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... on from each mode using an appropriate modal combination technique. 11) Scale the elastic deformation obtained from the analysis to allow for post-elastic deformations and check that the overall deformation of the structure and that the inter-storey drift limits are within acceptable limits. (Note: The overall building deformation checks usually control boundary edge clearance requirements to avert blocks pounding each other. The inter-storey drift limits control the onset of non-structural damage (a Serviceability Limit State criteria) and also the necessity to consider second order P-& # 61508; effects. ) 7. 3. The 'Conventional' Earthquake Design Procedure The conventional engineering design approach is to use the actions for members derived from the above elastic analysis as the basis for determining the dimensions and structural capacity. Significant changes in dimension will affect the building stiffness and may require re-analysis. The resulting sizes are then checked against those assumed during the analysis and provided a reasonable match is attained, the design verification process is considered complete.
Earthquake design has three important distinctions from other loadings. Firstly there is the acceptance that damage to both non-structural and some structural elements will occur, but collapse is to be avoided (refer to section 3). Secondly earthquakes are highly variable dynamic events which designers tend to simplify into a set of quasi-static lateral loads. This approach enables relatively simple analysis and design, but noticeably departs from reality. It is therefore important to build into the structure a degree of toughness or robustness which will avoid the development of undesirable collapse mechanisms.
Thirdly, although there is geological and seismological understanding of how earthquakes are initiated and how the energy release mechanisms translate into surface ground motion, earthquakes still inherently contain a higher level of uncertainty than do other forms of loading. Several modern earthquake design standards, particularly those which apply to regions of moderate or high seismicity [ 8, 9, 10, 12 ], permit designers to make special provisions to accommodate the anticipated level of damage, provided they also take additional measures to ensure the collapse prevention mechanisms are robust enough to avoid overloading. This can be achieved when the designer takes control of the structure, and dictates which and where post-elastic mechanisms are to occur. The designer should also ensure the post-elastic demands are within levels acceptable for the material being used and that undesirable possible collapse mechanisms are suppressed within the elements themselves and within the structure as a whole. This approach is known as Capacity Design. 8. The Capacity Design Philosophy for Earthquake Resistance 8. 1.
General Approach The capacity design philosophy for earthquake resistant design involves the following procedures: 1) Clearly define plastic hinge regions within the structure and the structural mechanism which is to be employed within those regions. (Note these are usually flexural mechanisms located within the end zones of beam elements, the base region of cantilever shear walls, the link beams of coupled shear walls and the mid-span region of eccentrically braced frames. ) 2) Detail the capacity of the elements within the plastic hinge zones so that their dependable strength matches AS CLOSELY AS POSSIBLE the combined gravity and earthquake loads derived from the conventional structural analysis. 3) Detail these zones so that other, less desirable post-elastic mechanisms are suppressed. When the flexural behaviour is chosen as the preferred post-elastic mechanism, the PROBABLE OVER STRENGTH OF THE SECTION WITHIN THE PLASTIC HINGE ZONE (including the strength contribution which may be made to the section by secondary components such as slab reinforcing) needs to be considered, with the dependable strength of other, less desirable failure mechanisms having a DEPENDABLE STRENGTH WELL IN EXCESS OF THIS. The following are considered unreliable (and thus undesirable) failure mechanisms with the recommended precautionary measures being indicated within brackets; o the loss of anchorage (by avoiding bar laps within the plastic hinge zones) o shear failure within the element (by the provision of closely spaced transverse reinforcing stirrups within the plastic hinge zone so as to assure diagonal shear crack development within the concrete is inhibited. ), o buckling of the flexural compression steel (by closely spacing well anchored lateral ties) o loss of axial load carrying capacity (by containing concrete column cores and avoiding buckling of main column reinforcing steel by using closely spaced, well anchored lateral ties). 4) The resulting structure has now effectively been tuned by the design process so that the post-elastic deformations will only occur within the well detailed plastic hinge zones of the structure, and that all other potential failure mechanisms have been suppressed, REGARDLESS OF THE INTENSITY OF GROUND SHAKING. 8. 2. The Implications of Capacity Design By using a capacity design technique, CONTROL OF THE STRUCTURE IS WITHIN THE HANDS OF THE DESIGNER. Through the selection of a preferred post-elastic mechanism, which is detailed in a manner to ensure the inelastic deformation demand of the structure occurs with in pre-assigned plastic hinge zones, it is possible to detail those zones so they accept the deformation demands placed upon them while ensuring the structure remains elastic elsewhere. In so doing undesirable potential collapse mechanisms are avoided.
This approach dictates the strength hierarchy throughout the structure. Fundamental to achieving this goal is that the over-strength capacity of sections within the plastic hinge zones must be realistically assigned. THE OLD ADAGE THAT STRONGER IS BETTER NO LONGER APPLIES. Within plastic hinge zones it is important that the dependable strength matches as closely as practicable the imposed actions as derived by analysis and that the over-strength capacity of the section within the plastic hinge zone be controlled to fall within the design range assumed (typically 30 % for reinforcing steel). This places a responsibility on the material supplier to satisfy both the minimum strength range specified AND THE DEPENDABLE SYSTEM OVER-STRENGTH. This is a relatively new concept and requires changes by material suppliers to match these specifications.
With these provisos, the adoption of the capacity design philosophy will ensure that the principle objective of earthquake resistant design, namely avoidance of building collapse under severe earthquake attack, is satisfied. It has been reasonably argued that this is indeed the only method which can assure compliance with this fundamental performance objective, particularly, considering the uncertainties of the random character of earthquake induced ground motion, the large influence that local site effects will have on this motion, and the ongoing rather crude engineering modelling methods available to simulate the actual post-elastic dynamic response of the building to that motion. The author commends the excellent text prepared by Professors Play and Preistley [ 11 ] as further reading on the capacity design approach to be used for reinforced concrete structures. 9. Earthquake Resistant Structural Systems Three types of earthquake resistant structural systems are generally available. 9. 1. Moment Resisting Frames: Moment resisting frames typically comprise floor diaphragms supported on beams which link to continuous columns.
The joints between beam and columns are usually considered to be 'rigid'. The frames are expected to carry the gravity loads through the flexural action of the beams and the propping action of the columns. Lateral loads, imposed within the plane of the frame, are resisted through the development of bending moments in the beams and columns. Framed buildings often employ moment resistant frames in two orthogonal directions, in which case the column elements are common to both frames. Moment resisting frames are well suited to accommodate high levels of inelastic deformation.
When a capacity design approach is employed, it is usual to assign the end zones of the flexural beams to accept the post-elastic deformation expected, and to design the column members such that their dependable strength is in excess of the over-strength capacity of the beam hinges, thereby ensuring they remain within their elastic response range regardless of the intensity of ground shaking. Moment resisting frames are, however, often quite flexible. When they are designed to be fully ductile, special provisions are often needed to prevent the premature onset of damage to non-structural components. 9. 2. Shear Walls The primary function of shear walls is to resist lateral loads although they are often used in conjunction with gravity frames and carry a proportion of gravity loads.
Shear walls fulfil their lateral load resisting function by vertical cantilever action. By reference to Figure 7 it can be seen that both the shear force and bending moment generated by the earthquake actions increase down the height of the building. Since shear walls are generally both stiff and can be inherently robust, it is practical to design them to remain nominally elastic under design intensity loadings, particularly in regions of low or moderate seismicity. Under increased loading intensities, post-elastic deformations will develop within the lower portion of the wall (generally considered to extend over a height of twice the wall length above the foundation support system).
This can result in difficulties in the provision of adequate foundation system tie-down to prevent uplift. The design of rocking foundations is common with shear walls, although care is required to ensure permanent rotational offsets are avoided following an earthquake. As outlined in Section 8. 1, good post-elastic response can be readily achieved within this region of reinforced concrete or masonry shear walls through the provision of adequate confinement of the principal reinforcing steel and the prohibition of lap splices of reinforcing bars. Shear wall structures are generally quite stiff and, as such inter-storey drift problems are rare and generally easily contained. The shear wall tends to act as a rigid body rotating about a plastic hinge which forms at the base of the wall. Overall structural deformation is thus a function of the wall rotation.
Inter-storey drift problems which do occur are limited to the lower few floors. A major shortcoming with shear walls within buildings is that their size provides internal (or external) access barriers which may contravene the architectural requirements. This problem can be alleviated by coupling adjacent more slender shear walls. The coupling beams then become shear links between the two walls and with careful detailing can provide a very effective, ductile control mechanism. 9. 3.
Braced Frames Frames which employ diagonal braces as the means of transmitting lateral load are common in low-rise and industrial buildings. The bracing elements are typically inclined axially loaded members which traverse diagonally between floors and column lines. They are very efficient in direct tension and may also be detailed to accept axial compression although suppression of compression buckling requires careful assessment of element slenderness. Two major shortcomings of braced systems are that their inclined diagonal orientation often conflicts with conventional occupancy use patterns (either internally or across windows or external fabric penetrations); and secondly they often require careful detailing to avoid large local torsional eccentricities being introduced at the connections with the diagonal brace being offset from the frame node. A variation on this form of lateral resisting system is the eccentrically braced frame.
This system employs a horizontal 'K' form of bracing with the central zone of the 'K' acting in flexure as the tension / compression legs of the brace drive the beam element into direct flexure. 10. The Importance & Implications of Structural Regularity 10. 1. General Most Standards outline certain provisions relating to both the vertical regularity of the structure and also the plan regularity. These usually apply to the appropriateness of several assumptions implicit in the distribution pattern of the loading or the torsional effects. 10. 2. Vertical Regularity Ideally the capacity of the structure should follow the shear and bending moment pattern of the structure shown in Figure 7. Substantial departures from this ideal typically result in the onset of premature post-elastic deformations often concentrated at over one level.
When this occurs, elements within the one level degrade, attract additional (post-elastic) deformation and a soft-storey mechanism develops with collapse often being the inevitable result. The vertical regularity check is intended to avoid abrupt changes in overall strength or stiffness at any particular level. Where such provisions are not met, then a more detailed analysis will be required to ensure that post-elastic deformation capacity at each level can be met without unacceptable loss of strength or post-elastic deformation demands in excess of their capacity. It is wise to avoid abrupt curtailment of reinforcing steel at one level of a reinforced concrete frame or substantive changes in a column section.
It is better to introduce such changes gradually, over several floors, thereby allowing a smooth transition between sections to develop. Obviously it is undesirable to curtail shear walls above their base as this also induces a very real potential for soft-storey development. 10. 3. Horizontal Regularity. The random, three dimensional motions generated by earthquakes is usually simplified into two transverse orthogonal components with the vertical response typically being ignored. The transfer...
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Research essay sample on Earthquake Loads Resistant Design Of Buildings
