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steelwise
december 2007
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pushing the Envelope
by James c. Parker, P.e., alec s. Zimmer, P.e., and christoPher m. hewitt
Careful attention to façade attachment details pays off in the long run.
O
ONE Of ThE MOST ChaLLENgINg DETaILS faCINg out of the wall. Water that makes it past the cladding runs down
STRUCTURaL DESIgNERS IS ThE SUppORT Of a bUILD- the drainage plane and out of the wall through weep holes at the
INg façaDE. The complexity of this attachment is not to be base or at flashing joints. AISC’s Design Guide 22 and its seminar
overlooked, and the entire design team should be engaged in on façade attachments to steel frames give attachment strategies to
developing a strategy for supporting the façade system in order to accommodate the challenges of each of these wall systems.
prevent unplanned changes and problems in the field. The struc-
tural designer should then strive to develop slab edge and spandrel Design of Slab Edge Conditions
beam designs that are consistent with this strategy. In designing slab edge conditions, there are two fundamental
approaches: Either the slab cantilevers over the spandrel beam to
building Envelope Systems carry any loads applied to the overhang, or the designer does not
Before devising a support strategy, it is important to understand count on the slab as a cantilever, and loads applied to the overhang
the function of the façade system. There are three basic concepts are carried by a structural steel plate or assembly attached to the
for exterior walls to mitigate water infiltration: reservoir walls, bar- spandrel beam. Generally, using the slab to carry the loads as a
rier walls, and cavity wall systems. While each system is designed cantilever will be more economical than relying on the spandrel
to keep water out of a building, each approaches the problem beam alone, but the slab must have sufficient strength to carry
differently. Understanding the type of system that the structural the load without increasing the thickness of the floor slab system.
frame is supporting is important for determining appropriate goals Otherwise, a bent plate or other steel assembly must be designed
for the structural support. to transfer façade loads to the spandrel beam. Several examples
Older masonry bearing wall buildings employ the reservoir sys- of these appropriate details are included in the design guide and
tem. In traditional thick stone and brick bearing wall buildings, the seminar.
walls act as reservoirs and allow a certain amount of water infiltra-
tion into the wall. The walls are thick enough to prevent an unac- pour Stops and bent plate Edge Details
ceptable amount of water from entering the interior space, and The use of a light-gage metal pour stop as part of the façade
the water later escapes by weeping and evaporation. Though good support system is generally limited to slabs that have a clear over-
in its day, this system is heavy and is no longer considered to be hang less than 12 in. and slabs that have sufficient strength to sup-
cost-effective.
Most modern commercial walls are constructed as either bar-
rier or cavity systems. Barrier wall systems are just that—barriers
designed to keep water out of the wall and to rely on the exterior
surface and joint seals to prevent (or mitigate) water infiltration
into the building.
Alternatively, cavity walls have a drainage plane and waterproof-
ing barrier behind the exterior cladding and a means to divert water
typical forces at slab edge conditions.
exterior wall systems for mitigating water infiltration. design loads on light-gage metal pour stops or bent plates.
december 2007 MODERN STEEL CONSTRUCTION
And, for practical reasons, the spandrel nation with the architectural and mechani-
design must consider: cal systems, which can limit their use. Roll
➜ overall depth and flange width beams ensure a rectangular grid of girders
➜ the interferences with the slab edge and and beams, but designers must be sure to
the connections design their connections to the spandrel
➜ interferences with window heads beams to have some level of moment resis-
➜ interferences with mechanical systems tance. Both approaches, however, are effec-
such as ducts and piping tive ways to restrain torsion on spandrel
➜ interferences with shade pockets or beams and thereby reduce spandrel beam
other architectural features rotations.
➜ flange interferences with the façade
spandrel beams must system accommodating Tolerances and
resist a variety of load
conditions. ➜ clearances for fireproofing and façade adjustments
elements Every façade attachment design must
port superimposed loads. Typically, in this ➜ constructability of the spandrel allow for adjustment to accommodate
type of slab edge, the façade attachment tolerances and deflections. Steel framing
Because composite spandrel beam sizes
will be made directly into the slab or span- tolerances are generally larger than façade
are often controlled by post-composite
drel beam and will not engage the pour tolerances, and the designer must be care-
deflections, cambering composite spandrel
stop. Accommodating such an approach ful to provide sufficient adjustability in the
beams does not tend to reduce the beam
may become difficult at corners and col- design to accommodate these tolerances,
weights for most façade systems. Camber-
umn joints. When used, the light-gage which may accumulate. A good attachment
ing adds to the tolerances that the design of
metal pour stop should be designed for the detail will provide the means to:
the façade system must accommodate, and
wet weight of concrete, concrete fluid pres-
it should typically be avoided for spandrel ➜ adjust the slab edge in or out
sure on the vertical leg, and 20 psf of live
beams. relative to the spandrel beam,
load. When these conditions are such that
➜ adjust the location of the façade
a light-gage element can not accommodate
Wind and Seismic Loads system in or out relative to the slab edge,
the loads, the designer must employ the
For the base building components, the ➜ adjust the location of the façade
use of a bent plate at the slab edge. The
structural engineer of record can often system up or down relative to the
bent plate may be designed to be a pour
use a simple and conservative design when slab edge,
stop, a means to attach the façade, and a
planning for façade wind and seismic ➜ and adjust the location of the
means to transfer the façade loads to the
loads without adding undue cost. Nega- attachments both vertically and
primary structure.
tive wind pressures on the wall, combined horizontally.
with gravity load eccentricities, will often
Design of Spandrel beams Designers must also provide sufficient
control the design of the façade attach-
As one might suspect, spandrels must clearances between the steel frame and
ment, particularly those attachments made
have sufficient strength and stiffness to the façade that accommodate tolerances,
to the bottom flange of the spandrel beam
resist the applied loads, but must also be relative movements and rotations, ther-
as shown in the below figure. The IBC
designed to accommodate architectural mal insulation, fireproofing, and access to
requires components and cladding to be
and practical limitations of the system. install the façade component.
designed for seismic forces and the attach-
For strength, the spandrel design must
ments to accommodate relative seismic
consider: forces from Restraint
displacements, which can be large. These
The façade system must be designed to
➜ superimposed floor loads requirements should not be overlooked
➜ loads arising from weak-axis bending and can control wind loading for heavy
➜ torsion on the member and its connec- wall systems.
tions
➜ offsets between the centerline spandrel Kickers and Roll beams
and of the column When façade loads are eccentric to the
➜ eccentric façade loads supporting spandrel beams, significant
eccentricity exists on a cantilevered slab.
For serviceability, the spandrel design
There are a number of strategies that the
must consider:
designer can use to help to resist the tor-
➜ deflection due to superimposed dead
sional moments imposed on the spandrel
loads and live loads
beam. Commonly, either a roll beam or
➜ rotation of spandrel beams and façade
a kicker is provided to restrain the beam
supports
against rotation. Each system has advan-
➜ long-term creep of composite sections
tages and disadvantages. In addition to
➜ relative displacements of the structural
their role as a torsional brace, kickers can
frame with respect to the façade
also be used as an effective solution for
➜ tolerances of the façade and structural
resisting façade forces applied between
frame Façade attachments must be designed to
floor levels; however, they require coordi-
resist lateral and gravity loads.
MODERN STEEL CONSTRUCTION december 2007
eccentric loads on spandrel beams can be resisted by kickers (top) or roll beams (bottom). Façade systems and related details must
account for fabrication and erection tolerances.
accommodate relative movement between approached with an earnest understanding
stories, and when that movement is of its complexity, and by initiating a dialogue
restrained the restraining element is highly among the structural engineer, architect,
loaded. When possible, it is best to avoid façade system designer, and contractor to
restraint altogether. When it can not be arrive at the most practical and economical
avoided, restraint forces can be difficult solution for a project. With open communica-
to predict because of cracking, creep, and tion and the appropriate degree of care, most
attachment stiffness. It is also important to façade attachment problems can be avoided
be cautious of the potential for inadvertent from the start, making the project successful
restraint caused by friction, movement, or for the entire construction team.
other sources, which can have dire conse-
quences when not accounted for. James Parker is a senior principal and Alec
Zimmer is a senior staff engineer at Simpson
Conclusions Gumpertz & Heger, Inc. Chris Hewitt is a
Designing façade systems can be a senior engineer wtih AISC.
challenging effort. The process should be
this steelwise qualifies for 1 Pdh (0.1 ceU)
through aisc’s “extra credit” program. to pur-
chase your extra credit, log on to www.aisc.org/
extracredit and take the online quiz. need more
ceU credits? register for the aisc seminar “Façade
attachments to steel Frames,” coming to a city
near you! Visit www.aisc.org/seminars for details.
MODERN STEEL CONSTRUCTION december 2007
