The Distinctive Corbel - Masonry Institute of Michigan

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Distinctive
Corbel
A projection of successive courses of
masonry, upward and outward, from the
outside face of the masonry wall, a corbel
adds visual interest to a wall. Corbels are
commonly found near the top of the wall
to form a cornice, or at floor lines to
form a water table.
Because corbelling introduces eccentricity
and bending stresses into the masonry,
the Code (TMS 402/ACI 530/ASCE 5
Masonry Standards Joint Committee
“Building Code Requirements for Masonry
Structures”) limits the outward projection
of a corbel. Prescriptive provisions (rules
of thumb) are provided therein for corbels
in masonry that is not loadbearing. Corbels
in masonry that is loadbearing are required
to be engineered in accordance with the
allowable stress design, strength design, or
prestressed design chapters of the Code.
Code Changes
Prior to the 2008 edition of the Code,
corbels were only permitted to be
constructed in masonry walls of solid
units. A change in the 2008 code permits
either solid units or hollow units that are
solidly filled with mortar or grout to be
used. Corbels can be formed in solid
walls, single wythe (vertical layer of
masonry units) walls, or hollow walls.
Prescriptive Code limitations on corbel
projections include the maximum
projection of a single course (horizontal
row of masonry units) and the maximum
total projection of the corbel. Those
limitations are as follows:
ARTISTIC, STRUCTURAL
PROJECTION RICHLY
DRESSING BUILDINGS
by ROCHELLE C JAFFE, AR,
SE, FTMS, CCS, SMI
The maximum amount that one course
can project out from the outside face
of the course below is limited to onehalf the nominal height of the unit and
one-third the nominal thickness of the
unit or wythe.
• The nominal unit height is the
specified unit height plus the
specified mortar bed joint thickness.
• The nominal unit thickness is the
specified unit thickness plus the
specified mortar joint thickness.
The maximum amount that the total
corbel can project out from the face of the
wall below is limited to one-half the wall
thickness for solid walls and is limited to
one-half the wythe thickness for single
wythe walls and hollow walls.
Limitations on corbelling:
Limitations on corbelling:
Pc ≤ t/2
Pc ≤ d/2
p ≤ h/2
p ≤ h/2
p ≤ d/3
p ≤ d/3
Where:
Where:
Pc = allowable total horizontal
Pc = allowable total horizontal
projection of corbelling
projection of corbelling
allowable projection of one unit
p = allowable projection of one unit
t=
nominal wall thickness
d = nominal unit thickness
d=
nominal wall thickness
p=
(specified thickness plus the
thickness of one mortar joint)
(specified thickness plus the
h = nominal unit height
thickness of one mortar joint)
h=
(specified height plus the
nominal unit height
thickness of one mortar joint)
(specified height plus the
a = air space thickness
thickness of one mortar joint)
Ties shown for illustration only
Note: Neither ties nor headers shown
c
Reproduced from Commentary on Building Code Requirements
for Masonry Structures TMS 402-08/ACI 530-08/ASCE 5-08,
Figure 2. Limits on corbelling in walls with air space
Adapted from Commentary on Building Code Requirements
for Masonry Structures TMS 402-08/ACI 530-08/ASCE 5-08,
Figure CC-1.12.1
Figure CC-1.12.2
Figure 1. Limits on corbelling in solid walls
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• Solid walls include multi-wythe walls in which the wythes are
bonded either by unit headers or by metal ties across a solidly
filled collar joint.
• Single wythe walls include the veneer wythe or a single wythe
of solid or hollow units. Hollow walls include masonry-bonded
hollow walls and cavity walls (multi-wythe walls with
an air space between wythes).
Photo courtesy of CWS Architects
For single wythe walls and hollow walls, there is an additional requirement that the back face of the corbelled section of masonry remain
plane within one inch. That is, it is not permissible to step out the units
in each course
“A change in the 2008 code
permits either solid units or
hollow units that are solidly filled
with mortar or grout to be used.”
by more than one inch without filling in the space that is created
behind the corbelled course. Ideally, the back face of the corbelled
masonry should be maintained in plane, but the Code provision
includes recognition of construction tolerances and the impracticality
of filling small slivers of masonry.
The prescriptive corbel limitations imposed by
the Code for masonry that is not loadbearing are illustrated in Figures
1 and 2.
Rochelle C Jaffe, Senior Vice President/Quality Officer
in Asset Preservation at NTH Consultants, Ltd in Northville,
specializes in investigation, evaluation and rehabilitation of
existing, deteriorated and damaged masonry structures.
She has over 25 years experience in this field. A licensed
architect (MI,IL) and licensed Structural Engineer (IL), she is
also a Certified Construction Specifier (by CSI) and a
Certified Special Inspector of Structural Masonry (by ICC).
Jaffe holds a Master of Architecture Degree (structures) from
the University of Illinois,Chicago. She received her Bachelor
of Science Degree in Liberal Arts and Sciences (mathematics)
from the University of Illinois, Champaign-Urbana.
Jaffe is a member of the American Concrete Institute (ACI)
and American Society of Civil Engineers (ASCE). Now a
Fellow member of The Masonry Society (TMS), she is past
Corbels add aesthetic interest to walls of the Bay
Area Transit Authority Center, Traverse City. Corbel details are
based upon historical patterns found at nearby old state hospital constructed in the early twentieth century by eastern
European immigrant masons. This paticular corbelling pattern
added depth and shadowplay at the top of the wall while
helping to direct water dripping from the cornice away from
the expansive windows on the wall below.
member of the Board of Directors, past Editor of the Journal,
serves on the Design Practices Committee, Construction
Practices Committee and is past chairperson of the Architectural
Practices Committee. She has actively participated in the TMS/
ACI/ASCE Masonry Standards Joint Committee, the group
that authors the Building Code Requirements for Masonry
Structures (TMS 402/ACI 530/ASCE 5), for more than
20 years.
Jaffe has authored or co-authored numerous magazine
articles and texts, as well as numerous technical reports
related to investigations of masonry structures with material,
construction and architectural or structural design deficiencies.
She has spoken at a variety of seminars and other technical
functions sponsored by national and local organizations.
248-324-5257, rjaffe@nthconsultants.com.
ROI/ADDING VALUE TO COMMUNITIES 2009
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