[scalziand]

Looking Down at the Sears Tower

An 1,486'-high observation deck in the Miglin-Beitler Tower will enable visitors to look down at the 1,454' Sears Tower

By Charles H. Thornton, P.E.; Udon Hungspruke; and Robert P. DeScenza, P.E.

At 1,999'-11 1/2" to the tip of its spire, the Miglin-Beitler Tower will provide a new cap for Chicago's skyline-while establishing new records for both the world's tallest building and the world's tallest non-guyed structure.

A simple and elegant integration of building form and function has emerged from close coordination of architectural, structural, and development team members. The resulting cruciform tube scheme offers structural efficiency, superior dynamic behavior, ease of construction, and minimal intrusion of leased office floors for this 125-story office building. But what really distinguishes it from its existing brethren in the very small family of 100+ story buildings is its relatively small plates that will enable smaller-sized firms to rent entire floors.



Structural System

The challenge of the design team was creating an economic and buildable structural system capable of resisting vertical and lateral loads for a supertall building with a relatively small footprint. The challenge was met with a composite system exploiting the advantages of both steel and concrete. The stiffness of high-strength concrete was combined with the advantages of a steel floor system, including its inherent strength, speed of construction, and flexibility to allow tenant changes.

The cruciform tube structural system has six major components:

+A 62'-6" x 62'-6" concrete core with walls of varying thickness. The interior cross walls of the core are generally not penetrated with openings. This significantly contributes to the lateral stiffness.

+Eight cast-in-place concrete fin columns are located on the faces of teh building and extend up to 20' beyond the 140'x140' tower footprint. They vary in dimension from 6'-6"x 33' at the base to 5'-6"x15' at the middle to 4'-6"x13' near the top.

+Exterior steel Vierendeel trusses consisting of horizontal spandrels and two vertical columns are located at each of the 61' wide faces on the four sides of the building between the fin columns. Exterior steel vierendeel trusses are used to pick up each of the four cantilevered corners of the building. These vierendeel trusses provide additional resistance to lateral forces as well as improving the resistance of the entire structural system to torsion. In addition, the trusses transfer deadload to the fin columns to eliminate tensile and uplift forces in the fin columns. All corner columns are eliminated providing for corner offices with undisturbed views. Connections between the steel vierendeel trusses and the concrete fin columns are typically simple shear connections, which minimize construction costs and expedite erection.

+Eight link beams connect the four corners of the core to the eight fin columns at every floor. These reinforced concrete beams are haunched at both ends for increased stiffness and reduced depth at mid-span to allow for passage of mechanical ducts. By linking the fin columns and the cor they enable the full width of the building to act in resisting lateral forces. In addition to link beams at every floor, sets of two-story deep outrigger walls are located at levels 16, 56, and 91. These outrigger walls enhance the interaction between exterior fin and columns and the core.

+The conventional structural steel composite floor system has 18" deep rolled steel beams spaced approximately 10' on center. A slab of stone concrete topping spans between the beams. The steel floor system is supported by the cast-in-place concrete elements.



+A 600'-tall steel framed tower tops the building. This braced frame will house observation levels, window washing equipment rooms, and an assortment of broadcasting equipment.

Lateral Forces

The proposed building and structural system has undergone extensive wind tunnel testing at RWDI in Guelph, Ontario. Pressure tap model, pedestrian level studies, high frequency force balance and aeroelastic models have been used to determine the static and dynamic behavior of the project under wind loadings.



In addition to providing ample resistance to all expected wind loads, the design has received a superior performance rating in its ability to virtually eliminate occupant perception of wind movements and accelerations.

Three separate computer programs, EASE-II, SAP90, and ETABS provided parallel checks on the accuracy and adequacy of 16 independent two-dimensional and three-dimensional static and dynamic computer analyses. The parallel sets of models were compared to validate computer approaches. Static displacements and dynamic mode shapes from the two sets of analyses were in very close agreement. Overall displacements, modal shapes and natural frequencies differed by less than 10%.

Although only UBC Zone I is applicable, the structural system was investigated for the effects of a UBC Zone 2 earthquake and was found satisfactory.

Foundations

The foundation system proposed for this project uses caissons varying from 8'-6" to 10'-6" in diameter. Each 95'-long caisson has a straight shaft and a rock socket a minimum of 6' into competent rock. The caissons are tied together with a series of grade beams. Passive pressure on the edge of these lugs and on the projected side surfaces of the caissons provides the base shear resistance for the tower.

Steel Vierendeel Trusses

On each of the four faces of the building, steel vierendeel trusses are employed to frame the 61' clear opening between the fin columns. The trusses consist of a W36 horizontal beam at each level with two W36 verticals. To eliminate stress produced by creep and shrinkage strains in the concrete fin columns, the verticals in the truss are provided with vertical slip connections. this has the added benefit of channeling all of the gravity loads on each of the building faces out to the fin columns to help eliminate uplift forces on the foundations.

The steel face Vierendeels are to be shop fabricated as horizontal trusses 12'-6" tall by 61' long. Field connections are simple bolted connections. this system allows for all of the welded connections to be shop fabricated, which results in an economical and elegant solution.

At each of teh corners of the Miglin-Beitler Tower, the floor slabs protrude beyond the fin columns by up to 26'. Again, it was desirable to channel the gravity loads from these areas to the fin columns to help eliminate uplift in the foundations due to lateral loads.

An added challenge was to frame the corners without having a vertical element at the corner, thus allowing the corner offices unobstructed views. The solution to this problem was a vierendeel truss, similar to the face vierendeels. The corner Vierendeels are to consist of a horizontal steel beam at each level with a vertical steel beam at the center of each face, thus allowing the corners of the building to be column free. Unlike the face vierendeels, all the vertical connections are not slip connections. This is to allow the corner vierendeels to resist unbalanced floor loads.

Topping the tower will be a 600'-tall steel-framed spire. The main structural framing consists of 12 exterior columns that cascade out at each of the setback levels. Each level of the spire contains horizontal bracing that stabilizes the structure. In addition, each of the elevations is typically x-braced, with the exception of a three story segment at the observation levels, where the design team desired to take advantage of the views.

Several steel vierendeel frames were analyzed to come up with an optimal solution that would minimize obstructions to the [ ] view. Topping off the spire is a section of 8' diameter steel tube. The tube is to be perforated with openings that allow for the installation of a wide range of broadcast equipment.

From Modern Steel Construction August 1991