CAP OF CONCRETE
Lawrence G. Griffis
Designers of a new, multipurpose arena in New
Orleans created its steel and concrete roof by drawing
on 19th-Century bridge technology
The roof of a new, $84 million arena in downtown New Orleans combines steel and concrete to produce a structural system that combines the best properties of both materials while performing like a reinforced-concrete arch bridge. The roof system's concrete components carry the large compression loads of an arch while the steel ties resist the thrusts.
The new arena is intended to host professional hockey and basketball teams, concerts, and other events not suited to the larger Superdome located nearby. The building, which will seat 17,500 for hockey and 18,500 for basketball, was designed by a joint venture of the local architecture firms Arthur Q. Davis and Partners, Billes-Manning Architects, and Hewitt Washington and Associates. The general contractor is the joint venture Manhattan/ Gibbs, also of New Orleans. Construction management services to the owner, the Louisiana Stadium and Exposition District, are being furnished by ILSI/CS joint venture of New Orleans; Walter P. Moore and Associates of Houston are the structural engineers.
The use of reinforced concrete dates back to the 19th century. The French gardener and inventor R. Jean Monier is recognized as the father of reinforced concrete. In the 1860s Monier began making large flower pots reinforced with steel wire mesh. He later expanded upon the principle to include an arch with a layer of wire near its thrust line. While the Frenchman may not have fully grasped the ramifications of his achievement, hundreds of reinforced-concrete arch bridges have since been built using this principle.
Students of concrete bridge design will also recognize the names Robert Maillart of Switzerland, Joseph Melan of Austria, and another Frenchman, Eugene Freyssinet, considered the father of prestressed concrete. Each of these engineers advanced the art of concrete bridge design in Europe using arches. But it was an engineer by the name of Conde B. McCullough, who, while serving as the head of Oregon's highway bridge division from 1919 to 1946, introduced the tied arch to the United States. He used it for a pair of 37 m spans at the Wilson Creek Bridge near Tillamook, Oregon, choosing the arch because the cost of building foundations able to sustain the thrusts would have been prohibitive.
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Walter P. Moore's engineers applied the concrete arch bridge concept to the roof of the new arena in New Orleans-with a few modern twists. Although the firm has designed more than 25 arenas nationwide, none of these have had composite sections for major roof elements. To the best of their knowledge, no one else has yet completed such a design. Designers modified the curved parabolic arch into a mansard roof to make it suitable for an arena.
The cross section of the compression chord takes the form of a hollow rectangular box, maximizing the component's strength-to-weight ratio. Engineers replaced the tie typically provided by the bridge deck surface with a pair of wide-flange steel shapes field bolted in the air. These steel ties also serve as supports for catwalks above the arena playing floor. Queen posts at the roof ridge lines counteract compression
chord forces and carry unbalanced rigging loads.
Engineers usually do not choose concrete for long-span roof designs because of its heavy dead load. Why was concrete specified here? First, the project arose at a time when steel prices were rising and lead times for steel orders were longer than usual in the region. To avoid delays, designers elected to minimize the steel
tonnage by finding a creative, low-cost alternative. Project engineers had learned from high-rise buildings that reinforced-concrete columns are six to eight times more economical than steel columns when carrying large
compression loads.
The compression chord of the tied-arch roof of this project is really just a high-rise building column laid on its side. There was no reason why the cost advantage conveyed by high-rise columns could not be applied to the arena roof, particularly if the chords could be cast on-site. Additionally, the hollow shape enabled contractors to save money by using conventional cranes during erection rather than specialized lifters.
During conceptual design, engineers developed a preliminary scheme of a concrete tied arch and a tied-arch steel-box truss system. Pricing by the construction manager showed a $430,000 savings in the final erected cost using the concrete tied-arch system instead of steel.
To minimize the costs of erecting the system, engineers limited chord bending and kept the concrete in compression as a column member, no a beam-column member with large flexural stresses. A beam column
would have had a deeper section and a greater weight, thereby reducing the economic advantage of concrete.
While prestressing the box girder was an option from a structural standpoint, it was unnecessary because the chord was primarily in compression. Project engineers kept the box girder in compression by loading
the tied-arch system only at the queen posts. Only the weight of the member itself caused flexural bending in the box girder. Designers conceived the roof plan so that long-span joists connected directly to four corner posts and to steel trusses above the sides of the playing floor. The trusses in turn rest on top of the box girder at the queen posts.
Another advantage of the concretied-arch system is related to the wind environment in New Orleans. Since the city is situated on the Gulf of Mexico in an area where severe hurricanes with 322 km/h wind gusts are a real threat, local structures have lateral load-resisting systems governed by wind loading. For this structure, the additional mass of the tied-arch system prevented any net uplift, even under the most severe hurricane loading. Thus, prestressing the tension was not necessary.
The primary design consideration in a tied-arch structural system is buckling of the compression chord. Designers established the width, depth, and wall thickness of the box girder based on buckling requirements and the desire to minimize system weight. In the early stages, project engineers contemplated using high-
strength concrete to reduce weight, but final analysis showed such a design would buckle at the given unbraced length (the distance from the support column to the queen post and between queen posts for strong axis buckling). The wall thicknesses Î 177.80 mm for the top and bottom flanges and 152.40 mm for the sidewalls-were governed by concrete placement considerations, including two required layers of reinforcing steel and local buckling. Thus, high strength concrete was unnecessary.
Designers settled on 41,370 kPa normal-weight concrete with a high water-reducing admixture for easier placement and prevention of honeycombing. The box sections were formed in three stages (top and bottom slabs and sidewalls) using construction joints at the tops and bottoms of the sidewalls.
Research at the University of Texas at Austin sponsored by the Texas Highway Department established criteria for determining wall thickness, local buckling limits, and reinforcement detailing. The research showed that maintaining a wall-length-to-thickness ratio no greater than 15.0 would ensure that a failure strain of 0.003 across the concrete cross section would be maintained and that local buckling would not be a design concern. In its final configuration, the box girder was 1.22 m wide and 1.98 m deep. These box girder proportions produced wall-length-to-thickness ratios of 5.1 and 10. 7 for the flanges and webs, respectively.
Unbraced lengths produced KL/r slenderness ratios of 36 and 21 for the strong and weak axes, respectively. The top of the box girder was kept below and unattached to the roof deck so it would not
vertically load the box girder. However, a brace supports the box girder's weak axis against side-ways buckling and allows vertical movement between the roof deck and the top of the box girder.
The stiffness EI was another important parameter for the box-girder design. Engineers need to know stiffness EI to assess the critical buckling load and to determine the bending moment amplified for the effects of member curvature as required by the American Concrete Institute building code. Although the code
includes approximate EI values, these were not precise enough for the critical box girder-chord design. Accordingly, engineers found more exact EI values from moment rotation curves generated by a computer program written as part of the Texas research.
When concrete members are subjected to large compression forces, they usually must be evaluated for elastic shortening, creep, and shrinkage. Project engineers assessed EI values at each stage of loading (including construction loads) and evaluated box-girder and overall system deflections. The tied-arch truss
proved to be a very stiff system. Upon release of the center shoring towers, roof deflection was calculated to be 88.9 mm. Additional overall deflection of the tied-arch system under creep and shrinkage effects is expected to be 58.42 mm after 10 years. Roof deflection under the prescribed 58.6 kg/ml roof live load is only 29.21 mm (equivalent to the length, L, divided by 3,506); under the 54,432 kg rigging load it is only 9.52 mm (L/10,750).
Aside from the weight of the roof itself, the most critical load for the tied-arch roof system is wind generated. Engineers performed a comprehensive investigation at the wind-tunnel laboratory of Cermak, Peterka, Petersen, Inc., in Fort Collins, Colorado, to gain a more accurate assessment. First they studied the wind environment of New Orleans in detail. The results showed that the basic wind speed (the largest 50-year wind speed from all directions at 10 m above the ground) was 6.5 percent greater than the wind
speed determined from the ASCE 7-95 (Minimum Design Loads for Buildings and Other Structures [Reston, Virginia: ASCE Press, 1996]) wind standard map (214.00 km/h peak gust versus 201.13 km/h peak gust).
However, when the directional properties of the wind were taken into account, as permitted by code when using a wind-tunnel study, it was determined that the highest directional wind speed was from a southerly direction and was only 84 percent of code. In terms of wind loading (pressure) rather than velocity, the largest value was only 70 percent of code. The wind tunnel study was a valuable tool not only in determining more accurate wind loads, but also in taking economic advantage of the lower loads.
The nearby Superdome (approximately 68.58 m northeast of the new arena) had a significant effect on the wind pressure patterns for the new arena. The flow over the curved Superdome surface resulted in a strong downward, rather than horizontal, flow toward the new arena from winds coming from the northwest. This
created positive (downward) roof pressures where there would normally be roof uplift. This load case had a significant effect on joist and joist-girder design for the roof.
The Superdome also affected arena wall pressures. If the arena were isolated, a northerly wind would create positive pressures on the arena's north face and negative pressures on its south face. However, as the winds come off the Superdome they cause a negative pressure in the space between the two structures,
generating a negative north wall pressure. In southerly winds the Superdome causes a back pressure between the two buildings, creating a positive north wall pressure instead of a negative pressure. These results were somewhat startling at first and could only have been predicted using a wind-tunnel study.
The wind-climate study showed that hurricane winds will originate in the southwest quadrant. Existing structures provide very little shelter from these wind directions, which affected the structural design of the tied-arch members. Another important aspect of the tied-arch design involved the load factor applied to wind loads. This load factor by code is 1.3. However, in the New Orleans wind environment, the use of a load factor of 1.3 implied less than a 200year mean recurrence interval storm, producing theoretical ultimate or irfailurelt loads. Instead, project engineers checked the roof structure for a 1,000-year storm with a load factor and a safety factor (phi) of 1.0.
Walter P. Moore's project specifications required a detailed written erection procedure Î to be submitted by the fabrication/ erection team and coordinated and approved by the general contractor. The erection plan included a step-by-step procedure describing the status of the structure at each stage, including bolted connections to be tightened, roof-deck attachment, column-support conditions, shoring-tower details, and jacking conditions. Engineers also demonstrated the structure's stability at all stages of the erection process.
Hurricane Georges struck just east of New Orleans in late September 1998, placing the project at risk: although the roof structure was partially erected and under construction at the time, no
damage was sustained.
The erection of the arena required four pile-supported shoring towers, one located at each queen post of each of the two tied- arch trusses. Shoring towers and their pile foundations could hold a maximum load of 318 metric tons. Each box-girder chord was cast on-site and erected in three pieces. The two sloping end sections, each approximately 37.80 m long, weighed 109.77 metric tons; the center flat section was 28.65 m long and weighed 85.28 metric tons. Each box-girder segment was fastened after erection using high-strength bolts 28.58 mm in diameter. The center planer trusses connecting to each queen post were 7.62 m deep and weighed 40.82 metric tons. Side trusses at each queen post weighed 17.24
metric tons.
Each box girder was cast on a concrete-pile-supported casting slab located in the arena playing floor area almost directly below the girder's final position. Special lifting pads, also pile supported, sat at the box girder lifts. Soil conditions in New Orleans are extremely poor and dictate that all slabs on grade be pile supported. Each box girder was erected in place using one crane with a 54.86m boom length on a 12.80 m radius.
The roof deck, together with all steel trusses and long-span roof joists, was installed prior to the release of the roof system by jacking at the four erection towers. Jacks released the load in 12 mm increments at each tower until the roof became self-supporting.
The design and construction of a long-span roof system require careful evaluation of several viable structural systems that support the architectural design objectives with respect to roof shape, height, and support locations. Additionally, market conditions must be evaluated in terms of the cost, availability of materials, local labor conditions, foundation conditions, schedule for construction, and opening-date delivery of the project to the owner. This process is a team effort involving the owner, the architect, the structural engineer, the construction manager, the general contractor, and the appro- priate subcontractors. For this project and the prevailing market conditions, the concrete tied-arch system proved to be the right choice.
Courtesy Walter P. Moore and Associates, Inc.
Lawrence G. Griffis, P.E., NLASCE, is the senior vice president and director of structural engineering for Walter P. Moore and Associates, Inc., headquartered in Houston.
THE ULTIMATE SPORTS ROAD TRIP
By: Andrew Kulyk & Peter Farrell
| New Orleans Arena Ranking by USRT |
| Architecture |  | 3.5 |
| Concessions | | 6 |
| Scoreboard | | 7 |
| Ushers | | 2 |
| Fan Support | | 6 |
| Location | | 7.5 |
| Banners/History | | 5 |
| Entertainment | | 6 |
| Concourses/Fan Comfort | | 7 |
| Bonus: Invocation | | 2 |
| Bonus: New Orleans Energy | | 2 |
| Total Score |  | 54 |
November 15, 2002 - After years of clamoring for a new venue in Charlotte, owners George Shinn and Ray Woolridge finally burnt every bridge left standing in North Carolina, and moved their team to open and waiting arms in New Orleans. After a 23 year absence, the NBA was back in New Orleans.
New Orleans Arena opened in 1999, and is a poster child for the motto "if you build it he will come". With the construction of a state of the art arena built to major league specifications, New Orleans was counting on the eventual arrival of a major league tenant to their new facility. That gamble was rewarded when the ownership team of Shinn and Woolridge bolted from Charlotte and relocated their team to New Orleans effective with the 2002-03 season.
Getting to the Venue
New Orleans Arena is located in the northwest corner of downtown New Orleans, and is
immediately adjacent to the Louisiana Superdome. Located on Girot Street, access to the
venue is via the grid of downtown streets, or directly off of I-10 by simply taking the
Superdome exits. There is public transportation throughout the downtown area, and the
venue is a brisk walk from most downtown hotels as well as the French Quarter. Parking
runs $10 in the Superdome garage, and you can find surface lots on the north and west
sides of the building as well.
Outside the Venue
The arena itself is an octagon shaped building, reminiscent of America West Arena, and the
south side along Girot Street serves as the building's main entrance. There is a second
entrance on the west side of the building. Both entrances are built with massive glass
facades which soar to the top of the building. At night the gray facades of the arena are
brightly floodlit, but the exterior of the building is devoid of any marquee or identification
signage, and this lends to sort of a bland appearance.
Girot Street is closed off to traffic on game days, and this area is roped off as a party zone
full of pregame activities and entertainment. A stage is set up with jazz and blues players,
there are hoops contests and even a tug of war pit. Refreshment and beer stands offer
pregame fare, and on this night there was even a rodeo display complete with live bulls
(perhaps because the Chicago Bulls were in town?). Given the warm winter weather here,
such an outdoor set up works nicely.
The Concourses
You enter the arena on the ground floor, walk through a small lobby and from there a long
escalator takes you high up to the main level of the building. The concourses are colored
pale gray, off white and pale turquoise, with terrazzo tile flooring and recessed lights in the
ceiling. Concession canopies pick up the color scheme, and again, the decor looks very
bland and austere, although bright and new. There are three concourse levels here - the
upper and lower concourses for the general public, and below the main level is a club
concourse and lounge. More on that later. A suite level overlooks the main concourse from
a balcony and is accessible via its own escalator.
The Seating Bowl
Seating is divided into lower and upper levels, divided by a separate suite level. The seats
here are checker colored in dark blue, light blue and gray. An older styled four sided
scoreboard hangs in the center with video boards and backlit ad panels. Along the upper
balcony are stationary ad panels and one color dot matrix boards offering in game stats and
out of town scores.
What makes the seating bowl is the 360 degree digital LED board which wraps around the
entire circumference of the suite level balcony. The Hornets utilize this display to the max,
offering a constant barrage of cool special effects, scrolling ads and bouncing logos
throughout the game. These guys use their board more creatively than anywhere else we
have seen.
Concessions
Voo Doo Barbecue offers the best dishes with a local flair, and of course, being New
Orleans both concourses are replete with frozen daiquiri stands. We also spotted a couple
items on the menu which we could not spell or pronounce, and being too timid to try we
passed. There is no team store in the arena, but small satellite merchandise carts scattered
throught the corridors. As we walked around the outside, there were two spaces in the
building at ground level that were under construction. One looks like a restaurant with
outdoor patio and the other looks like the future site of a team merchandise store.
Premium Seating
In addition to a separate suite level and concourse, the 100 level seats between the
baselines are designated as club seating. Tickets here run from $86-$136, and provide
access to a separate club concourse, which is located below the 100 level main concourse.
This exclusive area basically consists of north and south club lounges with bar service, a
couple concession stands and table seating. The two lounges are connected via a narrow
and austere corridor. Really nothing special to write home about here.
Banners/Retired Numbers
In a classy move, the Hornets brought their retired number of #13, Bobby Phills, over from
Charlotte. Added to the rafters in a moving ceremony on opening night was that of the late
"Pistol" Pete Maravich, the greatest player to ever play basketball in New Orleans as a
member of the New Orleans Jazz.
Slam Dunks, Assists, Fouls...
Slam Dunk - to the best feature of the building, those super 360 degree digital LED boards
in the seating bowl, well presented and a show to enjoy!
Assist - the USRT karma works again, as the home town Hornets run away with the game and
defeat the Chicago Bulls, 105-87.
Slam Dunk - A mention here on one of the finest attractions in downtown New Orleans - the
D- Day Museum. The movies, displays and exhibits are certain to stir the emotions of
anyone visiting.
Slam Dunk - to the many local fans on the "hornetsreport.com" message board who
provided us with lots of ideas and help in preparation for our visit here.
Foul - the heavy handed ushers and guest services people who need a few lessons in
customer courtesy. When we entered the building we were politely told at the security
check not to use flash photos while the game was in progress. (OK noted). As we toured
the building and snapped photos, a guest services person stopped us and berated us for
taking flash pictures in the corridor. Perhaps Jamal Mashburn will come barreling down the
corridor dribbling the ball any moment now???
Assist - Here is something new... at New Orleans Arena they don't just do the National
Anthem before the game, but also an INVOCATION, delivered by a member of the New
Orleans clergy. At the inaugural game the Mayor of the city gave the invocation, where he
asked God to bring a win over the visiting Utah Jazz and an NBA championship to the city of
New Orleans. God must be listening.... as of this writing the Hornets are a perfect 8-0 at
home. AMEN!!!!!!!
Foul - to ourselves, we had a hotel reservation at the Wyndham Hotel in downtown New
Orleans, and as we checked in we discovered that we were at the wrong hotel. Our
Wyndham was two blocks away and we had to repack all our gear, pay for parking and move
on. OOPS! Another USRT first.....
Assist - As in most arenas, there is an out of town scoreboard showing scores from around
the league. Here in New Orleans, they add a new twist... they also display scores from the
NBDL (National Basketball Developmental League). You can just imagine all the eyes
focused on that board as the Greenville vs North Charleston final popped up.
Summary
If one had to describe New Orleans Arena in just one word, it would have to be "ordinary". Bland, washed out colors (or really, lack of color) both inside and outside. Not much in the way of amenities, and premium level lounges that are left way behind in the dust compared to their peer venues. What makes this even more disappointing here is that this is NEW ORLEANS, where flash, color, pizzazz and flamboyance are the norm and part of the culture.
On the positive side, the Big Easy is not known to be the greatest of sports cities, yet the
local fans have really taken a shining to their Hornets, and those in attendance were very
loud and enthusiastic. Further upgrades, splashes of colors and better decor will raise the
score, but for now, we rate this venue somewhere in the lower end of the middle.
