Curtainwall Primer for Design Professionals
Matthew
Stuart, P.E., S.E., F.ASCE
Course Outline
Definitions of a curtainwall sometime differ between an Architect and an Engineer. It is therefore important that a structural engineer understand the different types of systems available and in use. This one hour online course will provide the user with a basic understanding of curtainwall systems as they relate to the structural design and detailing of the supports for this specialty type of building cladding.
This course includes a multiple choice quiz at the end.
Learning Objective
At the completion
of this course, the student will:
- Be familiar with the definitions
and nature of the two primary types of curtainwall systems in use;
- Know the important structural
design and detailing considerations associated with the support of curtainwall
systems; and
- Become familiar
with a number of different standard, typical wall sections associated with
several common framing and backup systems.
Course
Introduction
A curtainwall system is a specialized
type of cladding typically used in the construction of commercial and institutional
buildings. Definitions of a curtainwall sometime differ between an Architect
and an Engineer. It is therefore important that a structural engineer understand
the different types of systems available and in use. A clear understanding of
the curtainwall supplier's design criteria will enable the structural engineer
to properly design the supports required for this type of building cladding.
Course Content
I.
DEFINITIONS
Curtainwall: A wall
system which utilizes glass, either transparent or spandrel or both, and vertical
and horizontal mullions acting as structural members to transfer wind and gravity
forces to the building structure. The spandrel portion can be metal panel or
some other material. The entire system is supplied and installed by one contractor
and typically weighs 8 to 10 PSF. The manufacturer’s in-house engineering staff
typically provides the structural design and integrity of the curtainwall system.
There are two types of curtainwall systems.
Stick System: A curtainwall
system in which the mullions are installed first, and then the glass panels
are inserted into the mullion framing in the field. The vertical deflection
criteria is more stringent for this type of system than a unitized system. The
primary advantage of this system is it's lower cost when compared to the unitized
system.
Unitized System: A
curtainwall system in which the mullions are fabricated with the glass panels
in place, and then erected as individual panels. The primary disadvantage of
a unitized system is that it is more expensive than a stick system. The advantages
of the unitized system are
- The curtainwall erection time
is reduced.
- Unitized panels can tolerate
more vertical deflection in the structure due to their gasketed joints. The
gasketed joints also allow the curtainwall to form an incremental expansion
joint at each mullion which in turn provides for almost unlimited adjustment.
Non-curtain
wall systems:
Strip
Windows: Windows which form a continuous horizontal band along an elevation
of a building. Typically for strip windows, the structural support is provided
for the facade material above and below the window, leaving the window connection
to the window supplier. The architectural specifications should provide information
on the anticipated live load deflections for this system.
Punched
Windows: Individual windows which are "punched" into a building elevation.
The facade material is on all four sides of the window. For the punched window
system, structural support is typically provided for the entire facade system
and the window supplier is responsible for the support of the window from the
sill, jambs, and head framing. The structural support framing can either be
designed and detailed, or a performance specification can be provided. In the
later case the structural calculations, details and shop drawings are submitted
by an independent vendor and reviewed and approved by the project engineer of
record.
II.
IMPORTANT STRUCTURAL CONSIDERATIONS IN DESIGN
Wind
Loading: In conjunction with the architect, the design loading for the curtainwall
should be provided in the contract documents. Typically, this is done in the
curtainwall specifications in the form of narrative regarding loads and/or building
elevations showing loading. In all cases, this must be done using "components
and cladding" loads from building code. The local governing building code must
be reviewed for this information. For example, Ohio specifies one wind speed
for the entire state. Massachusetts however specifies individual wind speeds
for each county or city and divides the state into three zones. In determining
these wind loads, it is important to know what tributary area a particular mullion
has since this factors into the wind pressure used. Normally, you would assume
a mullion span and spacing in order to determine the tributary area.
Deflection
of the Supporting Structure: Anticipated building frame deflections (both
vertical and horizontal) must be coordinated with the curtainwall supplier.
This is typically done in the curtainwall specifications. The limiting deflection
of the mullions and glass themselves is typically based on industry standards.
The typical limiting vertical deflection in the curtainwall industry is only
1/4".
III.
IMPORTANT STRUCTURAL CONSIDERATIONS IN DETAILING
Allowance
for vertical deflections: Provide details to allow the building structure
to deflect vertically without imposing undue loading or movement to the curtainwall.
It must be stated clearly on the contract drawings where the gravity loading
is assumed to be supported and where the lateral support is assumed to be provided.
Connections to the structure can be from the screed angle, the concrete slab,
or a “free-floating” beam. It is important to note that the connections to the
building structure must be capable of supporting the components and cladding
loading. Torsion should also be considered. The “free-floating” beam connection
would typically be specified by the supplier to be provided at the top of the
beam which can impart a torsional load to the beam. The unanticipated impact
of curtain wall connections should be reviewed during shop drawing submittal
process.
IV.
DETAILING EXAMPLES
Composite
Slab
Bent
Plate
Concrete
Slab
Stud
Wall Backup
Course Summary
There are two types of curtainwall
systems; "stick" and "unitized". Although the stick system is cheaper, the unitized
system has the advantage of more flexibility relative to allowable deflection
of the supporting structure. Non-curtainwall systems such as strip windows and
punched windows are often incorrectly referred to as a curtainwall by architects.
The structural engineer-of-record should provide to the curtainwall manufacturer
the required component and cladding wind loads required. The structural engineer
and the curtainwall manufacturer should communicate with each other to ensure
that the deflection of the supporting structure is within the criteria established
by the material vendor.
Related Links
For additional technical informaion
related to this subject, please visit the following websites or web pages:
Curtainwall Connections -
HALFEN
http://www.modernsteel.com/Uploads/Issues/December_2007/122007_30772_steelwise_web.pdf
http://www.aisc.org/Template.cfm?Section=ePubs_Design_Guides1&Template=/MembersOnly.cfm&ContentID=35401
Quiz
Once
you finish studying the
above course content,
you need to
take a quiz
to obtain the PDH credits.
DISCLAIMER:
The materials contained in the online course are not intended as a representation
or warranty on the part of PDHonline.org or any other person/organization named
herein. The materials are for general information only. They are not a substitute
for competent professional advice. Application of this information to a specific
project should be reviewed by a registered professional engineer. Anyone making
use of the information set forth herein does so at their own risk and assumes
any and all resulting liability arising therefrom.
