Arc Welding is Essential to the Success of a Unique Structure By Miklos Peller, P.E.

Vice President Tilden, Lobnitz & Cooper, Inc. Cleveland, Ohio

A 5,000 seat stadium at Sea World, Or­lando, Florida was designed to give all seats an unobstructed view of the per­forming pool and stage. The roof framing system was comprised of an innovative network of structural steel trusses. The aesthetic and functional simplicity of the scheme afford so many benefits from the standpoint of the user that the owners ac­cepted the 7% cost premium.

T H E F A C I L I T Y

Sea World, Inc., wi th facilities located in Florida, Ohio and California is the lar­gest marine life theme park system in the world and a leader in the field of marine environments. Management at Sea World of Florida, Inc., located in Orlando, de­cided to incorporate an additional 20 acres of land in the theme park. This in­cluded, among other facilities, a new roofed stadium, seating 5000 spectators, and overlooking a 5-million gallon sea wa­ter pool system. (See Drawing 1)

The pool system consists of a 180' x 92' X 35' deep performing pool, a breed­ing and research pool covered by a shade structure, two medical pools for isolation and quick access purposes, and two addi­tional pools where the performing whales stay before shows. A stage structure sep­arates the performing pool f rom the other pools, with stainless steel gates for pas­sage between pools. A level below the stage structure is provided wi th portholes and electrical cable outlets for underwa­ter f i lming and video recording. The pools resist hydrostatic pressure from the high ground water table through post-tensioned soil anchors.

T H E C O N C E P T

The design team of the new facil i ty determined during the conceptual de­sign phase that the stadium's function can best be fulf i l led if a ful l view of the stage and the performing pool were pro­vided f rom any point in the seating area, unobstructed by intermediate vertical supports.

The structural engineer analyzed var­ious schemes of appropriate clear-span systems as well as more conventional ones to value-engineer the most economical roof system that would fu l f i l l the aesthet­ic as well as the functional requirements of the architectural design concept, and at the same time create a harmonious merger of the steel roof structure and the supporting concrete structure.

T H E R O O F FRAMING S Y S T E M

The roof framing system selected after the studies consists of an unusual and in­novative network of structural steel trus­ses. A 26-foot deep curved truss girder, spanning 276 feet along an inner concen-

trie circle wi th a 93-foot radius, resting on two massive 6 ' 0 " round concrete piers well outside the angle of vision from any seat. Radial trusses spanning 91 '6" , tapered in depth f rom 10' to 26' and spa­ced on 10-degree intervals, frame into the truss girder. Roof purlins are spaced at 11 '5" centers, and support an acoustical metal deck wi th a regular-weight concrete roof f i l l over it. The cost of this novel fra­ming system was merely 7% higher than a conventional column-supported roof. The aesthetics and functional simplicity of the scheme, however, afforded so many bene­fits f rom the standpoint of the user that the owners accepted the 7% cost premium.

T H E S T R U C T U R A L DESIGN

Since the girder truss is spanning along a curve, it requires torsional restraint a-long its length to maintain stability. The radial trusses are designed to provide this restraint. Even though each radial truss carries an equal tr ibutary roof area, the bending moments, shears and reactions are influenced by the torsional restraint requirement of the girder truss. Lateral design forces on the roof structure are re­sisted by a 25-foot high, 515-foot long curved concrete wall along the outer con­centric circle with a radius of 184'6", and the reinforced concrete seating frame.

Aero-dynamic analysis indicated that the upl i f t forces on the roof structure were of such magnitude under hurricane loading that weight in the form of a con­crete slab over the roof deck was needed to provide adequate gravity load to resist uplif t and to minimize stress reversals in the trusses. Acoustical engineers at the same time utilized the mass of this con­crete f i l l to eliminate background noise resulting from heavy rains which are typ­ical in Florida.

The resulting structure is a three-dimensional space frame, made up of a girder truss curved in plan (weighing ap­proximately 100 tons), 17 radial trusses framing into the truss girder at 10° inter­vals (each weighing approximately 9.5 tons), bottom chord struts and sloping bridging struts (weighing approximately 50 tons), and roof purlins (weighing ap­proximately 60 tons). A curved eave truss was created adjacent to the truss

girder bottom chord, designed to stab­ilize the framework against secondary for­ces. Top chords are stabilized by the dia­phragm action of the concrete slab, made composite wi th the trusses by the use of welded shear studs.

Standard rolled shapes were selected for truss members, and round pipe sec­tions were specified for bracing members, dictated by aesthetic considerations and equal strength requirements in any direc­t ion perpendicular to member axes.

T H E D E T A I L S

Given the mult i tude of member direc­tions, a major part of the structural de­sign effort was focused on the details of the member connections. A majority of the structural nodes have nine con­verging members from various directions at various angles, complicated by the pre­sence of sliding expansion joints at cer­tain locations. The detailing task involved five considerations in addition to load transfer requirements:

1. the necessity to keep the connec­tions simple and concise so that they would not dominate the aesthetics of the structure;

2. the minimization of connection ma­terial and their fabrication in order

to keep the cost of this structural scheme competitive with conven­tional short-span frames;

3. the facilitating of shop fabrication and field erection;

4. the minimization of materials and surfaces exposed to corrosive salt spray exposure;

5. the final goal of a simple, aesthet­ically organized system.

W E L D E D C O N N E C T I O N S

The above considerations led to the conclusion that welded connection details are the only appropriate details to achieve the desired results. Although welded trus­ses are common, the presented framing system is unique in view of the unusual force distribution and the unusual frame geometry. The system of welded connec­tions as designed resulted in simple yet disciplined structural nodes, emphasizing the lightness, effortlessness and airy ap­pearance of the frame which was one of the aesthetic design criteria. (See Figure 1)

Minimizing connection materials by judicious use of welded details meant less fabrication, reduced fabrication time and minimized material costs. The designers calculated that approximately 15 tons of fabricated connection material and about 25,000 bolt hole preparations and 10,000

Figure 1- A view of the roof structure, showing the radial trusses and the girder truss.

bolt installations were elinriinated f rom the structure by designing welded con­nections. Shop fabrication was minimized and simplified by the use of the indicated welded details to such an extent that the fabricator finished ahead of schedule.

Welded connections resulted in sub­stantial reduction of finished surfaces and interface surfaces and crevices, which meant reduced painting costs and greater confidence in the corrosion protection. (See Drawing 2)

Al l of the welds, with the exception of field splices, were made in the fabricator's shop, under controlled conditions. The specified quality control inspection of the welds was also carried out in the shop. Since the trusses were stored on the ground, it was possible to postpone the weld inspection to such time when nearly

all of the welds were completed, resulting in reduced quality control costs.

A l l connections were conceived and detailed as welded throughout wi th the exception of the field connection of the radial trusses to the main girder truss, and the strut connections. These were de­tailed as bolted for the fol lowing reason:

The radial trusses require the girder truss for vertical support, and at the same time the girder truss requires the strength of the radial trusses for torsional support. Four temporary support towers were used during the erection of the truss gir­der, resulting in five approximately equal spans. Erection of the radial trusses pro­ceeded from the end of the girder truss toward its mid-span. The temporary tow­ers were required to remain in place until the entire trusswork and the bracing

beams were installed, due to the frame geometry and the truly three-dimensional structural behavior of the system. Since time was critical and the erection towers limited construction activity at ground level, the erection of the roof structure had to proceed rapidly, and the connec­tion of the radial trusses to the girder truss had to keep abreast of the erection. The engineer selected bolted connection for these, eliminating extensive testing of welds at a critical t ime in the erection and at the same time eliminating the need for temporary welding platforms and ground level protection from welding sparks. Therefore, the structure was organized so that no major structural welding was done above ground at critical locations and at critical times.

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D E T A I L S O F T H E W E L D E D JOINTS

Details of the welding considered sev­eral factors. One was the choice of mater­ials. ASTM A 36 steel was used for struc­tural shapes up to the W12 series, and ASTM A 572 Grade 50 for W14 shapes. Using various grades of steel minimized abrupt thickness changes at welded spli­ces. Another factor to be considered was that the geometry of the joint should al­low run-off of rain water. The orienta­tion of the truss members and that of the stiffener plates were partially dictated by this consideration.

The joint configuration was organi­zed to avoid overlapping of welds and to minimize concentration of weld-related residual stresses. Weld locations were carefully selected and cope holes were ex­tensively detailed. Abrupt changes in ad­jacent material thicknesses were avoided. (See Figure 2)

The chord butt splices in the truss gir- ^ ' ^^ ^'^'^ ^^^"^^ ' ^^ Stadium der were detailed with edge plates and vertical stiffeners to avoid stresses from secondary effects which would develop inforce and stiffen the chords at the spli- dary stresses at those locations, and the a tri-axial tension stress state. Further- ces where they change direction in the same edge plates were used to connect the more, the edge plates were detailed to re- curved truss, reducing undesired secon- radial trusses. (See Section 1, Drawing 2)

The dimensions of the completed trus­ses ruled out their shipment as a comple­ted unit. The fabricator and the engineer worked out a system of field splices, en­abling transportation. The fabricator chose to completely assemble the truss in the shop to assure good fit-up which con­sideration was paramount in view of the complex geometry and the specified camber. Once assured of the f i t-up, the fabricator saw-cut the truss members at the field splices, and prepared the cut ends to specifications for a butt-welded field splice. (See Drawing 2) This re­sulted in truss segments of transportable dimensions.

The design of the truss members was rechecked for potential mid-length ec­centricities allowed by field assembly tol­erances. Upon delivery of the trusses to the project site, they were reassembled on the ground in a jig and rewelded at the field splice locations. Following accept­ance of the field splices after testing, the trusses were erected in their complete configuration. The field splices are not visible, their only mark is the round web

Figure 2-An interior joint at a slide-bearing expansion joint. Note field splice to the cope holes which were left open in the left of the joint. Round pipe sections are horizontal and diagonal bracing members. final product.

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Drawing 2

Q U A L I T Y C O N T R O L

Certification was required for all wel­ders for the detailed weld types. A l l welds needed to pass a visual inspection for AWS D1.1 compliance. Magnetic Particle Inspection ASTM E 109 was required for the root pass of all full-penetration butt welds. Ultrasonic Inspection ASTM E was required on all completed full-pene­tration welds and adjacent heat-affected zones in the base metal, and also for f i l let welds 3 /8 " or larger and for all welds that failed to pass visual inspection. The ful l procedure was required to be repeated in the cases of welds that failed to pass the

test. The same procedure was prescribed for the butt welds in the field splices.

In addition to the tests on the welds, steel for the W14 sections were specified with a low transition temperature based on Charpy V-notch impact tests.

Corrosion protection of the finished structure was of utmost concern since the structure is exposed to a corrosive envi­ronment. Development of corrosion or corrosion cells in highly stressed welded connections would be detrimental. Com­mercial blast cleaning surface preparation, a zinc-rich primer paint and a compatible epoxy-based finish paint was required in a dry f i lm thickness of 2.0 to 3.5 mills.

T H E R E S U L T

Welded connections achieved what was expected: the stadium roof structure eco­nomically achieved a clear statement of its purpose. The audience has an unob­structed view of the stage, and the ef­fortlessly clean lines and simple joint nodes and connections of the truss mem­bers enhance and complement the pri­mary function of the stadium.