High Rise Structures
A high-rise is a tall building or structure
- ·Buildings between 75 feet and 491 feet (23 m to 150 m) high are considered high-rises. Buildings taller than 492 feet (150 m) are classified as skyscrapers.
- The materials used for the structural system of high-rise buildings are reinforced concrete and steel. Most American style skyscrapers have a steel frame, while residential tower blocks are usually constructed out of concrete.
- High-rise structures have certain features. The structures are high & lead to higher vertical loads and higher lateral loads (mainly due to wind stress) in comparison with lower buildings.
LOADS ON THE HIGHRISE STRUCTURES
- Dead loads arise from the weigh to the individual construction elements and the finishing loads.
- Live loads are dependent on use depending on the number of stories; live loads can be reduced for load transfer and the dimensioning of vertical load-bearing elements.
· However, the reduction of the total live load on a construction element may not exceed 40%.
- Calculation of lateral loads should be carefully scrutinized.
- It generally arises from unexpected deflections, wind and earthquake loads
- It arises from imprecision in the manufacture of construction elements and larger components.
- Another cause is the uneven settling of the foundation at an in-homogeneous site.
- Any deflection produces additional lateral forces.
- High-rise buildings are susceptible to oscillation. It should not be viewed as statically equivalent loads, but must be investigated under the aspect of sway behaviour.
- Wind tunnel experiments are used to see the influence of the building?s shape on the wind load.
- The ability of wind loads to bring a building to sway must also be kept in mind. This oscillation leads both to a perceptible lateral acceleration for occupants, and to a maximum lateral deflection.
- Seismology (from the Greek seismos= earthquake and logos= word)
- scientific study of earthquakes
- propagation of elastic waves through the Earth.
- studies of earthquake effects, such as tsunamis
- diverse seismic sources such as volcanic, tectonic, oceanic, atmospheric, and artificial processes such as explosions.
- Produce different types of seismic waves.
- It travel through rock, and provide an effective way to image both sources and structures deep within the Earth.
There are three basic types of seismic waves in solids:
- P-and/or S-waves.
- The two basic kinds of surface waves (Raleigh and Love).
Pressure waves,/Primary waves /P-waves,
- Travel at the greatest velocity within solids and are therefore the first waves to appear on a seismogram.
- P-waves are fundamentally pressure disturbances that propagate through a material by alternately compressing and expanding (dilating) the medium, where particle motion is parallel to the direction of wave propagation.
Shear waves/secondary waves/S-waves,
- Transverse waves that travel more slowly than P-waves and thus appear later than P-waves on a seismogram.
- Particle motion is perpendicular to the direction of wave propagation. Shear waves do not exist in fluids such as air or water.
Type of High-Rise Structure
- Braced Frame
- Rigid Frame Structure
- Infilled Frame Structure
- Flat Plate and Flat Slab Structure
- Shear wall structure
- Coupled wall structure
- Wall-frame structure
- Framed tube structure
- The trussed tube
- Tube in tube or Hull core structure
- Bundled tube structure
- Core and Outriggers system
- Hybrid structure
- Braced frames are cantilevered vertical trusses resisting laterals loads primarily through the axial stiffness of the frame members.
- The effectiveness of the system, as characterized by a high ratio of stiffness to material quantity, is recognized for multi-storey building in the low to mid height range.
- Generally regarded as an exclusively steel system because the diagonal are inevitably subjected to tension for or to the other directions of lateral loading.
- Able to produce a laterally very stiff structure for a minimum of additional material, makes it an economical structural form for any height of buildings, up to the very tallest.
- Girders only participate minimally in the lateral bracing action-Floor framing design is independent of its level in the structure.
- Can be repetitive up the height of the building with obvious economy in design and fabrication.
Obstruct the internal planning and the locations of the windows and doors; for this reason, braced bent are usually incorporated internally along wall and partition lines, especially around elevator, stair, and service shaft.-Diagonal connections are expensive to fabricate and erect.
ACT Tower, Himatsu Japan
Rigid Frame Structure
Consist of columns and girders joined by moment resistant connections. Lateral stiffness of a rigid frame bent depends on the bending stiffness of the columns, girders, and connection in the plane of the bents. Ideally suited for reinforced concrete buildings because of the inherent rigidity of reinforced concrete joints. Also used for steel frame buildings, but moment-resistant connections in steel tend to be costly. While rigid frame of a typical scale that serve alone to resist lateral loading have an economic height limit of about 25 stories, smaller scale rigid frames in the for of perimeter tube, or typically rigid frames in combination with shear walls or braced bents, can be economic up top much greater heights.
- May be place in or around the core, on the exterior, or throughout the interior of the building with minimal constraint on the planning module.
- The frame may be architecturally exposed to express the grid like nature of the structure.
- The spacing of the columns in a moment resisting frame can match that required for gravity framing.-Only suitable for building up to 20 –30 storiesonly; member proportions and materials cost become unreasonable for building higher than that.
Fig. WTC OSAKA JAPAN
In-filled Frame Structure
Most usual form of construction for tall buildings up to 30 stories in height Column and girder framing of reinforced concrete, or sometimes steel, is in-filled by panels of brickwork, block work, or cast-in-place concrete. Because of the in-filled serve also as external walls or internal partitions, the system is an economical way of stiffening and strengthening the structure. The 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 in-filled frame.
Fig. Infilled Frame.
Flat-Plate and Flat Slab Structure
- Is the simplest and most logical of all structural forms in that it consists of uniforms slabs, connected rigidly to supporting columns.
- The system, which is essentially of reinforced concrete, is very economical in having a flat soffit requiring the most uncomplicated formwork and, because of the soffit can be used as the ceiling, in creating a minimum possible floor depth.
- Lateral resistance depends on the flexural stiffness of the components and their connections, with the slab corresponding to the girder of the rigid frame.
- Particularly appropriate for hotel and apartment construction where ceiling space is not required and where the slab may serve directly as the ceiling.
- Economic for spans up to about 25 ft (8m),above which drop panels can be added to create a flat-slab structure for span of up to 38 ft (12m).
- Suitable for building up to 25 stories height.
Shear Wall Structure
Concrete or masonry continuous vertical walls may serve both architecturally partitions and structurally to carry gravity and lateral loading. Very high in plane stiffness and strength make them ideally suited for bracing tall 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 shaft. well suited to hotel and residential buildings where the floor-by floor repetitive planning allow the walls to be vertically continuous and where they serve simultaneously as excellent acoustic and fire insulators between rooms and apartments. Minimum shrinkage restraint reinforcement where the wall stresses are low, which can be for a substantial portion of the wall.
- Tensile reinforcement for areas where tension stresses occur in walls when wind uplifts stresses exceeds gravity stresses.
- Compressive reinforcement with confinement ties where high compressive forces require the walls is designed as columns. Individual shear walls, say at the edge of a tall building, are design as blade walls or as columns resisting shear and bending as required.
- High strength concrete has enable wall thickness to be minimized, hence maximizing rentable floor space.
- Technology exists to pump and to place high-strength concrete at high elevation.
- Fire rating for service and passenger elevator shafts is achieved by simply placing concrete of a determined thickness.
- The need for complex bolted or side-welded steel connections is avoided.
- Well detail reinforce concrete will develop about twice as much damping as structural steel. This advantage where acceleration serviceability is critical limits state, or for ultimate limits state design in earthquake-prone area.
Action to be considered:-
- Shear wall formed around elevator and service risers requires a concentration of opening at ground level where stresses are critical.
- Torsional and flexural rigidity is affected significantly by the number and the size of opening around the shear walls throughout the height of the building.
- Shear wall vertical movements will continue throughout the life of the building.
- Construction time is generally slower than for a steel frame building.
- The additional weight of the vertical concrete elements as compared to steel will induce a cost penalty for the foundations.
- An increase in mass will cause a decrease in natural frequency and hence will most likely produce an adverse affect of the acceleration response depending on the frequency range of the building. But shear wall systems are usually stiff and cause a compensating increase in natural frequency.
Problem associated with formwork systems:
- A significant time lag will occur between footing construction and wall construction, because of the fabrication and erection on site of the moving formwork systems
- Time will be lost at the levels where wall are terminated or decrease in thickness, alignment of the shear walls are within tolerance.
- Regular survey check must be undertaken to ensure that the vertical and twist alignment of the shear walls are within tolerance.
- In general it is difficult to achieve a good finish from slip-form formwork systems, and hence rendering or some other type of finishing may be necessary.
Shear wall Structure
Coupled Wall Structure
- Consist of two or more shear walls in the same plane, or almost the same plane, connected at the floor levels by beam or stiff slabs.
- The effect of the shear-resistant connecting members is to cause the sets of wall to behave in their partly as a composite cantilever, bending about the common centroidal axis of the walls.
- Suited for residential construction where lateral-load resistant cross walls, which separate the apartments, consist of in-plane coupled pairs, or trios, of shear walls between which there are corridor or window openings. Besides using concrete construction, it occasionally been constructed of heavy steel plate, in the style of massive vertical plate or box girders, as part of steel frame structure.
Coupled shear walled structure
- The walls and frame interact horizontally, especially at the top, to produce stiffer and stronger structure. The interacting wall-frame combination is appropriate for the building in the 40 –60 story range, well beyond that of rigid frames or shear walls alone.
- Carefully tuned structure, the shear of 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 wall 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.
Wall frame structure
Majestic building, Wellington, New Zealand.
The lateral resistant of the framed-tube structures is provided by very stiff moment-resistant frames that form a “tube” around the perimeter of the building. The basic inefficiency of the frame system for reinforced concrete buildings of more than 15 stories resulted in member proportions of prohibitive size and structural material cost premium, and thus such system were economically inviable. The frames consist of 6-12 ft (2-4m) between centers, joined by deep spandrel girders. Gravity loading is shared between the tube and interior column or walls. When lateral loading acts, the perimeter frame aligned in the direction of loading acts as the “webs” of the massive tube of the cantilever, and those normal to the direction of the loading act as the “flanges”. The tube form was developed originally for building of rectangular plan, and probably it?s most efficient use in that shape.
Suitable for reinforced concrete and steel construction and has been used for building ranging from 40 to more than 100 stories. Aesthetically, the tube externally evident form is regarded with mixed enthusiasm; some praise the logic of clearly expressed structure while other criticizes the grid like façade as small-windowed and uninterestingly repetitious. Depending on the height and dimensions of the building, exterior columns spacing should be in order of 1.5 m to 4.5 m on center maximum. Spandrel beam depths for normal office or residential occupancy application are typically 600 mm to 1200 mm. Frame tube in structural steel requires welding of the beam-column joint to develop rigidity and continuity. The formation of fabricated tree elements, where all welding is performed in the shop in a horizontal position, has made the steel frame tube system more practical and efficient. The 110 story World Trade Center twin towers, New York are examples whereby the structuralist notion of a punched wall tube with extremely close exterior columns is architecturally exploited to express visually the inherent verticality of the high rise building.
The Trussed tube:
- The trussed tube system represents a classic solution for a tube uniquely suited to the qualities and character of structural steel.
- Interconnect all exterior columns to form a rigid box, which can resist lateral shears by axial in its members rather than through flexure.
- Introducing a minimum number of diagonals on each façade and making the diagonal intersect at the same point at the corner column.
- The system is tubular in that the fascia diagonals not only form a truss in the plane, but also interact with the trusses on the perpendicular faces to affect the tubular behaviour. This creates the x form between corner columns on each façade.
- Relatively broad column spacing can resulted large clear spaces for windows, a particular characteristic of steel buildings.
- The façade digitalisation serves to equalize the gravity loads of the exterior columns that give a significant impact on the exterior architecture.
Tube-in-Tube or Hull Core Structure
- This variation of the framed tube consists of an outer frame tube, the “Hull,” together with an internal elevator and service core.
- The Hull and core act jointly in resisting both gravity and lateral loading.
- The outer framed tube and the inner core interact horizontally as the shear and flexural components of a wall-frame structure, with the benefit of increased lateral stiffness.
- The structural tube usually adopts a highly dominant role because of its much greater structural depth.
- The concept allows for wider column spacing in the tubular walls than would be possible with only the exterior frame tube form.
- The spacing which make it possible to place interior frame lines without seriously compromising interior space planning.
- The ability to modulate the cells vertically can create a powerful vocabulary for a variety of dynamic shapes therefore offers great latitude in architectural planning of a tall building.
Core and Outrigger Systems
- Outrigger serve to reduce the overturning moment in the core that would otherwise act as a pure cantilever, and to transfer the reduced moment to columns outside the core by the way of tension-compression coupled, which take advantage of the increase moment arm between these columns.
- It also serves to reduce the critical connection where the mast is stepped to the keel beam.
- In high-rise building this same benefit is realized by a reduction of the base core over-turning moments and the associated reduction in the potential core uplift forces.
In the foundations system, this core and outrigger system can lead to the need for the following:
- The addition of expensive and labour-intensive rock anchors to an otherwise “simple” foundation alternative such as spread footing.
- Greatly enlarged mat dimensions and depth solely to resist overturning forces.
- Time-consuming and costly rock sockets for caisson systems along with the need to develop reinforcement throughout the complete caisson depth.
- Expensive and intensive field work connection at the interface between core and the foundation. This connection can become particularly troublesome when one considers the difference in construction tolerances between foundations and core structure.
- The elimination from consideration of foundation systems which might have been nsiderably less expensive, such as pile, solely for their inability to resist significant uplift.
- The outrigger systems may be formed in any combination of steel, concrete, or composite construction.
- Core overturning moments and their associated induced deformation can be reduced through the “reverse” moment applied to the core at each outrigger intersection. This moment is created by the force couple at the exterior columns to which the outrigger connect. It can potentially increase the effective depth of the structural system from the core only to almost the complete building.
- Significant reduction and possibly the complete elimination of uplift and net tension forces throughout the column and the foundation systems.
- The exterior column spacing is not driven by structural considerations and can easily mesh with aesthetic and functional considerations.
- Exterior framing can consist of “simple” beam and column framing without the need for rigid-frame-type connections, resulting in economies.
- For rectangular buildings, outriggers can engage the middle columns on the long faces of the building under the application of wind loads in the more critical direction. In core-alone and tubular systems, these columns which carry significant gravity load are either not incorporated or under utilized. In some cases, outrigger systems can efficiently incorporate almost every gravity column into lateral load resisting system, leading to significant economies.
The most significant drawback with use of outrigger systems is their potential interference with occupiable and rentable space. This obstacle can be minimized or in some cases eliminate by incorporation of any of the following approaches:
- Locating outrigger in mechanical and interstitial levels
- Locating outriggers in the natural sloping lines of the building profile
- Incorporating multilevel single diagonal outriggers to minimize the member?s interference on any single level.
- Skewing and offsetting outriggers in order to mesh with the functional layout of the floor space.
- Another potential drawback is the impact the outrigger installation can have on the erection process. As a typical building erection proceeds, the repetitive nature of the structural framing and the reduction in member sizes generally result in a learning curve which can speed the process along.
The incorporation of a outrigger at intermediate or upper levels can, if not approached properly, have a negative impact on the erection process. Several steps can be taken to minimize this possibility Provide clear and concise erection guidelines in the contract documents so that the erector can anticipate the constraint and limitation that the installation will impose. If possible, avoid outriggers locations or design constraints that will require “backtracking” in the construction process to install or connect the outrigger. The incorporation of intermediate outriggers in concrete construction or large variation in dead-load column stresses between the core and the exterior can in some cases result in the need to “backtrack”. Such a need can be minimized if issues such as creep and differential shortening are carefully studied during the design process to minimize their impact. Avoid adding additional outrigger levels for borderline force or deflection control.
Combination of two or even more of basic structural forms either by direct combination or by adopting different forms in different parts of the structure. This systems provide in-plane stiffness, its lack of Torsional stiffness requires that additional measures be taken, which resulted in one bay vertical exterior bracing and a number of level of perimeter Vierendeel “bandages” –perhaps one of the best examples of the art of structural engineering. Hybrid structures are likely to be the rule rather than the exception for future very tall buildings, whether to create acceptable dynamic characteristics or to accommodate the complex shapes demanded by modern architecture. High-strength concrete, consist of stiffness and damping capabilities of large concrete elements are combined with the lightness and constructability of steel frame exhibits significantly lower creep and shrinkage and is therefore more readily accommodated in a hybrid frame.