ARTISTIC, STRUCTURAL PROJECTION RICHLY DRESSING BUILDINGS by ROCHELLE C JAFFE, AR, SE, FTMS, CCS, SMI Limitations on corbelling: Pc ≤ t/2 p ≤ h/2 p ≤ d/3 Where: Pc = allowable total horizontal projection of corbelling p = allowable projection of one unit t = nominal wall thickness d = nominal wall thickness (specified thickness plus the thickness of one mortar joint) h = nominal unit height (specified height plus the thickness of one mortar joint) Note: Neither ties nor headers shown Figure 1. Limits on corbelling in solid walls Reproduced from Commentary on Building Code Requirements for Masonry Structures TMS 402-08/ACI 530-08/ASCE 5-08, Figure CC-1.12.1 Limitations on corbelling: Pc ≤ d/2 p ≤ h/2 p ≤ d/3 Where: Pc = allowable total horizontal projection of corbelling p = allowable projection of one unit d = nominal unit thickness (specified thickness plus the thickness of one mortar joint) h = nominal unit height (specified height plus the thickness of one mortar joint) a = air space thickness Ties shown for illustration only 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-1.12.2 34 the story pole ROI/ADDING VALUE TO COMMUNITIES 2009 Distinctive 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: The maximum amount that one course can project out from the outside face of the course below is limited to one- half 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. t t Corbel c the
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ARTISTIC, STRUCTURALPROJECTION RICHLYDRESSING BUILDINGS by ROCHELLE C JAFFE, AR, SE, FTMS, CCS, SMI
Limitations on corbelling:
Pc ≤ t/2
p ≤ h/2
p ≤ d/3
Where:
Pc = allowable total horizontal
projection of corbelling
p = allowable projection of one unit
t = nominal wall thickness
d = nominal wall thickness
(specified thickness plus the
thickness of one mortar joint)
h = nominal unit height
(specified height plus the
thickness of one mortar joint)
Note: Neither ties nor headers shown
Figure 1. Limits on corbelling in solid wallsReproduced from Commentary on Building Code Requirements for Masonry Structures TMS 402-08/ACI 530-08/ASCE 5-08, Figure CC-1.12.1
Limitations on corbelling:
Pc ≤ d/2
p ≤ h/2
p ≤ d/3
Where:
Pc = allowable total horizontal
projection of corbelling
p = allowable projection of one unit
d = nominal unit thickness
(specified thickness plus the
thickness of one mortar joint)
h = nominal unit height
(specified height plus the
thickness of one mortar joint)
a = air space thickness
Ties shown for illustration only
Figure 2. Limits on corbelling in walls with air spaceAdapted from Commentary on Building Code Requirements for Masonry Structures TMS 402-08/ACI 530-08/ASCE 5-08, Figure CC-1.12.2
34 the story pole ROI/ADDING VALUE TO COMMUNITIES 2009
Distinctive
A projection of successive courses ofmasonry, upward and outward, from theoutside face of the masonry wall, a corbeladds visual interest to a wall. Corbels are commonly found near the top of the wallto form a cornice, or at floor lines to form a water table.
Because corbelling introduces eccentricityand bending stresses into the masonry, the Code (TMS 402/ACI 530/ASCE 5Masonry Standards Joint Committee“Building Code Requirements for MasonryStructures”) limits the outward projectionof a corbel. Prescriptive provisions (rulesof thumb) are provided therein for corbelsin masonry that is not loadbearing. Corbelsin masonry that is loadbearing are requiredto be engineered in accordance with theallowable stress design, strength design, or
prestressed design chapters of the Code.
Code ChangesPrior to the 2008 edition of the Code, corbels were only permitted to be constructed in masonry walls of solidunits. A change in the 2008 code permitseither solid units or hollow units that aresolidly filled with mortar or grout to beused. Corbels can be formed in solidwalls, single wythe (vertical layer ofmasonry units) walls, or hollow walls.
Prescriptive Code limitations on corbelprojections include the maximum projection of a single course (horizontalrow of masonry units) and the maximumtotal projection of the corbel. Those limitations are as follows:
The maximum amount that one course can project out from the outside face of the course below is limited to one-half the nominal height of the unit andone-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 thewall below is limited to one-half the wallthickness for solid walls and is limited toone-half the wythe thickness for singlewythe walls and hollow walls.
tt
Corbel
c
the
ROI/ADDING VALUE TO COMMUNITIES 2009 the story pole 35
• Solid walls include multi-wythe walls in which the wythes arebonded either by unit headers or by metal ties across a solidlyfilled collar joint.
• Single wythe walls include the veneer wythe or a single wytheof solid or hollow units. Hollow walls include masonry-bondedhollow walls and cavity walls (multi-wythe walls with an air space between wythes).
For single wythe walls and hollow walls, there is an additional require-ment that the back face of the corbelled section of masonry remainplane within one inch. That is, it is not permissible to step out the unitsin each course
by more than one inch without filling in the space that is createdbehind the corbelled course. Ideally, the back face of the corbelledmasonry should be maintained in plane, but the Code provisionincludes recognition of construction tolerances and the impracticalityof filling small slivers of masonry.
The prescriptive corbel limitations imposed bythe Code for masonry that is not loadbearing are illustrated in Figures1 and 2.
Photo courtesy of CW
S Architects
Corbels add aesthetic interest to walls of the Bay Area Transit Authority Center, Traverse City. Corbel details arebased upon historical patterns found at nearby old state hos-pital constructed in the early twentieth century by easternEuropean immigrant masons. This paticular corbelling patternadded depth and shadowplay at the top of the wall whilehelping to direct water dripping from the cornice away from the expansive windows on the wall below.
“A change in the 2008 code permits either solid units or hollow units that are solidly filled with mortar or grout to be used.”
Rochelle C Jaffe, Senior Vice President/Quality Officerin Asset Preservation at NTH Consultants, Ltd in Northville,specializes in investigation, evaluation and rehabilitation ofexisting, deteriorated and damaged masonry structures.She has over 25 years experience in this field. A licensedarchitect (MI,IL) and licensed Structural Engineer (IL), she isalso a Certified Construction Specifier (by CSI) and aCertified Special Inspector of Structural Masonry (by ICC).
Jaffe holds a Master of Architecture Degree (structures) fromthe University of Illinois,Chicago. She received her Bachelorof 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
member of the Board of Directors, past Editor of the Journal,serves on the Design Practices Committee, ConstructionPractices Committee and is past chairperson of the ArchitecturalPractices Committee. She has actively participated in the TMS/ACI/ASCE Masonry Standards Joint Committee, the groupthat authors the Building Code Requirements for MasonryStructures (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 reportsrelated to investigations of masonry structures with material,construction and architectural or structural design deficiencies.She has spoken at a variety of seminars and other technicalfunctions sponsored by national and local organizations.248-324-5257, [email protected].
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