The rise of the steel frame skyscraper during the 20th century has introduced certain problems to the building industry – but also introduced standardized solutions and standardized methods of evaluation (e.g. local and national building codes requiring a minimum standard for design and construction). The failure of the three buildings belonging to the World Trade Center on 9/11 calls into direct question some of these standards. Despite the catastrophic loads the two main building were subjected to – a steel and concrete building that meets minimum standards of construction should easily be able to withstand such damage – especially, as in the case of the WTC buildings – since they were designed to be able to absorb precisely this kind of damage. Clearly our codes or specifications must be at fault in these cases- to say nothing of the third building to collapse – upon which no stresses of any sort were exerted…
Understanding steel frame & concrete high rise construction requires requires breaking the problem down:
Understanding Structural Loads & Design
Live and Dead Loads
Most basic engineering calcluations are based on ‘live loads’ and ‘dead loads – the former consist of variable loads (i.e. people and machines that are moved in and out of spaces over the years – or accumulated snow and force due to wind, etc) and the latter are constant loads (such as structure above each floor) that are constant. Both ‘live’ and ‘dead’ loads are added, taken as a minimum design load by which ‘safety multipliers’ are added according to building codes and engineering standards.
Specifications ensure that the quality of steel fabrication and the steel subcomponents and fireproofing, etc. meet a minimum standard according to the overall design. The fasteners and connection technology of steel members are at least as important of the steel specified in the structure, since they determine how effectively forces can transfer through the structure – ulitimately supported by the bedrock foundation of a given highrise.
These are the easiest to understand forces acting on the tall building. Steel must necesarily be thicker and stronger to accomodate the increased loads at lower points in the structure. A given floor need only be strong enough (in principle) to support the floors above it – so the 50th floor in a 60 floor tower won’t require a structural frame anywhere near as strong as the lowest floors of the same tower. Of course – this isn’t the only part of the picture. Diagonal and other bracing elements/ supporting members need to be much more numerous in lower floors – as do the number and size of fasteners (i.e. bolts, hold-downs, braces, welds, etc.)
Horizontal loads include the vertical transfer loads at horizontal joints (i.e. – at each floor or trancept) but also must take into account seismic activity and their associated loads, wind load, building settling, accidental vehicle impact, acts of terrorism, etc. Remember – a building is a semi permanent structure and must be able to sustain anything that gets ‘thrown’ at it and avoid catastrophic damage where possible. There are many ways to respond to such potential threat to buliding integrity:
Design Strategies against Catastrophic Damage (i.e. – ‘Force Protection’)
The concept of ‘structural redundancy’ is a very common method employed in nearly all built structures. In fact, any buildings except those on a single stilt (clearly the antithesis of ‘redundant’ and is illustrative of the advantages of redundancy) have a degree of redundancy already built in. A small single family home made from common wood framing techniques has this built-in by merit of having walls composed of multiple wood studs per unit wall length. Structural failure of a single stud will be compensated for by having it’s load ‘captured’ or transferred through the existing adjacent studs. Beyond this – by providing the largest number of structural transfer pathways through a given structure, catastropic damage will very likely be sustained with little danger to the occupants for nearly all conceivable catastrophe scenarios.
Especially since the 1945 incident involving a B-25 Mitchell bomber flying directly into the Empire State Building, both codes and designers of high rises have been especially cautious about warding off such perils (see also: redundant structures, below). Despite the failures presented by the World Trade Center catastrophe – the main towers of the WTC were actually designed with such peril in mind. A building with that degree of surface area must be expertly designed with plane impacts and a rather incredible amount of wind and other loads in mind. Each of the WTC towers were actually designed expressly for the purpose of handling multiple airliner impacts
Especially with high rise structures, and their high degree of built-in redundancy – a surprising amount of the building can be destroyed or removed before the possibility of collapse will exist. In many scenarios this may involve removal of up to 80% of the exterior structure of the building. In many cases even the complete removal of the exterior facade may still find the building without threat of collapse.
Structural redundancy through exterior curtain structural elements (i.e. World Trade Center Towers) can be a highly effective strategy. In such a scenario as the WTC buildings 1 and 2 (with exterior redundant structure AND a redundant core structure), where the exterior wall is composed of a very high number of rendundant load bearing members (see diagram below). The larger the number of redundant ‘pathways – the safer the building is and can be under catastrophic circumstances.
Design Method & Typology
The specific evolution of high rise building types, having occurred primarily from trial and error as well as case studies derived from computer and structural/numerical analysis have spawned a number of discrete types. Among these are:
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.
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.
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.
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.
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:
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.
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.
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.
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.
This var ation 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.
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.
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.
Skyscraper Design and Construction (wikipedia)