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Construction Systems for High-Rise Structures
High-Rise Construction
Introduction
The determination of the structural form of a high-rise building would involve the selection and arrangement of the major structural elements to resist most efficiently the various combinations of gravity and horizontal loading.
The principal objectives in choosing a building’s structural form are to arrange to support the gravity, dead and live loading, and to resist at all levels the external horizontal load shear, moment, and torque with adequate strength and stiffness.
Non-structural considerationsThe range of factors other than structural
considerations that influence the choice of structural form areInternal planningMaterialMethod of constructionExternal architectural treatmentPlanned locationRouting of the service systemsHeight & proportions of the building
Weight of steel in tall buildings
Height & Structural loading
In HRBs designed for a similar purpose and of the same material and height, the efficiency of the structures can be compared roughly by their weight per unit floor area.
The weight of the floor framing is influenced by the floor span and is independent of the building height, while the weight of the columns is approximately proportional to the height.
Buildings of up to 10 storeys can accommodate wind loading without increase in member sizes, because of the typically allowed increase in permissible stresses in Design Codes for the combined loading.
Height & Structural loading
For buildings of more than 10 storeys, the additional material required for wind resistance increases nonlinearly with height.
For buildings of 50 storeys and more the selection of an appropriate structural form may be critical for the economy and indeed the viability of the building.
Function of the building
A major consideration affecting structural form is the function of the building.
Modern office buildings call for large floor spaces that can be subdivided with lightweight partitioning to suit the individual tenant’s needs – the structure’s main vertical components are generally arranged as far as possible around the perimeter of the plan and internally in groups around the elevator, stair and service shafts.
Function of the building
The floors span the areas b/w the exterior and interior components, leaving large column free areas available for office planning.
The services are distributed horizontally in each storey above the partitioning and are concealed in a ceiling space. The extra depth required by this space causes the typical storey height in an office building to be 3.5m or more.
Plan of office block(tube-type)
Function of the building
In a residential building or hotel, accommodation is subdivided permanently and usually repetitively from floor to floor.
Therefore, continuously vertical columns and walls can be distributed over the plan to form, or fit within, the partitioning.
The services can then be run vertically, adjacent to the walls and columns or in separate shafts, to emerge in each storey either very close to where required, or to be distributed horizontally from there to where required, along the corridor ceiling spaces.
Plan of residential block
Function of the buildingWith the exception of the corridors,
therefore, a ceiling space is not required, and the soffit of the slab can serve as the ceiling.
This allows the storey heights in a typical residential building or hotel to be kept down to approximately 2.7 m.
A 40-storey residential building is, therefore, generally of significantly less height than a 40-storey office building.
Positioning of vertical members
With regard to horizontal loading, a HRB is essentially a vertical cantilever.
This may comprise one or more individually acting vertical cantilevers, such as shear walls or cores, each bending about its own axis and acting in unison only through the horizontal in-plane rigidity of the floor slabs.
Within the constraints of selected structural form, advantage may be taken of locating the main vertical members on plan so that the dead load compressive stresses suppress the lateral load tensile stresses, thereby avoiding the possibility of net tension occurring in the vertical members and uplift on the foundations.
Steel framingAppropriate for all heights of structure and,
because of its high strength-to-weight ratio, it has always been the material of construction for the tallest buildings.
Allows the possibility of longer spans, and of partial prefabrication, leading to reduced site work and more rapid erection.
Disadvantages: needs fire & rust protection, expensive to clad and requires costly diagonal bracing or rigid-frame connections.
RCC tall buildings
Introduced two decades after the first steel tall buildings.
Earlier structures were influenced in form by the skeletal, column and girder arrangements of their steel counterparts.
They differed in depending on the inherent rigid frame action of concrete construction to resist horizontal loading.
The subsequently introduced flat plate & flat slab forms and moment resistant frame, continued till late 1940s.
Shear wallsA major step forward in RC high-rise
structural form to resist horizontal loading.Freed the concrete HRBs, the 20 to 25
storey limitations of the rigid frame and flat plate systems.
The innovation and refinement of these forms with the development of HPC has allowed the height of concrete buildings to reach within striking distance of 100 storeys.
Construction Systems for HR StructuresBraced-Frame structuresRigid-Frame structuresInfilled-Frame structuresFlat-Plate and Flat-Slab structuresShear Wall structuresWall-Frame structuresFramed-Tube structuresOutrigger-Braced structuresSuspended structuresCore structuresSpace structuresHybrid structures
Braced-Frame StructuresThe lateral resistance of the structure is
provided by diagonal members. Horizontal shear is resisted by the horizontal
components of the axial tensile or compressive actions in the web members.
Generally regarded as an exclusively steel system because the diagonals are inevitably subjected to tension.
Sometimes concrete bracing of the double diagonal form are used where each diagonal designed as a compression member to carry the external shear.
Braced Frame
Braced Frame
Braced-Frame StructuresAdvantages: Able to produce a laterally
very stiff structure for a minimum of additional material makes it an economical structural form for any height of building.
Disadvantages: It obstructs the internal planning and
the location of windows and doors. Diagonal connections are expensive
to fabricate and erect.
Rigid-Frame Structures
Consist of columns and girders joined by moment-resistant connections.
The lateral stiffness of a rigid-frame bent depends on the bending stiffness of the columns, girders, and connections in the plane of the bent.
Advantage: Its open rectangular arrangement allows freedom of planning and easy fitting of doors and windows.
Disadvantage: In its typical 6 x 9 m bay size, rigid framing is economic only for buildings up to about 25 storeys. Above 25 storeys the relatively high lateral flexibility of the frame calls for uneconomically large members in order to control the drift.
Rigid-Frame
Rigid-Frame Structures
Ideally suited for RC buildings because of the inherent rigidity of RC joints.
Also used for steel frame buildings, but MR connections in steel tend to be costly.
Gravity loading also is resisted by the rigid-frame action.
Negative moments are introduced in the girders adjacent to the columns causing the mid-span positive moments to be significantly less than in a simply supported span.
Rigid-Frame Structures
Infilled-Frame StructuresMost usual form of construction for tall buildings
of up to 30 storeys in height in many countries.Column and girder framing of RC, or sometimes
steel, is filled by panels of brickwork, block-work, or cast-in-place concrete.
When an infilled frame is subjected to lateral loading, the infill behaves effectively as a strut along its compression diagonal to brace the frame.
Because the infills serve also as external walls or internal partitions, the system is an economical way of stiffening and strengthening the structure.
Infilled-Frame Structures
Infilled-Frame StructuresThe complex interactive behaviour of the
infill in the frame, and the rather random quality of masonry, has made it difficult to predict with accuracy the stiffness and strength of an infilled frame.
No method of analyzing infilled frames for design has gained general acceptance.
Because of the fear of the unwitting removal of bracing infills at some time in the life of the building, the use of the infills for bracing tall buildings has mainly been supplementary to the rigid-frame action of concrete frames.
Flat-Plate and Flat-Slab Structure
The simplest of all structural forms that consists of uniform slabs of 12-20 cm thickness, connected rigidly to supporting columns.
The system, which is essentially of RC, is very economical in having a flat soffit requiring the most un-complicated formwork and, because the soffit can be used as the ceiling, in creating a minimum possible floor depth.
Under lateral loading the behaviour of a flat-plate structure is similar to that of rigid frame, that is, its lateral resistance depends on the flexural stiffness of the components and their connections with the slabs corresponding to the girders of the rigid frame.
Two-way flat-plate
Two-way flat-slab
Flat-Plate and Flat-Slab StructureAppropriate for apartment and hotel
construction where ceiling spaces are not required and where the slab may serve directly as the ceiling.
The flat-plate structure is economical for spans of up to about 8 m, above which drop panels can be added to create a flat-slab structure for spans of up to 12 m.
Buildings that depend entirely for their lateral resistance on flat-plate or flat-slab action are economical up to 25 storeys.
When Code requirements for wind design were less stringent, many flat-plate buildings were constructed in excess of 40 stories, and are still performing satisfactorily.
Shear Wall StructuresConcrete or masonry continuous vertical walls may
serve both architecturally as partitions and structurally to carry gravity and lateral loading.
Very high in-plane stiffness and strength makes them ideally suited for bracing tall buildings.
Shear walls are entirely responsible for the lateral load resistance of the building.
Act as vertical cantilevers in the form of separate planar walls, and as non-planar assemblies of connected walls around elevator, stair, and service shafts.
Much stiffer horizontally than rigid frames, shear wall structures can be economical up to 35 storeys.
Shear Wall Structure
WYNN LAS VEGAS - Concrete shear wall structure with post-tensioned floor system, 50-story tower with 2,700 hotel rooms - the tallest hotel tower (543’ tall) on the Las Vegas Strip
Shear Wall Structures
In contrast to rigid frames, the shear walls’ solid form tends to restrict planning where open internal spaces are required.
Well suited to hotels and residential buildings where the floor-by-floor repetitive planning allows the walls to be vertically continuous and where they serve simultaneously as excellent acoustic and fire insulators b/w rooms and apartments.
In low-to-medium-rise buildings, if shear walls are combined with frames, it is reasonable to assume that the shear walls attract all the lateral loading so that the frame may be designed for only gravity loading.
Shear Wall StructuresIt is especially important in shear wall
structures to try to plan the wall layout so that the lateral load tensile stresses are suppressed by the gravity load stresses. This allows them to be designed to have only the minimum reinforcement.
Shear walls structures have been shown to perform well in earthquakes for which case ductility becomes an important consideration in their design.
Coupled Wall StructuresTwo or more shear walls in the same
plane, or almost the same plane, connected at the floor levels by beams or stiff slabs.
The effect of the shear-resistant connecting members is to cause the set of walls to behave in their plane partly as a composite cantilever bending about the common centroidal axis of the walls.
Results in a horizontal stiffness very much greater than if the walls acted as a set of separate uncoupled cantilevers.
Coupled Wall Structure
Coupled Wall StructuresCoupled walls occur in residential construction
where lateral-load resistant cross walls, which separate the apartments consist of in-plane coupled pairs, or trios, of shear walls b/w which there are corridor or window openings.
Although shear walls are obviously more appropriate for concrete construction, they have occasionally been constructed of heavy steel plate in the style of massive vertical plate or box girders as parts of steel frame structures.
They have been designed for locations of extremely heavy shear such as at the base of elevator shafts.
Wall-Frame StructuresWhen shear walls are combined with rigid frames
the walls, which tend to deflect in a flexural configuration, and the frames, which tend to deflect in a shear mode, are constrained to adopt a common deflected shape by the horizontal rigidity of the girders and slabs.
As a consequence, the walls and frames interact horizontally, especially at the top to produce a stiffer and stronger structure.
The interacting wall-frame combination is appropriate for buildings in the 40 to 60 storey range, well beyond that of rigid frames or shear walls alone.
Wall-Frame Structure
Wall-Frame Structures
An additional feature of the wall-frame structure is that in a carefully “tuned” structure, the shear in the frame can be made approximately uniform over the height, allowing the floor framing to be repetitive.
Although the wall-frame structure is usually perceived as a concrete structural form, with shear walls and concrete frames, a steel counterpart using braced frames and steel rigid frames offers similar benefits of horizontal interaction.
The braced frames behave with an overall flexural tendency to interact with the shear mode of the rigid frames.
Framed-Tube StructuresThe lateral resistance of framed-tube structures is
provided by very stiff MR frames that form a “tube” around the perimeter of the building.
The frames consist of closely spaced columns, 2 – 4m b/w centres, joined by deep spandrel girders.
Although the tube carries all the lateral loading, the gravity loading is shared b/w the tube and interior columns or walls.
When lateral loading acts, the perimeter frames aligned in the direction of loading act as the “webs” of the massive tube cantilever, and those normal to the direction of the loading act as the “flanges”.
Framed-Tube StructuresThe tube is suitable for both steel and RC
construction and has been used for buildings ranging from 40 to more than 100 storeys.
The highly repetitive pattern of the frames lends itself to prefabrication in steel and to the use of rapidly moving gang forms in concrete, which make for rapid construction.
Most significant modern developments in HR structural form which offers efficient and easily constructed structure for greatest of heights.
Framed-Tube
(a) Tube-in-Tube or Hull-core StructuresThe variation of the framed tube
consists of an outer framed tube, the “hull” together with an internal elevator and service core.
The hull and core act jointly in resisting both gravity and lateral loading.
In a steel structure the core may consist of braced frames, whereas in a concrete structure it would consist of an assembly of shear walls.
Tube-in-Tube
(b) Bundled-Tube Structures
Used for the Sears Tower in Chicago – the then world’s tallest building.
The Sears Tower consists of four parallel rigid steel frames in each orthogonal direction, interconnected to form nine “bundled” tubes.
In Sears Tower, advantage was taken of the bundled form to discontinue some of the tubes, and so reduce the plan of the building at stages up the height.
Bundled-Tube
Sears Tower
(c) Braced-Tube Structures
Another way of improving the efficiency of the framed tube, thereby increasing its potential for use to even greater heights as well as allowing greater spacing b/w columns, is to add diagonal bracing to the faces of the tube.
First used in a steel structure in 1969, in Chicago’s John Hancock Building and in RC structure in 1985, in New York’s 780 Third Avenue Building.
In the steel tube the bracing traverses the faces of the rigid structures, whereas in the concrete structure the bracing is formed by a diagonal pattern of concrete window-size panels, poured integrally with the frame.
Steel-braced tube & Concrete-braced tube
Chicago’s John Hancock Building
New York’s 780 Third Avenue Building
Outrigger-Braced Structures
Consists of a central core, comprising either braced frames or shear walls, with horizontal cantilever “outrigger” trusses or girders connecting the core to the outer columns.
When the structure is loaded horizontally, vertical plane rotations of the core are restrained by the outriggers through tension in the windward columns and compression in the leeward columns.
Outrigger-Braced StructuresThe effective structural depth of the
building is greatly increased, thus augmenting the lateral stiffness of the building and reducing the lateral deflections and moments in the core.
In effect, the outriggers join the columns to the core to make the structure behave as a partly composite cantilever.
Used for buildings form 40 to 70 storeys high, but the system should be effective and efficient for much greater heights.
Outrigger-braced structure & Outrigger-braced structure under
load
Suspended Structures
Consists of a central core, or cores, with horizontal cantilevers at roof level, to which vertical hangers of steel cable, rod, or plate are attached.
The floor slabs are suspended from the hangers.Advantages: Primarily architectural in that, except
for the presence of the central core, the ground storey can be entirely free of major vertical members, thereby allowing an open concourse, also, the hangers, because they are in tension and consequently can be of high strength steel, have a minimum sized section and therefore less obtrusive.
Suspended structure & sequence of construction
Two-tiered suspended structure
Core StructuresA single core to carry the entire gravity
and horizontal loading.In some, the slabs are suspended at each level
by cantilevers from the core. In others, the slabs are suspended b/w the core and perimeter columns, which terminate either on major cantilevers at intervals down the height, or on a single massive cantilever a few storeys above the ground.
Advantages: mainly architectural, in providing a column-free perimeter at the ground level and at other levels just below the cantilevers.
Core Structure
Space StructuresThe primarily load-resisting system of a
space structure consists essentially of a 3D triangulated frame – as distinct from an assembly of planar bents – whose members serve dually in resisting both gravity and horizontally loading.
Highly efficient, relatively lightweight structure with a potential for achieving the greatest heights.
The 76-storey Hong Kong bank of China Building is a classic example.
Core Structures
Core Structures
76-storey Hong Kong bank of China Building
Suspended Structures
Hybrid StructuresNew generation of “postmodern” buildings that are
emphatically non-regular in shape, with large scale cut-outs, flutings, facets, and crowns that defy classification.
Combinations of two or more of the basic structural forms have often been used in the same building, either by direct combination as in a suspended tube and outrigger system or by adopting different forms in different parts of the structure as in a tube system on three faces of the building and a space frame on a faceted fourth face.
With the ready availability of powerful computers and highly efficient structural analysis programs, an engineer is able to analyze a structure to suit a building of almost any conceivable irregularity.
Hybrid Structure
Hybrid Structure
Floor Systems
Floor Systems – Reinforced ConcreteOne-way slabs on beams or wallsOne-way pan joists and beamsOne-way slab on beams and girdersTwo-way flat plateTwo-way flat slabWaffle flat slabsTwo-way slab and beam
Floor systems – Steel framingOne-way beam systemTwo-way beam systemThree-way beam systemComposite steel-concrete floor systems
Burj Dubai Project
A Case Study
Burj Dubai- Artist’s Rendering828 meters tall.160 floors.Total floor area
4,60,000 square meters
Multi-use: residential, hotel, commercial, office, entertainment, shopping and leisure.
The Main TeamClient of Burj Dubai Tower - Emaar
Properties, a major developer of lifestyle real estate in the Middle East.
Construction Manager - Turner International
General Contractor - Samsung Joint Venture (consisting of Samsung, Korea base contractor; Besix, Belgium base contractor; and Arabtec, Dubai base contractor).
The DesignDerived from geometries of the desert
flower, which is indigenous to the region, and the patterning systems embodied in Islamic architecture.
The tower massing is organized around a central core with three wings. Each wing consists of four bays. At every seventh floor, one outer bay peels away as the structure spirals into the sky.
Unlike many super high-rise buildings with deep floor plates, the Y-shape floor plans of Burj Dubai maximize views and provide tenants with plenty of natural light.
The tower superstructure is designed as an all reinforced concrete building with high performance concrete from the foundation level to level 156, and is topped with a structural steel braced frame from level 156 to the pinnacle.
Structural System
Lateral Load Resisting System
The tower’s lateral load resisting system consists of high performance, reinforced concrete ductile core walls linked to the exterior reinforced concrete columns through a series of reinforced concrete shear wall panels.
The core walls vary in thickness from 1300mm to 500mm.
The core walls are typically linked through a series of 800mm to 1100mm deep reinforced concrete or composite link beams at every level.
Due to the limitation on the link beam depth, ductile composite link beams are provided in certain areas of the core wall system.
These composite ductile link beams typically consist of steel shear plates, or structural steel built-up I-shaped beams, with shear studs embedded in the concrete section. The link beam width typically matches the adjacent core wall thickness.
At the top of the centre reinforced concrete core wall, a very tall spire tops the building, making it the tallest tower in the world for all categories.
The lateral load resisting system of the spire consists of a diagonal structural steel bracing system at level 156.
Floor Framing System
The residential and hotel floor framing system of the Tower consists of 200mm to 300mm two-way reinforced concrete flat plate slabs spanning approximately 9 m between the exterior columns and the interior core wall.
The floor framing system at the tips of the tower floor consists of a 225mm to 250mm two-way reinforced concrete flat plate system.
The floor framing system within the interior core consists of a two way reinforced concrete flat plate system with beams.
Comparison of various structural systems
