoid layout problems, such as an never number of brick to either side of the Immediately after placement of the keystone, very slight downward displacement of the centering, termed easing, can be performed to cause the arch visitors to press against one another and compress the mortar Joints between them.

Easing helps to avoid separation cracks in the arch. In no case should centering be removed until it is certain that the masonry is capable of carrying all imposed loads. Premature removal of the centering may result in collapse of the arch. Centering should remain in place for at least seven days after construction of the arch. Longer curing periods may be required when the arch is constructed in cold weather conditions and when required for structural reasons.

The arch loading and the structural resistance of the arch will depend upon the amount of brickwork surrounding the arch, particularly the brick masonry within spandrel areas. Appropriate time of removal of centering for a structural arch should be determined with consideration of the assumptions made in the structural analysis of the arch. It may be necessary to wait until the brickwork above the arch has also cured before removing the centering. Workmanship All mortar Joints should be completely filled, especially in a structural member such as an arch.

If hollow brick are used to form the arch, it is very important that all face shells and end webs are completely filled with mortar. Brick masonry arches are sometimes constructed with the units laid in a soldier orientation. It may be difficult to lay units in a soldier position and also obtain completely filled mortar Joints. This is especially true for an arch with tapered mortar Joints. In such cases, the use of two or more rings of arch brick laid in rowlock orientation can help ensure full mortar Joints


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