A VISUAL CLASSIFICATION SYSTEM FOR MASONRY ARCH FAILURES

Thomas E. Boothbyl, Scott E. Nelson2, and Matthew 1. Scolforo3

1. ABSTRACT

Based on field survey, visual observation, and photographic record of over 70 masonry arehes, the prevailing failure patterns of masonry arehes on eommereial and residential buildings are identified. Most of the arehes surveyed are loeated in Alexandria, V A, Bellefonte, P A, Boston, MA, Georgetown, DC, and State College, PA Areh spans ranged from 1 m to 6.7 m (3 fi to 22 fi). Both load-bearing masonry and masonry veneer systems are investigated. In ali cases, only "struetural" arehes were eonsidered; that is, arehes supported by steel lintels or other struetural members were not included in our survey. Furthermore, only failures of unreinforeed masonry arehes were considered. The failures are classified by the type of rigid body mechanism observed, whieh is suggested by the eraeking and movement patterns of the masonry. The most prevalent failure modes and likely causes of failure are identified for the development of analysis procedures for masonry building arehes.

Keywords: Masonry, Areh, Failure, Briek, Stone.

lAssistant Professor, Department of Arehiteetural Engineering, The Pennsylvania State University, University Park, PA 16802.

2Graduate Student, Department of Arehitectural Engineering, The Pennsylvania State University, University Park, PA 16802.

3Staff Engineer, Briek Institute of America, Reston, V A 22091.

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2. INTRODUCTION

The mechanics of masonry arehes have been interpreted in a number of ways, most often foeusing on the behavior of the areh ring. The areh ring has been modelled eitber as a eontinuous, elastie member [1], or as an assembly of voussoirs, whieh ean be analyzed by the theory of rigid plastic struetures [2,3,4]. Most of the Iiterature on masonry arehes foeuses on bridge arehes. A very comprehensive overview of masonry bridge areh analysis metbods is given in Heyman [5]. Methods of analysis of bridge arehes bave been tailored · to typieal bridge configurations and loading conditions. More searce is Iiterature on the analysis of masonry building arehes [6,7,8]. Heyman also diseusses the behavior of stone masonry arehes in Gothie eathedrals [5]. The behavior of masonry arehes as a part of a building wall ean be quite eomplex, and is often a significant departure from the behavior of masonry bridge arehes. The eorrect interpretation of this behavior requires an understanding of the entire areh system; areh ring, spandrel, masonry above the areh, abutments, adjacent openings, and attaehed elements. In this study, the most common failures of areh systems in building applieations have been doeumented with the intention of grouping observed failures into eategories of behavior.

There are many varieties of masonry arehes, but the most eommon arehes used in residential and commereial building construetions are the jaek (flat) areh, segmenta I areh, semi-cireular areh, and eIliptieal areh. In our survey of masonry arehes, at least one example of areh failure was doeumented for eaeh of these areh varieties. So that the pieture is not skewed to only failures, we ean also say that many more than just one example of eaeh of these masonry areh types was observed in exeellent condition. As the reader may suspeet by the geographie areas surveyed, most of the areh failures doeumented were on older masonry buildings. The age of buildings surveyed ranged from newly construeted to the eighteenth century.

Loading conditions, forrn of building eonstruetion, and age of the building, or combinations of these faetors, are directly responsible for the failures we observed. Most of the masonry areh failures we observed are failures which affeet the serviceability of the masonry wall, rather than total areh eollapse. On older buildings, we observed many cases of eraeking of the masonry in the areh ring and above the areh and evidenee of repointing in these areas. While eraeking does not often cause collapse of the areh, it certainly impaets the water and air penetration resistance of the wall system and is an appearance issue.

The intent of this paper is to group the failure mechanisms that were observed into definable eategories. The failure eategories established are all assoeiated with displacement of the abutments. Theyare; 1) abutment displacement due to moisture and therrnal expansion of the masonry, 2) insufficient abutment stiffness, and 3) foundation settlement of abutments. Eaeh eategory eontains a variety of failure meehanisms of the areh system, whieh are diseussed in the following sections.

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3. ARCH FAILURE DUE TO MASONRY EXPANSION

The most commoo cause of slippage of voussoirs aod crackiog of masoory above the arch is attributed to moisture aod thermaI expaosioo of the masoory. OIder masoory buildings very ofteo do oot have expaosioo joints or other meaos of addressing the moisture and thermal expaosioo of masoory. Ofteo times, crack patterns arouod arches aod other waIl openiogs indicated unaccommodated expaosion of the masoory. Moisture and thermal expaosion of the masoory had the effect of lateral spreading of the arch abutments, which caused cracking aod voussoir slippage in the arch ring. Some window arches we observed had slipped to the point where they were being supported by the wooden frame of the window, as shown in Fig. 1.

- ----- ----­~.""'*'

.-----Fig. 1 - The voussoirs have displaced and are carried by the window frame.

Fig. 2 - A single voussoir near the crown of the arch has slipped downward.

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Slipping failure occurs when there is insufficient tensile bond strength and little frictional resistance between voussoirs in the arch ring. Tensile bond strength between voussoirs is either overcome by the abutment movement or is lost due to mortar deterioration. As a result, a voussoir or a series of voussoirs may slip from the initial position, as shown in Figs. 2 and 3, respectively.

,"

Fig. 3 - A sizeable portion of the arch ring has slipped. Notice the triangular area of masonry above the arch defined by the cracking pattem.

We observed that slippage occurs more frequently when one or more of the following is the case; 1) the voussoirs or mortar joints between voussoirs are not tapered, 2) the arch rise is toe small for the span and arch type, and 3) the skewback angle is greater than about 65 degrees. The potential for slippage failure reduces as the number of masonry units for a given span decreases, i.e. larger size voussoirs are used. This is because larger masonry units are typically cut at a greater taper, which increases the frictional resistance between the voussoirs. But even in arch systems where only a few large blocks were used, slippage failure was observed in this survey, as shown in Fig. 4.

Fig. 4 - A skewback angle of 85 degrees has caused slippage near the springing.

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4. INSUFFICIENT ABUTMENT STIFFNESS

Insufficient abutment stiffness is the most likely cause of total collapse of the arch. This is because deflection of the abutments will occur immediately afier the arch shoring is removed, without warning of distress. Surprisingly, most segmental arches we observed could withstand considerable deflection of the arch abutments without collapse. The arch abutment shown in Fig. 5 is constructed of four inch face brick attached to wood studs with corrugated metal ties.

Fig. 5 - Rotation of tall porch columns has caused abutment spreading and arch cracking.

The rigidity of the column connection at the base and the width of brickwork were insufficient for the height of the columns and the thrust force from the segmenta I arch, and rotation has taken place at the springing. This form of construction is common for brick masonry porch columns built today. While not documented in our survey, collapse of this form of construction has occurred when abutment rotations were too large [9].

Other examples of insufficient abutment stiffness were observed. The wood post abutment shown in Fig. 6 is too weak to resist the thrusts from the two segmenta I arches it supports. Cracks were observed in the arch ring and masonry above the arch to the right.

Even rather sizeable solid brick masonry arch abutments can be insufficient, depending on the arch thrust force. The abutments of the elliptic arch shown in Fig. 7 have displaced 50 mm (2 in.) from plumb at the spring line, and cracks were observed at the base of both abutments. Heavy stone pieces have added to the weight loading this arch and, consequently, the thrust force. Despite the lateral spreading of abutments, this 1810 arch still stands.

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Figo 6 - Rotation of the ornamental wood post has induced slippage of voussoirs in the arch to the right

Figo 7 - This brick and stone masonry archway has experienced spreading of the abutmentso

50 FOUNDATION SETTLEMENT OF ABU1MENTS

Differential ·settlement of the foundations of abutments of an arch can cause failure of the arch systemo Documentation of building arch failures due to differentiaI foundation settlement of abutments is not commono This is beca use spans are short, abutments typically rest on the same foundation, and proper foundation design precludes excessive differential settlement of abutmentso Besides poor foundation

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design and construction, likely causes of differential settlement of foundations are earthquakes, soil failures, building alterations, and adjacent excavation work. All of these, obviously, are not common events.

In the case of the larger span arch shown in Fig. 8, foundation settlement caused failure of the arch system. The arch failure was caused by excavation work beneath the building to the right. A sizeable amount of settlement of the building occurred, and this caused the myriad of cracks seen in the arch ring, masonry above the arch, and abutments. Two structural steel tubes have been installed to temporarily secure the arch system until it can be repaired.

'" / ')

liiI ~ '" ~ \ ~

Fig. 8 - The crackíng pattern in this arch is a clear reflection of the sizeable settlement of the building to the right Notice the magnitude of settlement by the displacement of the parape!.

6. INTERPRETATION

The method of analysis of masonry arches promoted by most researchers today is by investigating potential collapse mechanisrns. The collapse mechanism behavior of a masonry arch with rigíd abutments has been explained in detail by Kooharian [2], Heyman [3, 5] and others for hingíng mechanisms, as íllustrated in Fig. 9. This form of arch system failure mechanism has proven accurate for the analysis of masonry bridge arches. The fundamental idea of the collapse mechanÍSm is that, in order for the arch to fail, the arch must be in equilibrium in an admissible mechanism.

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(a) Hinging Mechanism (b) Sliding Mechanism

Fig. 9 - Collapse mechanisms of a masonry arch with rigíd abutments

Using this method of analysis, two conditions must existo First, that part of the arch-Ioad system moving downwards must release enough energy to overcome the weight of the adjacent parts of the arch being lifted upward to form a hinge mechanism. Second, the system must overcome the friction at the joints to permit sliding. Again, this collapse mechanism is based on an assumption of rigíd abutments, although the mechanism analysis can be modified to account for abutment movements [7].

In this investigation, there is little evidence of the collapse mechanism shown in Fig. 9. It is apparent fiam the failure mechanisms documented that lifting of portions of the arch ring does not take place in most building arches. Rather, abutment displacement induced failures have been observed. This is notable, because it requires a change in thinking with regard to arch analysis techniques for masonry building arches. For masonry bridge arches, assuming the abutments are rigíd makes sense, even though bridge abutments may likely displace. This is due to the large size of bridge arches and the magnitude of loads bridge arches carry. For building arches, however, abutment displacements and loss of mortar joint integrity are common concerns and the principal causes of arch failures.

7. CONCLUSIONS

From this visual study of common failure mechanisms for masonry building arches, we propose the following conc1usions.

1) The stability of a building arch depends upon the total arch system; arch ring, abutment, spandrel, masonry above the arch, and location of other wall openings, and not solely on the properties of the arch ring.

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2) The most common eause of failure of masonry building arehes is by abutment displaeement. Abutment displacement is eaused by moisture and thermal expansion of the masonry, deformatÍon due to areh thrust, and differentÍal foundation settlement.

3) Failure of the areh by slipping of voussoirs or whole seetions of the areh ring is the most common meehanism of failure observed on older masonry buildings.

4) Abutment displacement and slippage of masonry units does not always cause coUapse of the areh. Even for sizeable abutment displacements, eollapse is avoided if the voussoirs or the mortar joints between the voussoirs are suffieiently tapered, the rise of the areh is suffieient for the areh span and type, and the skewbaek angle of the areh is not greater than about 65 degrees.

The authors are eurrently developing a method of analysis for masonry building arehes based on the results of this survey. The analysis method wiU concentrate on the eonditions at the abutments and the configuration of the areh ring. The analysis method will reflect the tensile bond strength of the masonry, deterioration of mortar joints over time, taper of voussoirs and mortar joints between voussoirs, rise of the areh in relation to the span and areh type, and skewbaek angle. An imposed amount of horizontal abutment displaeement, whieh will simulate moisture and thermal expansion of the masonry, may become a mandatory analysis eonsideration for masonry building arehes in light of this survey.

8. REFERENCES

1. Pippard, AJ.S., Trantor, E. and Chitty, L "The Meehanics of the Voussoir Areh," Joumal of the Institution of Civil Engineers, v.4, 1936, pp. 281-306.

2. Boothby, T.E. and Brown, C.B., "Stability of Masonry Areh Piers and Arehes," Joumal of Engineering Meehanics, vol. 118, no. 2, February 1992, pp. 367-383.

3. Heyman, 1., "The Safety of Masonry Arehes," Intemational Joumal of Mechanical Seience, vol. 11, no. 4, 1969, pp. 363-385.

4. Kooharian, A, "Limit Analysis of Segmental and Conerete Arehes," Proceedings of the Ameriean Conerete Institute, vol. 84, no. 4, 1952, pp. 317-328.

5. Heyman, J., "The Masonry Areh, " Ellis Horwood Limited, Chiehester, England, 1982, 11 7 pp.

6. Boothby, T.E., "Stability of Masonry Areh Piers and Arehes Ineluding Sliding," Joumal of Engineering Meehanics, vol. 120, no. 2, February 1994.

7. Hendry, A W., "Struetural Masonry," Maemillan Edueation Ltd., London, England, 1990, pp. 246-263.

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8. "Technical Notes on Brick Construction," Nos. 31, 31A, and 31B, Brick Institute of Ameriea, Reston, VA, 1986.

9. Bailey, W.G., "FalJen Arehes," Magazine of Masonry Construetion, Oetober 1993, pp. 456-459.

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