Architectural design and practice Phần 6

Công nghệ và vật liệu mới vẫn được áp dụng mạnh mẽ trong kiến trúc hậu hiện đại, mà áp dụng chúng một cách khôn ngoan đầy cảm xúc hơn, nhằm nhấn mạnh các đặc thù của công trình và mối liên hệ của công trình đến khung cảnh tự nhiên văn hóa xã hội xung quanh.
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Architectural design and practice Phần 67.3 APPROXIMATE THEORY FOR LATERAL LOAD ANALYSIS OF WALLS SUBJECTED TO PRECOMPRESSION WITH AND WITHOUT RETURNS7.3.1 Wall without returnsHaving taken into consideration all the factors contributing to the lateralstrength of the wall, an approximate analysis (Hendry et al., 1971) can bedeveloped based on the following assumptions:• Elastic deflections of the wall supports are negligible.• Failure occurs by horizontal cracking at the top, centre and bottom of the wall, causing rotation about horizontal lines through A, B and C (Fig. 7.4).The forces acting on the top half of the wall at the point of failure areshown in Fig. 7.4. By taking moments about A (7.1) (7.2)where =precompressive stress, t=thickness of the wall which is subjectto precompression (in the case of a cavity wall with inner leaf loaded,thickness should be equal to the thickness of inner leaf only), L=length ofwall, h=height of wall, q0=transverse or lateral pressure and a=horizontaldistance through which centre of the wall has moved. If the compressive stress is assumed constant throughout the uplift ofthe wall at failure, the maximum pressure resisted by the wall is equal to (7.3)If the precompression increases on the wall with uplift of the building, asexplained above, it is possible for the moment of resistance, tL (t-a), toincrease, even though the moment arm (t-a) decreases—thus resulting inan increase in the maximum lateral pressure resisted by the wall.7.3.2 Wall with returnsIn the case of a wall with returns, part of the lateral pressure is transmittedto the return, thus causing axial and bending stresses in the return.©2004 Taylor & Francis Fig. 7.5 Simplified failure mechanism for walls with returns.Now substituting the value of q0 (wall with no return) from equation (7.3)into equation (7.5) (7.6)Similarly, for a wall with two returns (Fig. 7.5 (a)): (7.7) (7.8)From equation (7.3) (7.9)For various values of , the q1/q0 and q2/q0 plots have been shown in Fig.7.6 together with the experimental results. In the British Code of Practice BS 5628 the factors 1/[1-1/(3 )] and 1/[1-2/(3 ) ] are replaced by a single factor k . Table 7.1 shows thecomparison between factor k obtained from the theory and from thecode. From Table 7.1 it can be seen that the British code values are ingood agreement with the theoretical results. The theoretical values in©2004 Taylor & FrancisFig. 7.7 Effect of wall rotation: (a) basic rotation; (b) modified rotation (with highprecompression). =precompression; =half maximum uplift of wall with nocorner deformations; δh=elastic shortening.• Simply supported top and bottom, i.e. vertically spanning panel.• Simply supported on two edges, i.e. horizontally spanning panel.• Simply supported or continuous on three or four sides, i.e. panels supported on more than two sides of various boundary conditions.It will of course be realized that simple supports are an idealization ofactual conditions which will usually be capable of developing somedegree of moment resistance.7.5.1 Vertically or horizontally spanning panelsThe maximum moments per unit width for a wall spanning vertically orhorizontally can be calculated from:vertically spanning panel (7.10)horizontally spanning panel (7.11)©2004 Taylor & Francis Fig. 7.8 Prccompression versus maximum lateral pressure on 102.5mm wall of storey height.©2004 Taylor & Franciswhere w=design pressure, M x and My=maximum moments per unitwidth at midspan on strips of unit width and span h and L. Similarly, the moment of resistance per unit width of the panel can becalculated from the known value of the flexural tensile strengths inrespective directions as: My=fty Z (7.12) Mx=ftx Z (7.13)where f ty=allowable tensile strength perpendicular to the bed joint,ftx=allowable tensile strength parallel to the bed joint and Z=sectionalmodulus for unit width. In case of limit state design, the design bending moments per unitwidth in two directions will be (7.14) (7.15)where wk=characteristic wind load per unit area and f=partial safetyfactor for loads. The moment of resistance of the panel spanning vertically andhorizontally will be given by (7.16) (7.17)where fky and fkx are characteristic tensile strength normal and parallel tobed joints.7.5.2 Panels supported on more than two sides with various boundary conditionsThe lateral load analysis of masonry panels of various boundaryconditions is very complicated since masonry has different strength andstiffness properties in two orthogonal directions. Some typical values ofbrickwork moduli of elasticity on which the stiffness depends are givenin Table 7.2. The Britis ...