2010.09.08 - key considerations for the attachments of facades to structures
DESCRIPTION
fachadas en estructurasTRANSCRIPT
Key Considerations for the Attachment
of Facades to Structures
James Parker, PE, SE
Simpson Gumpertz & Heger Inc.
www.FindYourTechnology.com 2
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University of Massachusetts Medical Center
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AISC Design Guide 22
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Design Guide Objective
� To assist the practicing engineer in achieving
slab edge and spandrel beam details for
steel frames that are:
� Structurally sound
� Durable
� Economical
� Accommodating of façade requirements
Today’s Agenda
1. Fundamentals of Façade Performance
2. Design Criteria
3. Roles and Responsibilities
4. Tolerances
5. Slab Edges
6. Spandrel Beams
7. Masonry Veneer, PC Panels, Aluminum
Curtain Walls
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Three Key Take-Aways1. The design team needs to develop a
strategy for façade attachment and the
SER has a role in its development.
2. The current ASCE 7 and IBC have
explicit criteria for façade attachments
especially for seismic considerations.
3. The façade attachment strategy chosen
by the team will affect the design of slab
edges and spandrel beams.
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Fundamentals of Façade Performance
The building envelope encloses the building,
controlling the transmission of air, water, heat,
sound, and light both into and out of the building.
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The Façade and the Building Enveloperoofing
walls
foundations
doors
windows
all interfaces
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Fundamentals of Façade Performance
ReservoirReservoirReservoir BarrierBarrierBarrierCavityCavityCavity
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0 1000 A.D. 20001000 B.C.2000
Time Line
Load Bearing Masonry
Contemporary Curtain Walls
Transitional Masonry
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0 1000 A.D. 20001000 B.C.2000
Load Bearing Masonry
1250 B.C. 430 B.C. 530 A.D.
1220 to
1472
A.D.1796
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Load Bearing Masonry
• Walls are Thick
• Walls Support all Loads
• Floor “Rides” with Walls
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Load Bearing Masonry
• Walls Functions
as a Reservoir
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0 1000 A.D. 20001000 B.C.2000
Transitional Masonry
Time Line
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1900 19501890 1910 1920 1930 1940
Transitional Masonry Buildings
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Transitional Masonry Buildings
� Masonry Walls Still
Thick
� Floor & Wall Loads
Carried by
Structural Frame
�Wall Still Functions
as a Reservoir
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Transitional Masonry Buildings
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0 1000 A.D. 20001000 B.C.2000
Contemporary Curtain Walls
Time Line
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Contemporary Curtain Walls
20001950 1960 1980 199019701918
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Contemporary Curtain Walls
• Floor Loads Carried by Frame
• “Skin” Transfers Wind Loads to the Frame
• “Skin” Employs a Drainage Plane and Back-up Waterproofing
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Contemporary Curtain Walls
• “Skin” and Frame
Detailed to
Accommodate
Differential Movement
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Functional Components of the Exterior
Wall System
� Cladding
� Joints
� Insulation
� Water barriers and air
barriers
� Structure
� Interior finishes
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Concepts for Control of Water
Infiltration
� Barrier Walls
� Internal Drainage
Planes
� Cavity Walls
� PE Rain Screens
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Barrier Walls
� Still used today
�Old EIFS
�GFRC panels
� Precast wall panels
(but a bit of a reservoir
too.)
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Internal Drainage Planes
� Stucco
� Drainable EIFS
� Wood siding
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Cavity Walls
� Brick veneer
� Stone veneer
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Pressure Equalized Rain
Screens� Terracotta cladding
� Stone cladding
� Metal panel cladding
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Problems Associated with Support
and Anchorage
� Anchors or support clips interrupt the flashing or
water barrier without proper repair.
� Anchors causing conditions of poor drainage.
� Anchors not stiff enough to prevent differential
movement that tears barriers.
� Damage to barriers during erection and
installation.
� Constructability issues, coordination of trade
issues.
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General Design Criteria
Façade/Wall System Criteria
1. structural integrity;
2. provisions for movement; and
3. envelope performance.
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Primary Criteria for Attachment
� Structural Integrity
� Accommodating Movement
� Durability
� Accounting for Tolerances and Clearances
� Constructability
� Economy
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Conflicting Ideas
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Example Resources(there are others)
� Building codes
� 2006 IBC
� ASCE 7-05
� Material Standards/
Organizations
� AISC
� PCI
� ACI
� BIA
� TMS
� Façade Trade
Associations
� AAMA, Aluminum
Curtain Walls for
example
� Papers, Texts
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Structural Integrity
RedundancyDuctility
Strength
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Gravity Loads
� Façade dead load� Need to understand materials and system
� Façade live loads� Horizontal projections
� SER usually needs to estimate before wall is designed.
� Window washing activities.
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Gravity Load Eccentricities
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Wind Loads
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Wind Loads
� Wind tunnel testing
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Wind Loads
� Negative pressures combined with gravity eccentricities often control attachment design.
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Seismic Requirements
1. Seismic Forces
2. Relative Displacements
3. Ductility
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Seismic Loads
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Seismic Loads
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Seismic Loads
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Seismic Loads
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Seismic Loads
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Seismic Loads
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Seismic Loads
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Seismic Loads
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Seismic LoadsRelative Seismic Displacement
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Relative Seismic Displacement
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Forces from Restraint
� Best to avoid restraint
altogether.
� Predicting restraint
forces inexact.
� Cracking
� Creep
� Attachment stiffness.
� Watch out for
inadvertent restraint.
Bldg. Exp. Joint
Slotted Insert
Panel Joint
This bolt
fractured and
panel fell.
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Accommodating Relative Movement
� Spandrel deflections,
� Spandrel rotations,
� Column shortening,
� Bracket deflections,
� Inter-story dirft,
� Façade thermal,
� Façade moisture,
� Façade deformation
due to forces.
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Accommodating Relative Movement
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Accommodating Relative Movement
� Rules of thumb and code provisions for
flexural stiffness control façade material
cracking.
�L/360; L/600; etc.
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Accommodating Relative Movement� Joint criteria may control –example:� Say ¾ inch joint; allowable movement of ¼ inch, M=.33
� Say thermal and moisture is 1/8 inch; leaving 1/8 inch for structural movement.
� Say design load movement from 50% LL; then 100% LL allowable movement = ¼inch.
� This is L/960 and L/1440 on 20 ft and 30 ft spans, respectively.
Add slide of protruding sealant
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Accommodating Relative Movement
Inter-story Drift from Lateral Loads
� Common drift limits:
�Wind� H/400 (.0025H); or H/500 (.002H)
�Seismic� .025 H (10 times wind!)
� For a 12 ft story height:�Wind – 0.36 inches (but not less than ½”)
�Seismic – 3.6 inches
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Accommodating Relative Movement
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Accommodating Relative Movement
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Inter-Story Drift: Corners
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Limit States
� Code prescribed forces for safety:
�50 yr. recurrence interval for wind
�475 yr. recurrence interval for seismic
� Attachments must safely accommodate
forces.
� Joints must prevent hazardous damage;
falling hazards.
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Limit States
� Serviceability checks may allow lower
forces and drifts; for example joint sealant
movements.
� ASCE 7-02 Commentary suggest: D +
0.5L + 0.7W
�0.05 annual probability of being exceeded.
�0.72 probability of being exceeded in 25 yrs.
�0.92 probability of being exceeded in 50 yrs.
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Shear and Flexural Deformations
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Tolerances and Clearances
� Tolerances:
�Permissible amount of deviation from a
specified criterion: dimension, shape, location.
� Clearances:
�Space purposely provided between two parts
to allow for movement, accommodate
tolerances and provide access.
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Durability of the Attachment
� Attachments are usually hard to
inspect.
� Consider what happens if the
wall leaks.
� Consider how likely the wall is to
leak over time.
� Special attention to thin steel
parts or steel fasteners.
Constructability and Economy
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Summary of Criteria
� For Attachments:
�Structural Integrity
�Accommodating Movement
�Durability
�Accounting for Tolerances and Clearances
�Constructability and Economy
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Responsibilities for Façade
Attachments for New Buildings
� Owner
� Architect
� SER
� SSE
• Fabricator, Erector
• CM, GC
• Façade Contractor(s)
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Responsibilities
� SER (Structural Engineer of Record)
�For this presentation, we mean the design
professional responsible for the structural
design of the primary building structure.
� SSE (Specialty Structural Engineer)
�For this presentation, we mean the design
professional responsible for the structural
design of the façade and/or façade
attachments to the primary structure.
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Responsibility
� The design of the façade elements and
their attachments are often NOT in the
scope of the SER responsible for the
primary building frame.
�Yet the SER must understand the façade
system and the strategy for attachment to
design the primary structure.
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Responsibility
� The performance specified elements
including attachments will often be
designed by the SSE working for the
contractor (but could part of the design
team).
�The SSE may not become involved until after
the frame is designed, even fabricated, and
even erected!
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SER
� It’s in the project’s and SER’s best interest
to consult with the Architect about façade
attachments.
� Provides anticipated structural
movements.
� Designs frame and slab edge consistent
with attachment strategy.
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SER
� Structural drawings/specifications
� Delineate the structural elements from the attachment
items by the SSE.
� Indicate the assumptions/limitations of the locations
and magnitudes of the façade attachment loads.
� Indicate the fabrication and erection tolerances of the
frame.
� Provides sufficient adjustability in structural frame
details for façade attachment strategy chosen.
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Masonry
Veneer
Example
SER
SSE
Arch.
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Story-tall PC
Panel Example
SER
SSE
Arch.
SSE
SER
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Column Supported PC Spandrel
Panel Example
SSE
SER
SSE
SER
SSE
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Curtain Wall
Example
SSE
SSE
SER
SER
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Summary
� Communicate!
� Façade attachments are difficult because
every member of the design team has a
significant role in the planning, designing
and coordination.
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Accommodating Construction
Tolerances and Clearances
Adjustability must be provided between the
structural details and façade attachment details
to achieve a façade erected within acceptable
tolerances relative to the theoretical plane.
University of Southern Indiana
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Tolerances and Clearances
� Tolerances:
�Permissible amount of deviation from a
specified criterion: dimension, shape, location.
� Clearances:
�Space purposely provided between two parts
to allow for movement, accommodate
tolerances and provide access.
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Types of Tolerances
� Material Production Tolerances
� Fabrication and Assembly Tolerances
� Erection and Installation Tolerances
� Accumulated Tolerances
The AISC Design Guide includes summaries
of major façade materials and components.
The AISC Design Guide includes summaries
of major façade materials and components.
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Accumulated Tolerances
� Unlikely that all tolerances will vary to the
maximum allowed and all occur in the
same direction.
� However, no statistical data is usually
available to the designer about the
distribution of variation.
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Recommendations for Accumulated
Tolerances
� Understand the sources of variability.
� Understand the consequence of exceeding the
tolerance provisions in the details.
� Understand the costs associated with providing
means to accommodate the variability.
� For each project, the team should develop a
design criteria for addressing façade
accumulated tolerances.
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Slab Edge Conditions
The slab edge detail is an important
consideration when designing for façade
attachments.
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Factors that Influence the Design
� Type, weight and
location of façade
� Amount of slab
overhang
� Slab or deck capacity
� Application of façade
loads
� Similar conditions (or
not)
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Two Fundamental Approaches
� The slab or deck
cantilevers and picks
up load.
� The designer does
not count on the slab
or deck to carry loads.
Approach 1
Approach 2
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Approach 1: Slab Cantilever Resolves
Eccentricity
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Design of Slab Overhang
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Design of Slab Overhang
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Approach 2: Slab Cantilever Does Not
Resolve Eccentricity
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Kickers
1
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“Roll” Beams
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Design of Steel Spandrel Beams
The design of the spandrel beam is more than
selecting a wide flange shape that meets flexural
strength and stiffness criteria.
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General Design Considerations
� Flexural Strength
� Composite or
Noncomposite?
� Part of a Moment
Frame?
� Any weak axis
bending?
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General Design Considerations
� Flexural Stiffness
� Precomposite DL
� Post-composite DL
� Façade load
� Superimposed DL
� Superimposed LL
� Floor vibrations
� Creep, long-term
�Weak axis loads
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General Design Considerations
� Torsion?
� Resolved at columns?
� Kickers?
� Roll beams?
� Rotation and projected
translations?
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General Design Considerations
� Connection to
Columns
� Simple shear?
� Special copes, non
standard?
� Horizontal forces?
� Torsional forces?
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General Design Considerations
� Spandrel dimensions
� Depth
� Flange width
� Flange thickness
� Project consistency
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General Design Considerations
� Centerline location
� Column connections?
�Minimize façade
eccentricities?
� Clearances for
adjustments?
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Deflection and Movement Limitations
� Curvature
� L/360, L/400, L/600,
etc.
� Absolute magnitude
for joints
� Must consider rotation
as well as vertical
deformations
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Designing for Torsion
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Kickers remove torsion
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Kickers reduce torsion –
torsion in spandrel between
kickers.
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Roll beam reduces torsion.
Torsion in spandrel between
roll beam and columns.
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Design approaches
� Detailed guidance on
torsional stresses and
rotations of bare steel
wide-flange shapes.
� Rotation about center
of shape.
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Design approaches
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Center of Rotation
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Slab Resisting Torsion
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Effects of Rotation at Slab
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Modified Flexural Analogy
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Appendix A Study
� Three models� FEM
�Modified DG #9
�Modified Flex. Analogy
� Two spans� 10 ft
� 30 ft
� Two load shapes� Concentrated
� Uniform
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Conditions with Torsion
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Options for Increasing Rotation Resistance
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Masonry Cavity Walls
The strategy for supporting masonry cavity walls
starts with the decision for the location of the
horizontal movement joints.
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Volume Change
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Good Movement Joints
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Movement Joints
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Elevation
Strategies: Punched
Openings
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Strategies:
Strip Windows
Elevation
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Strategies:
Shelf at Slab
Elevation
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Vertical Movements
Design Vertical
Movements
Design Vertical
Movements
Note: Column shortening is important too for tall
building’s bottom story.
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Top of Wall Connections
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Plan Locations Of Hangers
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Potential Problems
� Inadequate provisions for the shelf angle adjustment cause the masonry to have too little bearing on the shelf angle.
� Inadequate provisions for the shelf angle adjustment cause the masonry cavity to be too wide for the specified masonry ties.
� Flashing design does not accommodate projection of bolts or fasteners into the cavity at the shelf angle.
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Potential Problems
� Inadequate soft joint size causes over
compression of sealant so that it bulges out.
� Inadequate clearance between the bottom of the
shelf angle and the top of the masonry veneer
results in bearing of the shelf angle onto the
masonry due to spandrel deflections.
� Support details for the shelf angles at corners
and atypical conditions are not clearly
documented in the design.
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Precast Concrete Wall Panels
The most important strategy for support of
precast concrete panels is to support the weight
of each panel on no more than two points.
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General Description
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Strategies For Support
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Seismic Forces
138
Field Adjustability
139
Fire Safing
� Approved materials
� Securely installed
� Prevents passage of
flame and hot gases.
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Potential Problems
� Erection sequence: brackets, blockouts,
and embedment plate coordination.
� Cantilevers without sufficient stiffness
which may deflect or rotate significantly
during erection.
� Division of responsibilities for designing
and providing attachment and support
components.
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Potential Problems� Joints in architectural elevations are not
coordinated with the points of load
application to the primary structure as
anticipated by the SER.
� Attachment details by the SSE designing
the attachments are such that the precast
panels deliver moments or otherwise load
the primary structure with eccentric loads
not anticipated by the SER designing the
primary structure.142
Potential Problems� Kickers that resolve eccentricity apply loads to lightweight roof elements, such as bar joists, which are not designed for the kicker loads and this result in unexpected rotations and flexibility of the precast support.
� Tie back connections are attached to clip angles fastened to the bottom of the bottom flange of spandrel beams without consideration to the effects on the spandrel beam.
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Aluminum Curtain Walls
Often the most important part of the aluminum
curtain wall design is anchorage adjustability to
the base building structure.
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Strategies for Support
� Easily accessible attachments
� Adjustability
� Limit eccentricity
� Block-outs of fire proofing
� Factory drilled bolt holes in curtain wall
� Welded field connections
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Strategies for Support
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Movement
� Critical to performance
� 4 to 5 times frame
temperatures
� L/175 common design
for out-of-plane
� L/360 common design
for in-plane
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Field Adjustability
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Field Adjustability
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Attachments
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Potential Problems
� Large gaps between the anchors and the primary building structure can result in excessive bending stresses.
� Coordination of locations for adjustment – If vertical and horizontal adjustments are to be made solely in the attachments of the curtain wall system to the primary building structure, slotted holes must be long enough to account for all of the required adjustment.
� If the curtain wall designer is relying on adjustments being made through the primary building structure rather than his attachments, the slotted holes that are provided may be inadequate in length.
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Potential Problems
� Coordination of bolted attachments to the
primary building structure. Locations of bolt
holes for curtain wall attachment should be
coordinated with the steel fabricator so that
holes can be made in the shop. Holes should
not be burned in the field.
� Mullion splices (responsibility of the SSE) –
should properly account for volume changes and
movement of the primary building structure.
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Key “Take-Aways”
1. The design team needs to develop a strategy, or
strategies, for supporting the façade elements from the
primary frame.
2. Given this strategy, the team needs to communicate
responsibilities and scope.
3. The SER needs to know the façade attachment strategy
and needs enough information from the façade designer
to anticipate the impact on the primary frame.
4. The SER needs to communicate the relevant frame
performance characteristics (principally deformations).
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Key “Take-Aways”
5. The SER should strive to develop slab edge and spandrel beam designs that are consistent with the façade attachment strategy.
6. Tolerances, façade movements and frame movements need to be considered in total. Strategy and responsibility need to be clearly communicated and accepted.
7. The SER’s documents for the primary structure should indicate pertinent assumptions about façade attachment loads.
Questions?