November 2009 - Fretz Construction

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The
Pavilion after
Hurricane Ike,
which left the
fabric structure
in tatters.
IMAGE: JEFF YOUNG
Teamwork, persistence, and the flexibility of tensile fabric and structural
steel, combine to allow the replacement and expansion of a landmark
amphitheatre in just 164 days.
H
urricane Ike — the third most
destructive hurricane to ever make landfall in the United States — roared into the
Texas Gulf Coast on September 13, 2008.
It inundated Galveston Island with record
flooding and left more than three million
households without power for up to several weeks. The storm obviously wasn’t a
patron of the arts. Eighty miles inland,
the Teflon-coated fibreglass fabric roof
over the reserved seating area of the Cynthia Woods Mitchell Pavilion was completely destroyed. This forced the
remainder of the 2008 concert season to
be abruptly cancelled.
A combination of Ike’s strong winds
and windborne debris shredded the roof,
causing more than US$3m in damage.
Yet the durable steel substructure survived intact. Taking advantage of the
reconstruction process, the Pavilion’s
board of directors decided to expand the
roof structure, more than doubling the
facility’s covered seating capacity from
2,479 to 6,387. With 9,653 lawn seats
after the US$9.5m renovation and expansion project, the total capacity of the
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venue has grown to 16,040 seats. In the
process, the fabric roof more than doubled in size from 30,000sqft to
82,000sqft.
How could this all occur by May?
Thanks to the dedication of a multi-disciplinary team of designers, fabricators,
and erectors, the venue re-opened after a
five month reconstruction project, twice
as large and more hurricane-resistant
than before, and in time for a rocking
concert by the Dave Matthews Band.
In reopening the fully replaced and
expanded roof structure just 164 days
after their first meeting, the design and
construction team members took full
advantage of the strength and flexibility
of a composite fabric material and structural steel, outstanding teamwork, and
the benefits of technology.
The Pavilion is located 27 miles north
of Houston in The Woodlands, and is
owned by The Center for the Performing
Arts at The Woodlands — a non-profit
organization that focuses on enriching
the culture of its local community.
George Mitchell, who founded The
Woodlands in 1974, and his wife, Cynthia Woods Mitchell, wanted to create a
facility that would host performing arts,
popular music, and community events.
They achieved their goal in April 1990
when the Pavilion first opened.
In recent years, the Pavilion has hosted approximately 75 events every season,
March through November. These include
ballet performances, a diverse range of
music concerts, and high school graduations. Since 1992, the Pavilion has been
the official summer home of the Houston
Symphony.
Educational outreach programs sponsored by The Center for the Performing
Arts at The Woodlands have exposed
approximately 40,000 students from seventeen area school districts to the performing and visual arts.
Background
Prior to the storm, the twenty-year-old
Cynthia Woods Mitchell Pavilion was
ranked for three consecutive years by
Pollstar magazine as one of the top six
amphitheatres in the world.
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Plan view from 3D model of the expanded structure,
with the structural components identified.
IMAGE: WALTER P MOORE
The venue was originally designed
both architecturally and structurally by
Horst Berger Partners in 1989. Fortunately, Horst Berger and his protégé, Joseph
DeNardis of DeNardis Engineering in
White Plains, New York, were also available to design the expanded fabric structure. Walter P Moore, headquartered in
Houston, TX, was the designer of the
steel substructure and the structural engineer-of-record, and FabriTec Structures —
a brand of USA Shade & Fabric Structures, Inc. in Dallas, Texas — performed
the fabric patterning and installation.
Fabric Skin & Steel Skeleton
Sheerfill II fabric, which is a composite
material made of fibreglass and polytetrafluoroethylene (PTFE) with a Teflon coating, was used for both the original and
expanded structures. The fabric, produced by Saint-Gobain Performance Plastics, has sufficient strength to withstand
design wind loads in addition to having
superior ability to transmit light without
the heat gain associated with traditional
glazing. This is especially important in the
Houston area, where outside temperatures are commonly above 90 degrees
Fahrenheit during the summer months.
The Teflon coating of Sheerfill II protects
it from staining, and the material maintains its original pre-stressed shape without relaxation throughout its typical
lifespan of 25 or more years. The thirteen
panels of Sheerfill II fabric used in the
newly expanded Cynthia Woods Mitchell
Pavilion weigh approximately 25,000lbs
in total.
Although the same type of fabric was
used for both the original and the
expanded structures, several design
aspects of the expanded fabric structure
make it more resilient to hurricane loads
than when it was first constructed. These
features include providing triangular fabric panels at the outer edges to avoid terminating ridge cables in edge catenary
cables, and reinforcing the fabric with
surface cables to act as rip stops. The surface cables panelise the new roof fabric
at the seams to limit damage from windborne debris to localized areas. The
expanded structure was also designed to
withstand up to a three-second wind
gust with a speed of 110mph, which is a
higher design wind load than the original
structure.
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The steel substructure involves compressive loads being transferred to spread
footings through A-frames, struts, and
vertical masts, while tensile loads are
transmitted through a network of ridge,
valley, and catenary cables. Lateral loads
are transferred through the compression
and tension members to the stage house,
where they are resisted by braced frames,
and through diagonal cables anchored to
new foundations.
The requirement of creating a large
open space using members with long
spans made structural steel an ideal
material for the project. Laced steel
struts and columns were used in both
the original and the expanded structures
to further improve the feeling of openness under the canopy, to minimize
impact on the sightlines, and for acoustical purposes.
The original structure was supported
by one row of A-frames and a series of
struts and cables. The expansion of the
roof area was accomplished by using a
second row of A-frames with connecting
struts and cables, while incorporating as
much of the original structure as possible.
The A-frames, which are trussed elements between the masts, create external
supports for various peaks of the roof.
The struts are nearly horizontal laced
members in compression that offer support points at the perimeter edge of the
structure and are critical for stability
through the braced frames located at the
stage house.
All of the original masts and Aframes, along with several of the original
struts, were re-used in the expanded
structure. The new A-frames span up to a
projected horizontal length of 130ft,
while the new struts span up to 85ft.
ASTM A586 structural strand cables of
5/8 in to 1 5/8 in diameter with Type A
inner and outer coating were used
throughout the structure, with breaking
strengths of 48,000lbs to 324,000lbs.
The stage house was extensively
retrofitted to allow it to provide much of
the lateral bracing for the expanded
structure. The retrofits involved adding
steel side plates to many wide flange
members, adding bracing members, and
exposing and retrofitting existing footings. In addition, several of the existing
connections were strengthened. The
retrofits were redesigned during the Ú
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Image from the Sketch-Up model of an exterior mast in
the second row. The 3D model was used extensively for
coordination between consultants.
IMAGE: LVJ SPECIALTY, INC.
Structural steel erection nearing completion, prior to
installation of the fabric.
IMAGE: WALTER P MOORE
Close-up of one of the second row masts. Three struts,
two A-frames, and five cables connect to this interior
mast.
IMAGE: WALTER P MOORE
Installation of a fabric panel showing temporary
netting and beige fabric that will be bleached white by
UV rays.
IMAGE: WALTER P MOORE
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TATTERS TO FINE TUNES
The first fabric panel installed.
construction process as existing conditions were discovered that differed from
the 1989 design drawings.
to speed up the project delivery schedule.
A mere seventeen days after design
began, foundation construction drawings
for the retaining walls were issued for
construction. One week later, steel trunk
drawings were issued to allow most of
the steel to be ordered and the trunks to
be fabricated while the design of the
connections was underway. Four days
later, cable drawings were issued so that
the cables integral to the steel structure’s
stability could be ordered. The remaining
foundation drawings were issued on
December 22, just six days after the cable
drawings, to allow existing foundations
to be retrofitted and new foundations
constructed where needed.
Once construction began on these
critical-path elements by the general contractor, Fretz Construction Co. of Houston, the design team turned its attention
to the stage house. Although the retrofit
of the stage house was urgent, because it
had adequate strength and stiffness to
keep the entire structure stable during
construction, its retrofit could wait until
the expanded roof fabric was in place
and full wind loads would be possible.
Design continued apace through the
Christmas and New Year holidays to
allow the stage house retrofit package to
be issued on January 4, 2009 — just 45
days after the start of design. This was
the last structural engineering package
issued. After this time, a few revisions
were issued to incorporate the final wind
tunnel test results, which were received
on January 23.
IMAGE: WALTER P MOORE
Speedy Engineering Design
Schedule
Photo from underneath The Pavilion showing a second
row exterior mast with connecting elements.
IMAGE: WALTER P MOORE
Abseilers working on connections at the A-frame peaks.
Structural cables are connected at the A-frame peaks,
while ridge and valley cables give the fabric its shape.
IMAGE: JEFF YOUNG
Image from the sold-out opening concert by the Dave
Matthews Band.
The most challenging aspect of the project was the fast-track schedule. On
November 6, 2008, Walter P Moore’s
structural engineers first visited the site to
assess the existing condition of the hurricane-damaged structure. When the first
meeting of the design team with the
owner and general contractor was held
on November 18, only 164 days
remained before the planned grand reopening event, a sold-out concert by the
Dave Matthews Band scheduled for May
1, 2009. To succeed and avoid the
US$1m penalty, the project team had to
compress structural assessment, redesign,
fabrication, shipping, erection, and structural strengthening into those valuable
164 days. This hyper-schedule demanded
activity durations measured not in
months or weeks, but in days. In the
process, the project team found ways to
retrofit and re-use many existing structural members to save time and reduce
costs.
Even though a wind tunnel analysis
was performed for the original structure,
another was required to accurately predict wind loads acting on the new structure due to the significant changes in
roof geometry and because current codes
do not address wind loads on structures
with shapes as complex as this. However,
the six-week lead time for the analysis did
not accommodate procurement deadlines
for the fabric panels, structural steel, and
especially the cables. Therefore, the
design was carried out on the basis of
preliminary wind loads from desktop
studies performed by the Canadian wind
engineering consultant, Rowan Williams
Davies & Irwin, Inc., that would later be
confirmed using the final wind tunnel
test results. Less than two weeks after
the initial project kick-off meeting, the
results from the desktop studies were
received by the design team on November 30, 2008. DeNardis Engineering provided the fabric interface geometry on
December 8 and the fabric details on
December 24.
The construction drawings for the
structure were issued in several packages
An Equally Ambitious
Construction Schedule
The construction schedule maintained
the same record pace as the engineering
design schedule. Fabrication of the structural steel members and cables began
almost as soon as the design drawings
were issued. Communication was vital in
maintaining the momentum of the project’s construction. Interaction between
the structural and fabric engineers, the
general contractor, and the steel detailer
— LVJ Specialty, Inc. of Lafayette, LA —
and fabricator — Alpha Fabricators, Inc.
of Houston — occurred on nearly a daily
basis to ensure that the project could proceed at maximum speed. Rey de la Reza
Architects, Inc. of Houston managed the
project for the owner.
IMAGE: JEFF YOUNG
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When construction began on December 1, it included seven-day working
weeks. Later, construction crews were at
work 24 hours a day. Demolition of the
existing concrete from the uncovered
reserved seating area and construction of
a concrete foundation for installation of
the tower crane occurred first. In midDecember, foundation retrofitting and
new construction were performed.
As many as thirteen lifts were used on
the site at one time to erect 75 tons of
steel in three weeks. During the week of
February 1-7, the first row of masts,
struts, pipe columns, tension cables, and
A-frames were installed. The masts, pipe
columns, and A-frames from the original
structure were re-used, with new struts
and tension cables. During the subsequent two weeks, the second row of
masts, struts, pipe columns, tension
cables, and A-frames were erected. Upon
erection of the steel structure, retrofitting
of the stage house was performed.
Installation and tensioning of the fabric occurred during a 50-day period,
which ended on April 26 — five days
before the opening concert. At that
point, just enough time was left to complete the seating, handrails, landscaping,
and final painting.
The completed restored and
expanded structure.
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Technological Tools
contractor and structural engineer to
discuss modifications of structural steel
connections.
Application of the latest technologies
enabled the project to be delivered ontime. As mentioned, wind pressures from
a desktop computer study were utilized
for design before the wind tunnel analysis could be performed. In addition, a
specialty membrane and cable-net program was employed for analysis of the
fabric roof, and an additional program
was used to pattern the fabric for installation purposes. Results from the fabric
roof analysis were imported into SAP
2000 for incorporation in the design of
the structural members.
Although AutoCAD was used for
documentation, Google Sketch-Up was
employed to create a three-dimensional
model of the steel structure and fabric.
This model was used by Walter P Moore
to determine the connection geometry
and cable lengths. Portions of the model
were exported into AutoCAD to facilitate drafting structural details. When the
fabric patterning shape was imported
into the Sketch-Up model, which included the structural steel, potential interferences between the fabric and the steel
structure were determined. The model
was also used to facilitate discussions
between the steel detailer and structural
engineer regarding the orientation of
the A-frames, and between the general
A Standing Ovation for
the Project
The Cynthia Woods Mitchell Pavilion
underwent a fast-tracked transformation
after Hurricane Ike left its roof in tatters.
With a concerted team effort, the
expanded amphitheatre opened on time
in May to begin its 2009 season as a
prime venue like it was before Ike’s glancing blows.
Horst Berger, the world-renowned
designer of both the original and the
expanded Pavilion, appreciates the success of the project team. “The space, in
spite of its huge dimensions, has retained
its sense of intimacy. I remember that, to
my pleasure, sitting under this roof at the
opening concert of the Houston Symphony [in 1990] felt as intimate as sitting in
Carnegie Hall. I think the new space is
even better,” he remarked. J
About the authors:
Tarek Ayoubi, P.E., is a principal and senior
project manager at Walter P Moore.
Rachel Calafell is a graduate engineer at
Walter P Moore. They can be reached at
+1 800.364.7300.
IMAGE: JEFF YOUNG
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