A How to prevent abutment failures in unreinforced structural arches
Transcription
A How to prevent abutment failures in unreinforced structural arches
How to prevent abutment failures in unreinforced structural arches This semicircular multiring brick arch experienced sliding failure. By Elizabeth Keating recent study documented the most common failures of unreinforced, structural masonry arches as part of a building wall (Ref. 1). After investigating over 70 arches in the District of Columbia, Massachusetts, Pennsylvania, and Virginia, the researchers* found that the failures fell into the following three categories: ■ Abutment displacement due to moisture and thermal expansion of the masonry ■ Insufficient abutment stiffness ■ Foundation settlement of abutments A At least one of the above failures was noticed for each of the most common arches used in residential and commercial building construction—jack arches (flat), segmental arches, and semicircular arches. In the older masonry buildings (over 100 years), the Glossary Abutment: The masonry that supports an arch at the skewback. Extrados: The upper curve of an arch. Intrados: The lower curve of an arch. Skewback: The inclined surface on which an arch joins the supporting wall. Skewback angle: The angle made by the skewback from the horizontal. Soffit: The bottom surface of an arch. Spandrel: Either of the triangular spaces between the exterior curve of an arch and a rectangular shape enclosing the arch. Spring line: The horizontal line where the arch intrados meets the skewback. Springing: The point where the skewback intersects the intrados. Thrust load: The horizontal load that results from vertical loads applied to an arch. *The researchwasconducted b yT h o m a s E. Boothby, Ph.D., P.E., assistant professor of a rc h itectural engineering, Penn State University; Scott E. Nelson, formerly a graduate student, Department of Architectural Engineering, Penn State University; and Matthew J. Scolforo, formerly a staff engineer with Brick Institute of America. most common mechanism of arch failure was slippage of voussoirs or whole sections of the arch ring. Loading conditions, form of building construction, and age of the building, or combinations of these factors, were directly res p o n s i b l ef o rt h ef a i l u re so b s e rv e d , most of which are failures that affect the serviceability of the masonry wall (rather than total arch collapse). Serviceability is the ability of the arch to support the imposed loads safely to assure its proper performance during inplace use. Arch loading conditions can be described as uniform or concentrated. Uniform loads derive from the triangular area of masonry above the arch opening. Concentrated loads occur when beams, girders, or trusses framing into the wall above the arch exert pressure on the arch. Arch design is influenced by: whether the wall is rectangular or square; whether it has many projected or recessed areas; or whether the wall is flat or curved. The study found many cases where the masonry had cracked in the a rc h ring and above the a rc h . Some of these a re a s had been repointed. While cracking does not often cause collapse of the arch, it impacts the water and air penetration resistance of the wall system and is unsightly. 1 The lateral spread and voussoirs’ slipping caused this Alexander, Va. window arch to rest solely on the wood window frame. Abutment displacement due to moisture and thermal expansion of the masonry In the arches examined, the most common cause of slippage of voussoirs and cracking of masonry above the arch was moisture and thermal expansion of the masonry. When this expansion is not taken into consideration in the planning stage, the arch abutments spread laterally until cracks appear and voussoirs slip in the arch ring. Some window arches had slipped to the point where they were being supported by the wood frame of the window (see Photo 1). Structural analysis of the arch should consider the location of expansion joints. When designing closely spaced multiple arches, vertical expansion joints should be detailed at a sufficient distance from the end arches so that the abutments adequately resist the horizontal arch thrusts, and overt u rn i n g of the abutments is avoided. Source: BIA Technical Notes, 31 Revised, figure 10 a, page 8. When designFigure 1. Illustration shows where expansion joints can ing long arcades, be positioned near structural arches. expansion joints should also be portant not to detail expansion placed along the centerline of joints too close to the arch and its abutments between arches where abutments, as this would affect necessary. In this case, horizontal the arch’s integrity adversely (see thrusts from adjacent arches will Figure 1). Also, vertical expansion not be counteracting (you don’t joints should not be placed in the have to take into consideration masonry directly above a structhe arching action of the adjacent tural arch or in close proximity to arch); so the effective abutment the springing (Ref. 2). length should be halved, and the Slipping failure occurs as a reresistance of each half of the sult of both insufficient shear abutment to overturning should bond strength and too little fricbe tested by putting a load on the tional resistance between voustop of it (Ref. 2). soirs in the arch ring. When this One of the most significant benoccurs, the shear bond strength efits of expansion joints is that between voussoirs is either overthey help prevent cracking of the come by the abutment movement brickwork and also reduce the or is lost due to mortar deteriorasize of wall sections. Reduction tion (see Photo 2). of wall size has a very important Abutments may move due to effect upon the performance of lack of resistance to the horizonstructural brick masonry arches. tal thrust from arch loading. When The state of stress in a structural this happens, the arch tries to debrick arch and the surrounding flect downwards (flatten out) due masonry is very sensitive to the to imposed loads, and the mortarrelative movements of the abutunit bond can break, causing the ments. voussoirs to move. Abutment deFor structural arches, it is imsign is critical to the successful p e rf o rm a n c eo fa rc h e s . Abutments 2 need arching-action resistance to control the horizontal thrust of the loads supported. M o rt a r deterioration can be controlled only by choosing a good quality mortar. It should always be specified in accordance with ASTM C 270 Standard SpecifiThe keystone of this semicircular cation for Mortar for Unit Masonry stone arch experienced sliding failby either the proportion or propure and spreading of the abutments. erty specification. Portland ce- ment-lime mortars are permitted greater stresses than those permitted for masonry cement. Richard T. Kreh Sr., a Frederick, Md.-based author and masonry consultant, recommends that a portland-lime cement m o rt a r (Type N) be used for best results, if possible, for the mortar joints in arches. “The lime in the mortar will bond better to the bricks without shrinking,” he says. “The a b i l i t yo fc e m e n t - l i m em o rtar to reknit or reseal itself if hairline cracks develop is known as autogenous healing,” explains Kreh. “Rainwater and atmospheric carbon dioxide will react with the mortar to provide this feature.” Autogenoushealingoccurswhen rainwater is absorbed into the mortar joint, dissolving hydrated lime. The calcium hydroxide in the hydrated lime reacts with atmospheric carbon dioxide to produce calcium carbonate (in effect, “limestone”). “This is the primary reason that you will see very few cracks in old historic buildings that have a high lime mortar,” says Kreh. “For this reason, I think that this type of mortar would be highly beneficial for mortar joints in arch work. Even though an arch might not be at the point of collapse, cracked mortar joints would still be very unsightly.” Slippage occurs more frequently when one or more of the following conditions is present: 1. The voussoirs or m o rt a r joints between voussoirs are not tapered. The amount of tapering of the arch brick is determined by the arch type, arch dimensions, and the desired appearance. Some brick manufacturers already have standard wedge-shaped brick specifically for arches. Usually the amount of tapering can be determined by graphical analysis or by using principles of trigonometry based on the radius of the arch desired. If the mortar joints have not been tapered enough (wider at the extrados and narrower at the intrados), the mason can put pressure on them to achieve the right tapering. 2. The arch rise is too small for the span and arch type. The minimum arch rise-to-span ratios for various types are based on rules of thumb. Jack arches are relatively flat and generally do not require a lintel support unless the arch spans more than 6 feet. Segmental Source: BIA Technical Notes, 31 Revised, figure 8, page 7. arches should have a rise-toFigure 2. For arches with horizontal skewbacks, such as span ratio of a) and b), the most desirable spring line location is coincident with a bed joint in the abutment. For others, the 0.15 or m o re . spring line should pass about midway through a brick Semicircular course in the abutment to avoid a thick mortar joint at arches do not the springing. have such requirements, since failure. In other words, when largthey are essentially a half c i rc l e er voussoirs are used, there is with a rise-to-span ratio of 0.5. more frictional resistancebetween The rise is established by the the voussoirs because larger maheight of the intrados above the sonry units are typically cut at a spring line. The span is the width greater taper. However, this is not of the arch opening. a guarantee that there will be no 3. The skewback angle is greatslippage. er than about 65 degrees. For flat Insufficient abutment stiffness and semicircular arches, the skewback angle should be at the spring Insufficient abutment stiffness line location coincident with the is the most likely cause of total bed joint in the abutment. For collapse of the arch because, if segmental arches, the spring line abutment deflection (bending) ocshouldpass a b o u tm i d w a yt h ro u g h curs, it does so immediately after a brick course in the abutment arch shoring (wood template) is (see Figure 2). removed, without warning of disThe smaller the number of matress. If the abutment is not desonry units used for a given span, signed appropriately to handle the less the potential for slippage the horizontal thrust of the arch and its imposed loads, the arch can fail when the template is re3 moved. It is recommended that the wood shoring be left in place until the masonry attains at least 75% of its ultimate strength, usually seven days. However, most segmental arches (examined in this study) that experienced insufficient abutment stiffness withstood a l o to fd e f l e c t i o no ft h e arch This arch abutment is made of 4abutments without collapsing. inch face brick attached to wood If the abutment is not designed studs with corrugated metal ties. correctly, arching action may roThe rotation of the tall porch columns caused the abutment to tate the arch off its support. Ususpread and the arch to crack. ally this occurs at the m o rt a r f o rc i n gt o resist t h el o a d si mposed or by proH1=resisting thrust in pounds viding more maVm=allowable shearing stress in the masonry sonry mass for wall in psi the columns) or n=the number of resisting shear planes provide an adjacent wall to the x=the distance from the center of the skewback to the end of the wall in inches a rc h to help resist horizontal t=wall thickness in inches thrust requireB=spring line ments. The horizontal thrust must be calculated so it doesn’t exceed the allowable stresses in the masonry abutment. The horizontal thrust for each arch must Source: BIA Technical Notes, 31A, figure 4, page 4. be calculated to Figure 3. How to calculate resisting thrust in pounds. determine how substantial the abutment must joint of the arch ring at the spring be. Horizontal thrust resistance is line. This action may cause tendeveloped by the mass of masonsile stresses in the arch, which ry on each side of the a rc h . It is can add to the rotation problem. found by calculation with the forIt is best to ensure that the resulmula, H=vmnxt (see Figure 3). For tant arch loading falls inside the more information on the design of middle third of the arch section to horizontal thrust resistance, refer prevent rotation. to Brick Institute of America (BIA) One arch abutment documentTechnical Notes 31A (Ref. 3). ed in the study was made of 4 F o rj a c k arches, abutmentlength inch face brick attached to wood should equal the span length for studs with corrugated metal ties o n e s u rf a c e a nd half the span (see Photo 3). The rigidity of the length for two. For segmental column connection at the base arches, abutment length should and the width of the brickwork equal 0.66 times the span length was insufficient for the height of the columns and the thrust force 4 from the segmental arch;consequently, rotation took place at the springing. This form of arch construction is common for brick masonry porch columns built today, some of which have collapsed when abutment rotations became too large (see “Fallen Arches,” October 1993, pages 456-459). Column supports for arches must be rigid enough to control lateral movement of the horizontal thrust. They must also resist This ornamental wood post was too the potential flexural, compresweak to resist the thrusts from the sive, or shear stresses that may be two segmental arches it supported. imposed on the abutment. To corWhen the post rotated, it caused rect insufficient stiffness of the the voussoirs to slip in the arch on abutment, a contractor should eithe right, and cracks appeared in ther stiffen the columns (by reinand above the arch ring. H1=vmnxt for one surface and 0.33 times the span length for two. For semicircular arches, abutment length should equal 0.4 times the span length for one surface and 0.2 times the span length for two surfaces. In order to qualify as two surfaces, the abutment must extend to the crown of the arch. Other examples of insufficient abutment stiffness were found. In one case, the wood post was too weak to resist the thrusts from the two segmental arches it supported. Cracks appeared in the arch ring and the masonry above the arch to the right (see Photo 4). Even rather sizeable solid-brickmasonry arch abutments can be insufficient, depending on the arch thrust force. One prime ex- 5 Despite the lateral spreading of abutments, this brick and stone arch from 1810 still stands. ample of this is shown on the abutments of the elliptical arch that were displaced 2 inches from p l u m ba tt h es p r i n gl i n e ,a n dw h i c h had cracks at the base of both abutments (see Photo 5). Heavy stone pieces added to the weight loading this arch and to the thrust force. Foundation settlement of abutments Differential settlement of the foundations of arch abutments can cause failure of the arch system, but building a rc h failures are rare because spans are short, abutments typically rest on the same foundation, and p ro p e r foundation design precludes excessive differential settlement of abutments. If the foundation does not settle, the abutment will not deflect, rotate, or slide due to arch loading. A continuous load path must be followed for loadbearing elements of construction. The load p a t hi st h ep a t ha l o n gw h i c hl o a d s are imposed from the arch to the abutments. Then the loads are transferred from the abutments to the foundation wall (which bears on footings) to the surrounding soil. Differential settlement of foundations can become a problem if the foundations are designed by “guess.” This phenomenon is caused by the relative direction of the settlement and location of the settlement with respect to wall length. It could be the result of improperly preparing the soil on which the foundation abutments are bearing; the use of inappropriate soils based on design conditions for bearing-load purposes; or not defining all loading conditions (seismic events, building alterations, or excavation work) in the design of the foundation wall system. These are just some of the potential causes of d i ff e re ntial settlement. Proper design of the foundation should be based on engineering principles through the use of ACI 530/ASCE 5/TMS 402 Building Code Requirements for Masonry Structures (MSJC Code) for masonry and ACI 318 Building Code Requirements for Structural Concrete and Commentary for concrete. Proper foundation design should follow the principles of mechanics and engineered design. It is always material-specific. There are certain allowable loads for masonry that are different from concrete, and there are certain allowable loads for materials based o nt h e t y p eo fc o n s t ru c t i o n ,w h e t her the foundation is of a hollow or solid nature, what type of mortar is used in construction, and so on. The use of design standards is necessary for proper design of foundation wall systems. Standards determine whether reinforcing is necessary or not, due to the prescribed building loads that must be considered during the design phase of a project. “One method of strengthening an abutment, if it does not interfere with the interior design of the building, is to have a brick pilaster attached to it,” says Kreh. It is also a good idea to have the abutment reinforced with steel rods and concrete in the center. The larger mass of the abutment will allow it to withstand the compressive and lateral pressure exerted against it.” Proper construction of masonry foundation wall systems involves the complete filling of all mortar joints intended to receive mortar. “If the back of the arch does not show, it would help to parge (plaster a coat of mortar to) the back of the a rc h to ensure that all mortar joints are filled solid,” advises Kreh. Also, the complete filling of all spaces designated to be grouted, as for reinforced masonry, is necessary. All mortar and grout materials should be mixed properly, and any necessary reinforcing steel should be placed properly. Masonry should be erected within prescribed tolerances. “Bond strength and adhesion of mortar to the brick or stone are very important in a masonry arch to prevent moisture f ro m entering the joints,” says Kreh. These are just a few of the critical items for successful-performing masonry foundations. Besides poor foundation design and construction, other likely causes of differential settlement of foundations are earthquakes, soil failures, building alterations, and adjacent excavation work. Where the arch span is larger, the foundation can settle and cause the failure of the arch system. In one arch examined, excavation work was done beneath the building to the right of the arch. There was a lot of settlement of the building, and many cracks appeared in the arch ring, in the masonry above the arch, and in the abutments. Two structural steel tubes were installed temporarily to secure the arch system until it was repaired. “Even though the abutment may rest on a good solid foundation or footing, it is very important that the soil or earth around it is welldrained to prevent any erosion that could result in the shift or movement of the masonry abut- ment,” says Kreh. The complete picture The stability of a building arch depends on the total arch system (arch ring, abutment, spandrel, masonry above the arch, and the location of other wall openings), and not solely on the properties of the arch ring. “Good masonry workmanship and practices have to be followed in arch construction,” says Kreh. “It is especially important that the mortar joints between the voussoirs that form the a rc hr i n ga re filled completely.” Abutment displacement and slippage of masonry units does not always cause collapse of the arch. Even for sizeable abutment displacements, collapse can be avoided if the voussoirs or the mortar joints between the voussoirs are sufficiently tapered, the rise of the arch is sufficient for the arch span and type, and the skewback angle of the arch is not greater than about 65 degrees. As a result of this visual classification system study (and other research), the Reston, Va.-based BIA and the Vienna, Va.-based Consulting Engineers Corp. created a computer program and manual, called ARCH, to be used for the structural analysis of unreinforced brick masonry segmental, semicircular, and jack arches. References 1. Thomas E. Boothby, Scott E. Nelson, and Matthew J. Scolforo, “A Visual Classification System for Masonry Arch Failures,” presented at the 10th International Brick Masonry Conference, Calgary, Canada, July 1994. (Arches supported by steel lintels or other structural members were not included in the survey.) Copies of the proceedings are available from The Masonry Society, 3775 Iris Ave., Suite 6, Boulder, CO (303-939-9700). 2. BIA Technical Notes, 31 Revised, pages 8-9, Expansion Joints, Brick Institute of America, 11490 Commerce Park Dr., Reston, VA 22091 (703-620-0010). 3. BIA Technical Notes, 31A, Brick Institute of America. PUBLICATION #M970193 Copyright © 1997, The Aberdeen Group All rights reserved