The infill is brick masonry. The properties of brick masonry are described in art. 2.2.5.2. Lateral loads are applied to Infilled Frame model and calculated infill stresses due to wind load for the model frame upto 8th story because more stress at lower portion. The stresses are
presented in Tables 5.21 and 5.22. The maximum compressive stress is found in infilled material is 2332 kN/sq.m which is 4.5% less than allowable limit [2442 kN/sq.m, (BNBC)] and the maximum tensile stress found in infilled material is 282 kN/sq.m which is 19.5 % less than allowable limit [350 kN/sq.m, (BNBC)].
The shear strength of brick masonry is represented in Codes of Practice by a static friction type of equation (Coull, 1991)
fs = fbs + µ σ c
together with a maximum limiting value (0.40 N/mm2, BNBC).
Where,
fbs = bond shear stress = 0.025 √f’m N/mm2 (5.2) σ c = compressive force
This relationship holds good up to value of σ c = 2 N/mm2 (2000 kN/ m2) µ = 0.40 (average value)
The maximum shear stress found in infilled material is 540 kN/sq.m, which is 35 % greater than allowable limit [400 kN/sq.m (BNBC)].
From above results, it is found that the shear stress has exceeded the allowable limit. Hence, the use of infill masonry in high rise building is limited. When it is used as infill to the frame, the stresses in infill are to be carefully checked against lateral loads. For over stresses in brick infill, reinforced may be used as infill which allowed by ACI and UBC [The crushing strength (f’m) of brick masonry is taken 12.5 kN/sq.m (1813 psi)].
From the analysis of structural models, it is found that if the rigid frame model is filled by brick masonry then the moments in connecting beam substantially reduces at lower level of building height (Table 5.16 and 5.17) and the stiffness of Infilled model increases (about 40%). The brick masonry is cheap and easy in construction than RC work. It can be used economically as structural system if the stresses in infill material do not exceed the allowable limits.
Table 5.21 Infilled masonry normal stress due to wind load
Floor F
x, kN/sq.m, parallel to bed joint Fy, kN/sq.m, normal to bed joint
Level 300x600 mm 300x750 mm 300x900 mm 300x600 mm 300x750 mm 300x90 0 mm 1 98 -355 98 -363 97 -371 70 -2078 260 -2332 163 -2284 2 154 -401 137 -399 124 -399 282 -1490 138 -2106 32 -2053 3 160 -392 132 -390 114 -391 170 -1726 40 -1894 0 -1853 4 132 -390 142 -383 127 -385 32 -1554 0 -1703 0 -1671 5 143 -345 124 -345 112 -348 0 -1374 0 -1502 0 -1479 6 149 -319 132 -321 122 -325 0 -1222 0 -1337 0 -1323 7 147 -297 132 -299 124 -303 0 -1080 0 -1181 0 -1175
8 142 -274 130 -276 123 -280 0 -941 0 -1031 0 -1031
[+ve value in tension and –ve value in compression, beam size in mm, column size 300x750 mm]
Table 5. 22 Infilled masonry shear stress due to wind load
Floor Shear stress, Fxy in kN / sq.m
Level 300 x 600 mm 300 x 750 mm 300 x 900 mm 1 492 467 450 2 540 497 467 3 518 478 452 4 495 458 436 5 436 404 387 6 405 377 364 7 377 354 344 8 347 328 321
[Column size: 300 x 750 mm, Variable beam size: 300x600, 300x750, 300x900 mm]
5.7 Summary
A short direction bay of a 16-storied office building is considered for lateral load analysis. Wind load and Earthquake load are considered as lateral loads.
The specified bay is modeled by three structural systems, namely, a. Rigid Frame structure, b. Infilled Frame structure, c. Coupled Wall structure.
The Coupled Wall Structure is modeled into three structural models as i. Wall Element model ii. Wall Element model with auxiliary beam iii. Equivalent Wide Column model. The total five models are then analyzed by STAAD-III, a package program.
The maximum to gradually minimum stiffness is found in Equivalent Wide Column, Coupled Wall (with auxiliary beam), Infilled Frame, Rigid Frame and Coupled Wall respectively.
When the Coupled wall system is modeled by membrane finite elements, then shear wall’s (in-plane frame) connecting beams require a special consideration. Membrane elements do not have a degree of freedom to represent an in-plane rotation of these corners, therefore, a beam element connected to node of a membrane element is effective only by a hinge. As a result the walls deflect as free cantilever.
When the relative stiffness is greater, there exists larger bending moment in the connecting beams. Maximum bending moment develops in beams along building height at nearly H/3 for Rigid Frame for different beam and column sizes. There is no considerable change in maximum beam moments due to changes of beam and column sizes (Fig. 5.10 and 5.11). In Wide Column and Coupled Wall model, the maximum bending moment develops at nearly H/3 along building height and gradually increases along height with increased beam size (Fig. 5.12).
For Infilled Frame analyzed under wind load, it is found that the stresses of different types are close to allowable limit for the frame under consideration. The maximum compressive stress in infilled material is found to be 4.5% less than allowable limit. The maximum shear stress, however, found exceed the allowable limit by about 35%. The maximum tensile stress found in the infilled material is again about 19.5% less than the allowable limit.
Chapter 6
CONCLUSION & SUGGESTION
6.1 General
The ability to model high rise buildings successfully for analysis requires an understanding of their behavior under lateral loads. A good grasp of the techniques of modeling serves as an aid in generally assessing high rise building behaviour and subsequent selection and development of structural forms for such buildings.
In modeling a structure for analysis, only the main structural members are idealized and it is assumed that the effects of nonstructural members are small and conservative. Additional assumptions are made with regard to the linear behavior of the materials, and the neglect of certain member stiffness and deformations, in order to further simplify the model for analysis. In more accurate modeling, the columns and beams of frames are represented individually by beam elements. Shear walls are represented by assemblage of membrane finite elements. Certain reductions of a detailed model are possible while still producing an acceptable accurate solution. These reductions include halving the model to allow for symmetrical or anti symmetrical behavior or representing the structure by a planar model and conducting a two dimensional analysis.
6.2 Conclusions
A typical bay of a high rise building is considered for lateral load analysis. Wind and earthquake loads are imposed on the model frame as lateral loads. The loads are adopted in analysis as it is considered in design. The specified bay is modeled by three structural systems. They are a. Rigid Frame model, b. Infilled Frame model and c. Coupled Wall model. The Coupled Wall is modeled into three structural sub models, which are i. Wall Element model ii. Wall Element model with auxiliary beam and iii. Equivalent Wide Column model.
On the basis of results of analysis the following conclusions are made,
♦ The lateral deflection of the model is found minimum in Equivalent Wide Column model and the deflection value gradually increases for Coupled Wall model, Infilled Frame model, Rigid Frame model and Coupled Wall model (without auxiliary beam) respectively.
♦ Maximum bending moment in connecting beam develops in between H/3 to H/2 along model height from base level for different model with different member sizes.
♦ Stiffness of model increases with increased beam sizes or column sizes or both.
♦ Compressive stress, tensile stress and shear stress decrease in infilled material with increased beam size.
♦ In finite element method of analysis for Coupled Wall model, the finite membrane element and finite beam element connection is such that either it is a hinge or rigid joint.
In rigid connection (actual case), auxiliary beam must be considered in the connection of the model. If the auxiliary beam is not considered, the wall behaves as a free cantilever under lateral load, which is not representative of the real response.
♦ The maximum compressive stress and maximum tensile stress in infill brick masonry are found somewhat below the allowable limits as per BNBC values. However, the maximum shear stress is exceeded the allowable limit by 12.5% in infill brick masonry. Hence, the brick masonry wall may need strengthening with wire mesh (retro-fitting) or reinforced masonry may be used instead of masonry infilled frame can be used in high rise buildings of reasoned height as shear wall with proper analysis and design.
♦ Shear stress in infill material is critical in high rise building compared to tension in lateral load analysis.
♦ For severe lateral loads caused by wind load and or earthquake load, the reinforced shear wall is obvious. Because, it produces less deflection and less bending moment in connecting beams under lateral loads than all others structural system.
♦ The stiffness of Rigid Frame model found in the analysis is 42% to 65% of Coupled Wall and stiffness of Infilled Frame model found in the analysis is 86% to 91% of Coupled Wall.
The maximum moment in connecting beam of Rigid Frame model found in analysis is 31% greater than Coupled Wall model and the maximum moment in connecting beam of Infilled Frame is 15.5% greater than Coupled Wall model. The robust construction cost of RC wall makes the building cost higher. The efficient structural system is infilled rigid frame structure if the stresses are within the allowable limits, whether it is reinforced plaster or reinforced masonry.
6.3 Recommendations for Future Study
The following recommendations are made for future study on the basis of lateral load analysis of a 2-D bay for16-storied high rise building as
follows,
♦ In order to establish the influence of floor height of building, a similar investigation should be carried out in future.
♦ Three dimensional models study can be carried out for similar investigation.
♦ The study is performed only with uniform beam and column along height but it can be proposed to further investigations with various sizes along height.
♦ The investigation can be extended for cross bracing for every floor of a rigid frame and in filled frame in building.
♦ Laboratory investigation for infilled frame can be made, where the column and beam cast against infill and the column and beam cast prior to infill.
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