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international projects

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By Brad MalMsten, P.e.

WhILE ThE NEvER-ENDINg RaCE or worlds tallest building hasshited to the Middle East or now, tallest on the continent is nothingto sneeze at. Upon its completion next year, Tower A o the Federation

Tower complex in Moscow, Russia will be the tallest building in Europeat 1,181 t (its architectural central spire will reach 1,470 t).The 93-story Tower A is mainly concrete (as is the 62-story Tower

B), but major steel raming at multiple crucial areas o the towerincluding an atrium at the topwas essential to bringing the towertogether and allowing it to vie or the title o Tallest in Europe.Roughly 5,500 tons o structural steel in all made this happen.

Tower a Structure

Outrigger levelscomplicated structural interchanges involvingcolumn transers, shiting o wind loads rom core to exterior, andrebalancing o building weightare the hubs o the towers load net-

work. These hubs are built in steel, and the result is open space or

equipment, access to resh outside air, and a system o trusses capableo supporting ully reversed loading.

The trusses link all o the exterior columns together into onegiant perimeter belt and outrigger trusses link the concrete cen-tral core to this belt. This linkage maniests the network o loadpaths that is the cornerstone o the buildings plan to maximizethe resiliency o the structure as a whole, stabilize it against lateralloading, and mitigate the potential or progressive collapse.

The steel truss systems are placed at the one-third and two-thirdsheights o Tower A and share oor space with mechanical rooms andreuge areas. These areas naturally separate the building into distinctfre protection zones as well as occupancies. Steel raming also allowedclear spacethat would have been impossible to match with concrete

wallsacilitating the MEP design and the layout o reuge oors.The complex geometry o the tower also made these particular

areas more conducive to steel raming. The extraordinary amounto reinorcing steel that would be required and the curvature othe exterior belt made rebar detailing difcult and execution cum-bersome, and thus prone to non-conormance. On the other hand,angle changes were easily handled by steel truss work, with gussetplates careully detailed and shop-abricated to bend around thecolumn anges to make the required curvature.

At about 20 t deep, the 33rd-oor outrigger system is shallowerthan the designers would have preerred due to other design constraints,but even more reason to take advantage o the ability o steel to handle

Steel steps in to help Moscows Federation Tower reach

new heights and become Europes tallest building.

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july 2009 MODERN STEEL CONSTRUCTION

incredibly large orces in relatively small space. Some outrigger diago-nals see design axial orces as high as 3,500 tons, and the belt truss mem-bers are designed or up to 1,700 tons. The entire truss system weighs

roughly 2,000 tons and is comprised almost entirely o W14 sectionsconnected by ange-bolted gussets. Where trusses intersect, built upbox sections were used in order to simpliy the connection details.

The 61st-oor truss system is about 43 t deep and doubles as atranser truss or hal o the perimeter columns. At this level, out-rigger diagonals see axial orces as high as 5,400 tons and the belttruss members up to 3,500 tons. This group o outriggers and belttrusses weighs in at about 3,000 tons. Again, W14 sections wereused where possible, and the most extremely loaded pieces werebuilt up rom 5-in.-thick, 30- to 34-in.-wide plates.

Lod Network

The term load network is used above to indicate the level o

interconnectivity o the structural elements o Tower A. As in all build-ings designed to resist progressive collapse, creation o alternate loadspaths was desired. In the case o the Federation Tower, steel trussesallow two levels to be orce hubs in the overall load network o thestructure, providing not just one load path and one potential alternateload path, but an unlimited number o potential load paths.

Continuity and ability to handle reversed loading in all o thestructural elements in the building is a major theme o the design.

With this in mind, the steel trusses were not only encased in theconcrete core walls and columns within height o outriggers, butalso truss verticals were extended one oor up and down into theconcrete structure above and below. This ensures the best behavioro the building as a whole in common or unusual circumstances, as

well as ensures that each steel piece is loaded in a manner consis-tent with the design intent. Stubbing up and down allows the con-crete to unload smoothly into the steel trusses, and or the trussesto unload smoothly into the concrete columns and walls.

Cp Structure

The Tower A cap was destined to be a highlight o the project.Curved sides and roo meet above a fve-star hotel lobby and exclusive

VIP space, and the opportunity to do something special called. Steelabout 500 tons o itprovided the answer in the orm o an integrated,transparent crown.

The stunning glass box that caps Tower A is designed so that theglazing mullions also act as part o the load bearing structure. Inte-grating the structure, glazing, cleaning, drainage, shading, fre protec-

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tion, and MEP systems results in maximumtransparency and allows the occupant torealize the openness and beauty o the spaceinstead o its many components, most o

which will be ully exposed. The 16,000-sq.-t open space eatures built-up aade mul-lions, solid-plate roo mullions, two central

super columns built rom 4-in.-thick plates,and exposed triangular super trusses.

Perhaps no building in the worldso clearly demonstrates the benefts ohybridizationor how even in a concretebuilding, there are still areas where steel isthe only option.

Brad Malmsten is an associate with ThorntonTomasetti and an AISC Professional Member.

Structurl Enineer

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