2010.09.08 - Key Considerations for the Attachments of Facades to Structures

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


1250 B.C. 430 B.C. 530 A.D.

1220 to

1472

A.D.1796

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• Walls are Thick

• Walls Support all Loads

• Floor “Rides” with Walls

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• 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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� Masonry Walls Still

Thick

� Floor & Wall Loads

Carried by

Structural Frame

�Wall Still Functions

as a Reservoir

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0 1000 A.D. 20001000 B.C.2000


Time Line

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20001950 1960 1980 199019701918

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• 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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• “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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� 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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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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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


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:

�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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� 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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� Torsion?

� Resolved at columns?

� Kickers?

� Roll beams?

� Rotation and projected

translations?

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� Connection to

Columns

� Simple shear?

� Special copes, non

standard?

� Horizontal forces?

� Torsional forces?

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� Spandrel dimensions

� Depth

� Flange width

� Flange thickness

� Project consistency

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� 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

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Field Adjustability

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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?

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2010.09.08 - Key Considerations for the Attachments of Facades to Structures