Convention center’s bold architectural style is

supported by creative structural engineering.

One Challenge,
Several Answers By Greg Diana, P.E.
photos by Datum Engineers

T
The new Irving Convention Center at Las Colinas is at The native soil in Las Colinas is highly expansive, with potential
the intersection of Northwest Highway and John Carpenter Free- vertical rise (PVR) values in excess of 5 in. However, because large
way in the heart of Las Colinas, Texas. It is visible from the Four areas of the exhibit floor space are open, with no sensitive finishes,
Seasons Resort and Club and its championship golf course, immedi- partitions, or doors, engineers used a mix of foundation systems for
ately across the highway, where the Byron Nelson Championship is the ground floor structure. In the large, open exhibit space, a slab-on-
played every year. The city wanted to take advantage of this promi- grade foundation over 12 ft of moisture-conditioned soils was used.
nent location with its high visibility to market the use of the facility. This reduced the predicted heave due to expansive clay to 1 in. and
The owner’s goals were to obtain statement architectural allows the exhibit floor to economically support 350 psf live loads. A
expression and an identifiable landmark structure with high vis- structured pan-joist system over a crawl space was used in the main
ibility while maintaining optimal functionality of their marketable entry lobbies, prefunction space, first floor office space, and other
space. The building’s architects, RMJM (formerly Hillier), pro- ground floor areas with sensitive finishes and lower live load require-
vided a striking stacked and rotated design that accomplished the ments to isolate the floor structure from the highly expansive soils.
owner’s goals in an exciting way. The upper building structure contains two additional floor
The master site plan includes the convention center and park- levels. One level supports 20 meeting rooms, each approximately
ing garage, along with a 350-room hotel and a 190-room boutique 1,000 sq. ft. Eight of the meeting rooms are created using move-
hotel, due to complete construction in 2011, plus a future perform- able partitions, which, when opened, create a 60-ft by 120-ft junior
ing arts center, residential, and retail space. The lower podium con- ballroom to allow greater flexibility for use of the floor space. The
tains the main exhibit hall, along with office and mechanical space. top floor is primarily for the main ballroom. The column-free ball-
The exhibit space is approximately 190 ft by 270 ft, column-free, room is approximately 115 ft by 180 ft, plus a separate prefunction
with 35 ft of clear headroom above the exhibit floor. The office area. This level also contains the kitchen space and a mechanical
space is divided between the main floor level and a mezzanine. The mezzanine. The elevated structure is rotated 20° from the orthog-
mechanical space is on an additional mezzanine level above the onal podium grid, causing the corners to cantilever out beyond the
office space. lower building spaces.

MODERN STEEL CONSTRUCTION november 2010

 The 20º rotation of the upper portion of the Irving Convention Cen- The initial pricing was based on conventional truss shapes of
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ter with respect to its base creates a stunning, signature presence, various depths, up to 20 ft. It quickly became apparent that this
leveraging its high-visibility location.
concept would require more steel and possibly not achieve adequate
deflection and vibration performance. It would also require A993
Between the podium and the upper structure, an outdoor ter- Grade 65 high-strength steel, which would need to be imported.
race level connects to the ground level below and the meeting Given the lead time for the high-strength steel and the cost associ-
level above via exterior stairways. This terrace level also cantilevers ated with the extra tonnage, these conventional structural systems
above the two main glass entrances in the southwest and southeast negatively impacted both the budget and the construction sched-
corners of the podium. An interior/exterior concrete elevator tower ule. In order to make the supporting structure deeper, the building
serves all floors and creates part of the architectural expression. would have to grow taller vertically, which would create additional
The podium and elevated structures are clad on all sides with a cost in copper skin and mechanical systems for heating and cooling
combination of embossed and perforated copper paneling. These per- the larger volumes.
forations create a lantern effect on the south facade, causing the illu- The engineer began exploring structural steel options that
minated interior to shine through the perforations and silhouetting would both eliminate the need for imported steel and reduce the
the exterior steel structure behind the copper panels. The perforations tonnage. The first proposal was to use a set of segmented catenary
also create a view from the interior to the surrounding skyline. trusses. Rather than being limited to the space below the meeting
level and above the 35-ft exhibit headroom, this proposal would
Engineering Considerations and Project Goals extend the structural system to the ballroom level, creating a struc-
Two key goals had to be met to make this project a success: tural system that would be 35 ft deep rather than 20 ft deep. The
• Meet the owner’s budget challenges while providing the archi- added depth also would improve vibration and deflection perfor-
tect’s unique design, without sacrificing building performance mance. The primary disadvantage of this system was the disruption
or functionality. that the catenary chord would cause to the meeting room floor
• Provide the building to the owner in time for the required spaces, which the architect would need to work around.
opening date. The second proposal was to use arch trusses that would extend
Early in the design phase of the project, the building construc- to the underside of the ballroom level, similar to the catenaries.
tion cost estimates exceeded the owner’s construction budget of This system had similar advantages to the catenary—similar steel
$85 million by 25%. The design team needed to eliminate cost tonnage required, improved deflection performance over conven-
from the building without impacting the function of the various tional truss systems, and all domestically-produced steel. The main
building spaces and uses. During this phase, engineers worked to disadvantage was also the same—the overhead arch chord would
economize several key areas of the structure. disrupt floor space on the meeting level.
Additionally, the owner wanted to begin preselling the space up to The solution was to use a combination of these two truss
two years prior to the building opening date. This required a commit- options. The majority of the floor is supported by three catenary
ment from the entire design and construction team to meet the aggres- trusses, spaced at 30 ft to 60 ft on center, along with one arch truss
sive opening date long before construction documents were issued. at one end. The catenary truss chords are located between meet-
The structural engineer worked with the owner and design ing rooms and in back-of-house spaces and away from useful floor
team to create a strategy for achieving both of these goals, while space. This approach coordinated the structural and architectural
also improving the building’s performance. requirements to reduce the disadvantage of the deeper catenary
trusses. On the west end of the floor, the catenary would have
Solution 1: Long-Span Elevated Floor Structure extended outside the building; therefore, the arch was used on this
The stacked-and-rotated design meant that multiple floors, plus end. This combined solution eliminated approximately $3 million
the roof, would have to be supported above the column-free exhibit from the construction budget and allowed the use of all domesti-
space on the first floor. In order to achieve this, Datum-Gojer pro- cally available structural steel, while also improving deflection and
posed a system of long-span trusses on a 30-ft module. The trusses vibration performance. In order to reduce sway due to unbalanced
span the 190-ft direction of the exhibit floor. live loading conditions, additional diagonal bracing was provided
within the truss, below the meeting room level and in the exposed
exhibit ceiling space.
This deep catenary-like truss is one of three spanning the 190-ft
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direction of the new Irving Convention Center’s exhibit floor.

Greg Diana, P.E., is a structural engi-

neer in the Dallas office of Datum
Engineers. He joined the firm in 1999
and currently serves as project manager.
His project experience includes conven-
tion centers, healthcare, churches, and
higher education facilities. Greg holds
a bachelor’s degree in civil engineering
from the University of Texas at Austin.

 november 2010 MODERN STEEL CONSTRUCTION

 Three catenary trusses support the majority of the floors above the
➤
exhibit hall, with an arch truss at the end of the building where the
catenary would have extended outside the building.

Given the exposed nature of the exterior trusses from the inte-
rior and through the perforated copper cladding from the exterior,
the architect was greatly interested in the exterior appearance of
the trusses. The truss web members needed to be coordinated with
the regularly-spaced copper panel joints as well as the randomly
located column supports. Over several weeks involving both archi-
tectural and structural input, a truss layout was devised that met
both the structural and the architectural requirements.
Because each of the four corners cantilever, the bottom chord is
in compression and requires midspan support. However, the four
perimeter trusses extend below the ballroom level but not to the
meeting room level, with the steel below that cantilevering beyond
the structural bottom chord. The bottom chord of the truss is pulled
Solution 2: Long-Span Roof Structure away from the fourth floor structure, so struts were used to brace the
The second challenge was to reduce the required tonnage on compression segments of the bottom chord back to the structure.
the four perimeter trusses clad in copper and supporting the high These trusses vary in overall depth from 20 ft to 62 ft, with
roof. The rotated grid at the upper structure caused the four cor- a maximum structural depth of 42 ft. The upper box is 282 ft by
ners of the building to cantilever beyond their supports. The layout 296  ft, and the longest cantilever is 117 ft. By working directly
of the occupied spaces also greatly reduced the number of support with the architect, Datum-Gojer was able to reduce the structural
locations that extend to the ground without interrupting the various cost by more than $600,000 while keeping the building’s exterior
occupancies within the building. Finally, three of the four corners appearance intact.
are upturned and all four corners cantilever, and the architectural
look prevented the use of supports at the corners. Solution 3: Terrace and Main Entries
After studying column opportunities on each floor, four column Early architectural renderings of the two main entries showed
locations were identified that would make the box stable. However, the entry glass spanning from the ground floor to the soffit of
the southeast face of the elevated structure remained unsupported, the terrace level without additional structural backup. While the
spanning almost 300 ft. To reduce this span and improve deflection most economical way to frame this would have been to introduce
performance, a fifth support was needed. Datum-Gojer decided structural columns behind the glass to create a conventional beam-
to make use of the architecturally exposed concrete elevator core. and-column floor system, the added elements would have greatly
Using a truss to cantilever from an interior column, over the con- disrupted the architectural appearance. To avoid the additional
crete elevator core, and out to the southeast face of the elevated columns, the engineer proposed to cantilever the floor structure
structure reduced the span of the southeast truss to 190 ft. at these two corners.

The concrete elevator core provides a fifth support point on the Cantilevering the floor structure over the two main entries allowed
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south side of the structure, reducing the required truss span from the area to remain column-free.
300 ft to just 190 ft.

MODERN STEEL CONSTRUCTION november 2010

➤
Many of the convention center’s steel connections, particularly in
the perimeter trusses, are very complicated.

Because the longest cantilever is approximately 153 ft, the bot-

tom chord of the truss would see a significant compression force.
The bottom chord of the trusses also creates the soffit of the entry
and braces the copper cladding and entry glass under wind load-
ing. Therefore, a horizontal bracing truss was provided in the soffit
behind the main bottom chord to reduce the unbraced length of
the main truss cantilever bottom chord and to take the imposed
wind forces. A 3-in. deflection joint was provided at the head of
the curtain wall to isolate the glazing system from the long canti-
lever support structure above. This system allowed the architect to
economically maintain the desired appearance at the primary front
door to the building.
working to complete and provide steel based on the sequence of
Solution 4: Long Spans and Vibration Control erection and the erection timeline provided by the steel erector.
The long-span floor support conditions created a need for seri- Through this process, the engineer was able to issue 90% of the
ous study of vibration. The engineer, along with the contractor steel tonnage in the first mill order package.
and steel fabricator, reviewed and considered several structural The mill schedules indicated that certain shapes would be clos-
floor-framing systems: normal weight versus lightweight concrete ing well ahead of the seven-week window. In particular, column
floors, purlins spaced at 7ft 6 in., 10 ft, and 15 ft, and conventional sections in the W14×90 through W14×132 group would close at
wide-flange versus castellated beams. The vibration performance the end of four weeks. The following week, W36×231 through
for the meeting room and ballroom occupancy and building uses W36×441 would close. These two early mill closings meant that
also needed to be weighed against the costs associated with provid- design of columns and floor trusses would need to be completed
ing a stiffer structural system. after only four and five weeks, respectively.
The engineer proposed using castellated beams at 15 ft spacing Subsequent to the mill order package, the design team issued sev-
with a lightweight concrete slab. This system provides improved eral other advanced bid, permit, and construction packages, includ-
vibration performance for the same structural cost as a similar ing foundations, concrete, and miscellaneous metals. The engineer
wide-flange system. The lightweight concrete slab could be thinner also issued weekly detailing packages, one sequence per week, for the
than a normal weight slab and still achieve the required two-hour mill-ordered steel until the final “Issued For Construction” package
fire separation. This change alone resulted in significant savings to was sent. This process allowed the steel fabricator to begin issuing
the overall project because the heavier, normal weight floors would shop drawings well ahead of the for-construction drawings. Approx-
have required more steel tonnage and larger, deeper piers. Addi- imately 15% of the steel on the project was reviewed, approved, and
tionally, the increased purlin spacing reduced the number of steel in fabrication prior to the final construction package.
pieces, decreasing fabrication and erection time while improving
vibration performance. Conclusion
The project is currently under construction and on schedule to
Solution 5: The Fast Track Process be completed in January 2011. The solutions provided by Datum-
At the end of the design development process, the design team Gojer were instrumental in maintaining the construction schedule.
met with the owner, contractor, and steel fabricator to discuss the In addition, the construction cost was significantly reduced from
budget and the remaining schedule. The contractor stated that to the original construction cost estimates. The building is now well
meet the owner’s required opening date, the building would need within budget, and the structural solutions played a key role in
to be issued for construction in just seven weeks. Given the level of achieving the necessary savings in addition to contributing to the
completion of the design at that time, along with the complexity of owner’s desire for an identifiable landmark facility. 
the building, everyone agreed that was an impossible task. Owner’s Representative
While brainstorming ways to meet the owner’s schedule, the Beck Group, Dallas
engineer noted that certain elements of the project were time-
Architect
critical. In particular, the concrete and foundation elements required
RMJM (formerly Hillier), Princeton, N.J.
only a minimal amount of time from design to construction, while
structural steel procurement, fabrication, and delivery would Structural Engineer
require far more lead time. Additionally, not all of the steel would Datum Gojer Engineers, LLC, Dallas (Datum Engineers and
be required on the first day of construction as steel erection was Charles Gojer & Associates)
scheduled to take several months. The length of time between the Steel Fabricators
first steel order and the last steel delivery allowed the steel to be North Texas Steel, Fort Worth, Texas (AISC Member)
issued in multiple packages. W&W Steel, Oklahoma City, Okla. (AISC Member)
The design and construction team agreed to issue a minimum Steel Erector
of 60% of the steel tonnage for mill order within the contractor’s Bosworth Steel Erectors, Inc., Dallas (AISC, IMPACT and SEAA
seven-week window. Engineers worked with the steel fabricator Member)
to determine the longest lead items for fabrication, while also

 november 2010 MODERN STEEL CONSTRUCTION