Structure and Architecture This Page Intentionally Left Blank Structure and Architecture Angus J. Macdonald Department of Architecture, University of Edinburgh. The major theme of this book is the relationship between structural design and architectural design. The various aspects of this are brought together in the last. This book explores the potential of structure, that is beams, columns, frames, struts and other structural members, to enrich architecture. At the most basic level I.
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Division of Structural Mechanics, LTH, Lund University, Box , SE 00 Lund , could make architecture develop in better collaborations and if the structure. tension (a system, greater than the sum of its parts). On a philosophical level, structures can strengthen architectural design as shown on the example of an. Birkhäuser – Publishers for Architecture. P.O. Box , Basel, . Vertical loadbearing structures in solid construction – Plan concepts. Vaulted.
Each of these has disadvantages. Rigid joints are the most convenient from a space-planning point of view but are problematic structurally because they can render the structure statically indeterminate see Appendix 3. Diagonal elements and diaphragms block the framework and can complicate space planning. In multipanel arrangements, however, it is possible to produce stability without blocking every panel. The row of frames in Fig. A single rigid joint is in fact sufficient to provide stability.
Note that in the case illustrated the resistance of transverse horizontal load is achieved by the insertion of rigid joints in the end bays. A threedimensional frame can therefore be stabilised by the use of diagonal elements or diaphragms in a limited number of panels in the vertical and horizontal planes.
In multi-storey arrangements these systems must be provided at every storey level.
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None of the components which are added to stabilise the geometry of the rectangular frame in Fig. Such elements are called bracing elements. Arrangements which do not require bracing elements, either because they are fundamentally stable or because stability is provided by rigid joints, are said to be selfbracing. Most structures contain bracing elements whose presence frequently affects both the initial planning and the final appearance of the building which it supports.
The issue of stability, and in particular the design of bracing systems, is therefore something which affects the architecture of buildings. Where a structure is subjected to loads from different directions, the elements which are used solely for bracing when the principal load is applied frequently play a direct role in resisting secondary load.
The diagonal elements in the frame of Fig. Because real structures are usually subjected to loads from different directions, it is very rare for elements to be used solely for bracing. The nature of the internal force in bracing components depends on the direction in which the instability which they prevent occurs. In Fig. Because the direction of sway due to instability cannot be predicted when the structure is being designed, the single bracing element would have to be made strong enough to carry either tension or compression.
The resistance of compression requires a much larger size of cross-section than that of tension, however, especially if the element is long3, and this is a critical factor in determining its size.
It is normally more economical to insert both diagonal elements into a rectangular frame 3 This is because compression elements can suffer from the buckling phenomenon. The basic principles of this are explained in elementary texts on structures such as Engel, H.
See also Macdonald, Angus J. Structural requirements cross-bracing than a single element and to design both of them as tension-only elements. When the panel sways due to instability the element which is placed in compression simply buckles slightly and the whole of the restraint is provided by the tension diagonal.
The compressive diagonal buckles slightly and carries no load. It is common practice to provide more bracing elements than the minimum number required so as to improve the resistance of three-dimensional frameworks to horizontal load.
The framework in Fig. A load applied parallel to the long side at this end of the frame would also cause a certain amount of distress as some movement of joints would inevitably occur in the transmission of it to the vertical-plane bracing at the other end.
In practice the performance of the frame is more satisfactory if vertical-plane bracing is provided at both ends Fig. This gives more restraint than is necessary for stability and makes the structure statically indeterminate see Appendix 3 , but results in the horizontal loads being resisted close to the points where they are applied to the structure. Another practical consideration in relation to the bracing of three-dimensional rectangular frames is the length of the diagonal elements which are provided.
These sag in response to their own weight and it is therefore advantageous to make them as short as possible. For this reason bracing elements are frequently restricted to a part of the panel in which they are located.
Structure and Architecture
The frame shown in Fig. Frame a is stable but will suffer distortion in response to horizontal load on the side walls. Its performance is enhanced if a diagonal element is provided in both end walls b. The lowest framework c contains the minimum number of elements required to resist effectively horizontal load from the two principal horizontal directions. Note that the vertical-plane bracing elements are distributed around the structure in a symmetrical configuration. Figures 2.
Another common arrangement, in which floor slabs act as diaphragm-type bracing in the horizontal plane in conjunction with vertical-plane bracing of the diagonal type, is shown in Fig.
When the rigid-joint method is used it is 13 Structure and Architecture Fig. Vertical-plane bracing is provided in a limited number of bays and positioned symmetrically on plan. All other bays are linked to this by diagonal bracing in the horizontal plane at every storey level. This eliminates the need for horizontal-plane bracing altogether, although the floors normally act to distribute through the structure any unevenness in the application of horizontal load.
The rigid-joint method is the normal method which is adopted for reinforced concrete frames, in which continuity through junctions between elements can easily be achieved; diaphragm bracing is also used, however, in both vertical and horizontal planes in certain types of reinforced concrete frame.
Loadbearing wall structures are those in which the external walls and internal partitions serve as vertical structural elements. They are normally constructed of masonry, reinforced Fig. Structural requirements concrete or timber, but combinations of these materials are also used.
In all cases the joints between walls and floors are normally incapable of resisting bending action in other words they behave as hinges and the resulting lack of continuity means that rigid-frame action cannot develop. Diaphragm bracing, provided by the walls themselves, is used to stabilise these structures.
A wall panel has high rotational stability in its own plane but is unstable in the out-ofplane direction Fig.
The arrangement is inherently stable. For this to be effective the structural connection which is provided in the vertical joint between panels must be capable of resisting shear4. Because loadbearing wall structures are normally used for multi-cellular buildings, the provision of an adequate number of vertical-plane bracing diaphragms 4 See Engel, H.
It is unusual therefore for bracing requirements to have a significant effect on the internal planning of this type of building. The need to ensure that a structural framework is adequately braced is a factor that can affect the internal planning of buildings. The basic requirement is that some form of bracing must be provided in three orthogonal planes. If diagonal or diaphragm bracing is used in the vertical planes this must be accommodated within the plan.
Because vertical-plane bracing is most effective when it is arranged symmetrically, either in internal cores or around the perimeter of the building, this can affect the space planning especially in tall buildings where the effects of wind loading are significant. They are subjected to internal forces that generate stresses the magnitudes of which depend on the intensities of the internal forces and the sizes of the elements.
The structure will collapse if the stress levels exceed the strength of the material. They must not rupture when the peak load is applied; neither must the deflection which results from the peak load be excessive. The requirement for adequate strength is satisfied by ensuring that the levels of stress which occur in the various elements of a structure, when the peak loads are applied, are within acceptable limits.
This is chiefly a matter of providing elements with crosssections of adequate size, given the strength of the constituent material. The determination of the sizes required is carried out by structural calculations. The provision of adequate rigidity is similarly dealt with.
Structural calculations allow the strength and rigidity of structures to be controlled precisely. They are preceded by an assessment of the load which a structure will be required to carry. The calculations can be considered to be divisible into two parts and to consist firstly of the structural analysis, which is the evaluation of the internal forces which occur in the elements of the structure, and secondly, the element-sizing calculations which are carried out to ensure that they will have sufficient strength and rigidity to resist the internal forces which the loads will cause.
In many cases, and always for statically indeterminate structures see Appendix 3 , the two sets of calculations are carried out together, but it is possible to think of them as separate operations and they are described separately here. Loading standards are provided to assist with this but assessment of load is nevertheless one of the most imprecise parts of the structural calculation process.
The maximum load could occur when the building was full of people, when particularly heavy items of equipment were installed, when it was exposed to the force of exceptionally high winds or as a result of many other eventualities. The designer must anticipate all of these possibilities and also investigate all likely combinations of them. The evaluation of load is a complex process, but guidance is normally available to the designer of a structure from loading standards5.
These are documents in which data and wisdom gained from experience are presented systematically in a form which allows the information to be applied in design. To understand the various processes of structural analysis it is necessary to have a knowledge of the constituents of structural force systems and an appreciation of concepts, such as equilibrium, which are used to derive relationships between them.
These topics are discussed in Appendix 1. In the analysis of a structure the external reactions which act at the foundations and the internal forces in the elements are calculated from the loads. This is a process in which the structure is reduced to its most basic abstract form and considered separately from the rest of the building which it will support. An indication of the sequence of operations which are carried out in the analysis of a simple structure is given in Fig.
The reason for this is explained in Appendix 3.
In these circumstances the analysis and element-sizing calculations are carried out together in a trial and error process which is only feasible in the context of computer-aided design. The different types of internal force which can occur in a structural element are shown in Fig.
As these have a very significant influence on the sizes and shapes which are specified for elements they will be described briefly here. The diagram shows the pattern forces which result from gravitational load on the roof of a small building.
Similar breakdowns are carried out for the other forms of load and a complete picture is built up of the internal forces which will occur in each element during the life of the structure.
In this way all of the internal forces in the structure are determined. In large, complex, statically indeterminate structures the magnitudes of the internal forces are affected by the sizes and shapes of the element cross-sections and the properties of the constituent materials, as well as by the magnitudes of the loads and the overall b c Fig.
The cut produces a free-body-diagram from which the nature of the internal forces at a single cross-section can be deduced. The internal forces at other cross-sections can be determined from similar diagrams produced by cuts made in appropriate places.
The shear force on the cross-section 1. Structural requirements resulting sub-elements are marked.
If these were indeed the only forces which acted on the sub-element it would not be in a state of equilibrium. For equilibrium the forces must balance and this is clearly not the case here; an additional vertical force is required for equilibrium. As no other external forces are present on this part of the element the extra force must act on the cross-section where the cut occurred. Its magnitude at the cross-section where the cut was made is simply the difference between the external forces which occur to one side of the crosssection, i.
Once the shear force is added to the diagram the question of the equilibrium of the sub-element can once more be examined. In fact it is still not in a state of equilibrium because the set of forces now acting will produce a turning effect on the sub-element which will cause it to rotate in a clockwise sense.
For equilibrium an anticlockwise moment is required and as before this must act on the cross-section at the cut because no other external forces are present. The moment which acts at the cut and which is required to establish rotational equilibrium is called the bending moment at the cross-section of the cut. Its magnitude is obtained from the moment equation of equilibrium for the free-body-diagram.
The Free Form Design (FFD) in steel structural architecture – aesthetic values and reliability
Once this is added to the diagram the system is in a state of static equilibrium, because all the conditions for equilibrium are now satisfied see Appendix 1. Shear force and bending moment are forces which occur inside structural elements and they can be defined as follows. The shear force at any location is the amount by which the external forces acting on the element, to one side of that location, do not balance when they are resolved perpendicular to the axis of the element.
The bending moment at a location in an element is the amount by which the moments of the external forces acting to one side of the location, about any point in their a b d Fig. In the simple beam shown here shear force and bending moment are the only internal forces required to produce equilibrium in the element isolated by the cut. These are therefore the only internal forces which act on the cross-section at which the cut was made.
In the case of the portal frame, axial thrust is also required at the cross-section exposed by the cut. Shear force and bending moment occur in structural elements which are bent by the action of the applied load.
Beams and slabs are examples of such elements. One other type of internal force can act on the cross-section of an element, namely axial thrust Fig. This is defined as the amount by which the external forces acting on the element to one side of a particular location do not balance when they are resolved parallel to the direction of the element.
Axial thrust can be either tensile or compressive. In the general case each cross-section of a structural element is acted upon by all three internal forces, namely shear force, bending moment and axial thrust. In the element-sizing part of the calculations, cross-section sizes are determined that ensure the levels of stress which these produce are not excessive.
The efficiency with which these internal forces can be resisted depends on the shape of the crosssection see Section 4. Bending moment is normally large in the vicinity of mid-span and near rigid joints.
Shear force is highest near support joints. Axial thrust is usually constant along the length of structural elements. In present-day practice these calculations are processed by computer and the results presented graphically in the form of bending moment, shear force and axial thrust diagrams for each structural element. In other words, the size of the cross-section must allow the internal forces determined in the analysis to be carried without overloading the structural material and without the occurrence of excessive deflection.
The calculations which are carried out to achieve this involve the use of the concepts of stress and strain see Appendix 2. In the sizing calculations each element is considered individually and the area of crosssection determined which will maintain the stress at an acceptable level in response to the peak internal forces.
The detailed aspects of the calculations depend on the type of internal force and, therefore, the stress involved and on the properties of the structural material. As with most types of design the evolution of the final form and dimensions of a structure is, to some extent, a cyclic process.
If the element-sizing procedures yield cross-sections which are considered to be excessively large or unsuitable in some other way, modification of the overall form of the structure will be undertaken so as to redistribute the internal forces. Then, the whole cycle of analysis and element-sizing calculations must be repeated.
If a structure has a geometry which is stable and the cross-sections of the elements are sufficiently large to ensure that it has adequate strength it will not collapse under the action of the loads which are applied to it.
It will therefore be safe, but this does not necessarily mean that its performance will be satisfactory Fig. It may suffer a large amount of Structural requirements separate issue and is considered separately in the design of structures. The deflection which occurs in response to a given application of load to a structure depends on the sizes of the cross-sections of the elements6 and can be calculated once element dimensions have been determined.
If the sizes which have been specified to provide adequate strength will result in excessive deflection they are increased by a suitable amount.
Where this occurs it is the rigidity requirement which is critical and which determines the sizes of the structural elements. Rigidity is therefore a phenomenon which is not directly related to strength; it is a 6 The deflection of a structure is also dependent on the properties of the structural material and on the overall configuration of the structure. In this chapter the factors which affect the basic requirements of structures have been reviewed.
The achievement of stable equilibrium has been shown to be dependent largely on the geometric configuration of the structure and is therefore a consideration which affects the determination of its form. A stable form can almost always be made adequately strong and rigid, but the form chosen does affect the efficiency with which this can be accomplished.
So far as the provision of adequate strength is concerned the task of the structural designer is straightforward, at least in principle. He or she must determine by analysis of the structure the types and magnitudes of the internal forces which will occur in all of the elements when the maximum load is applied.
Cross-section shapes and sizes must then be selected such that the stress levels are maintained within acceptable limits. Once the cross-sections have been determined in this way the structure will be adequately strong. The amount of deflection which will occur under the maximum load can then be calculated. If this is excessive the element sizes are increased to bring the deflection within acceptable limits.
The detailed procedures which are adopted for element sizing depend on the types of internal force which occur in each part of the structure and on the properties of the structural materials. The physical properties of materials determine the types of internal force which they can carry and, therefore, the types of element for which they are suitable.
Unreinforced masonry, for example, may only be used in situations where compressive stress is present. Reinforced concrete performs well when loaded in compression or bending, but not particularly well in axial tension. The processes by which materials are manufactured and then fashioned into structural elements also play a role in determining the shapes of elements for which they are suitable.
These aspects of the influence of material properties on structural geometry are now discussed in relation to the four principal structural materials of masonry, timber, steel and reinforced concrete. The range of different types of masonry is large due to the variety of types of constituent. Bricks may be of fired clay, baked earth, concrete, or a range of similar materials, and blocks, which are simply very large bricks, can be similarly composed. Stone too is not one but a very wide range of materials, from the relatively soft sedimentary rocks such as limestone to the very hard granites and other igneous rocks.
All have certain properties in common and therefore produce similar types of structural element. The physical properties which these materials have in common are moderate compressive strength, minimal tensile strength and relatively high density. The very low tensile strength restricts the use of masonry to elements in which the principal internal force is compressive, i.
In post-and-beam forms of structure see Section 5. Notable exceptions are the Greek temples see Fig. Even so, most of the elements which span horizontally are in fact of timber and only the most obvious, those in the exterior walls, are of stone.
Where large horizontal spans are constructed in masonry compressive form-active shapes must be adopted Fig. Where significant bending moment occurs in masonry elements, for example as a consequence of side thrusts on walls from rafters or vaulted roof structures or from out-ofplane wind pressure on external walls, the level of tensile bending stress is kept low by making the second moment of area see Appendix 2 of Structural materials Fig.
But this work is directed toward ends fundamentally alien to itself; its purpose is not to benefit society or edify mankind but rather serve as a site for the accumulation of capital. Either that, or the built object merely rematerializes the ghost of that which already floated up from the base, ideological figments and fragments that outlive the historical epochs from which they first arose. These now nestle into mortar, stone, and brick.
All that melted into air is made solid once again. Hardly anything could be further from the truth. The architectural legacy of the modern age is at least as impressive as that which preceded it — whether one begins, as Kaufmann did, with the French revolutionary architects of the eighteenth century, or reaches further back, like Tafuri, to the city-states of the Italian Renaissance.
Modernism itself was nothing but the self-conscious attempt to take hold of the forms and forces unleashed by modernity, as the spirit of the times comprehended in concrete. Even if its sociohistoric mission was tragically cut short, the greatest examples of modern architecture are on par with any of the iconic structures of classicism, or for that matter the Gothic or Baroque. Brilliant formal and technical solutions have been offered to address prob- lems previously thought insoluble under capitalism, and who knows how many architectural innovations that may take place before this chapter of history finally closes.
Yet the fact remains that any social good that results from the erection of a given building is entirely incidental to its primary function: namely, as a reservoir for the storage, collection, and augmentation of value amassed over time. It may double, temporarily, as a conduit for the movement of capital as money or commodities through space.
Or else it might serve as a site for circulation, the sphere in which the surplus-value of goods forged in the fires of production is realized in exchange. A revolution worthy of the name would not only allow architecture to liberate its oc- cupants and passersby, but would simultaneously entail the liberation of architecture.
Enough for now. The scope of inquiry — the social — has been delimited and defined.This building is supported by a reinforced concrete framework. It may double, temporarily, as a conduit for the movement of capital as money or commodities through space. The detailed procedures which are adopted for element sizing depend on the types of internal force which occur in each part of the structure and on the properties of the structural materials.
Bending-type internal force occurs if it is applied at right angles to the longitudinal 37 Structure and Architecture Fig. These normally have better technical properties than plain sawn timber elements such as that shown in a. Alastair Hunter 34 Steel is manufactured in conditions of very high quality control and therefore has dependable properties which allow the use of low factors of safety in structural design.
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