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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
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