07-07-2010, 08:49 AM
Present post is in reference to post #11. I will come back to post #19. later.
If you should take a look at this structure, you will notice that the ultimate limit state condition or strength will not govern the design. The dimensions of the frame components will be determined, rather by serviceability condition, particularly deflection.
If we should adopt the span/effective depth ratio of 26 (BS 8110) for continuous beam and concentrate at the platform (30m span), then required effective depth = 30000/26 = 1154mm; add cover and the dimension of the link plus diameter of 25mm diameter main reinforcement (say a total of 100mm), we have 1254mm which is practically 1260mm. if we should follow strictly the provision of the BS 8110, we will have to modify this further for the fact that 30m is outside the base length of 10m (actually, it should be between 1.5 and 3 times the figure calculated above if we should proportion the beam following to the letter, the stipulation in the standard). If we are to follow the weak beam strong column doctrine, what will be the right dimension for the column, if we cannot hide the strength of the column in the reinforcement? In that case, the column should be at least as thick as the beam i.e., if we have a beam of say 450mm x 1260mm (as estimated for the platform as above), then the column should be of dimension at least of 450mm x 1260mm. Considering the span of the column of 6.5m (which could also be treated as another span on a continuous beam with the central platform), we will notice that it will amount to the waste of material if we should provide this dimension, when a column of dimension 450mm x 325mm could have served our purpose (if we had used the slenderness ratio of 20 for the column i.e. 6500/20 = 325mm). Note that this is in the absence of the provision that Mr.col should be greater than or equal to 1.2 Mr.bm as stipulated in the code. (Mr.col = capacity of the column and Mr.bm= capacity of the beam). So if we are to put this fact into consideration, we will have to further increase the dimension of the column, thus the moment of inertia as to have an Mr.col greater than or equal to 1.2 Mr.bm. Though this may serve the provision of the code, but my question is:- “Is it necessary to meet up with this requirement even in the light of the analysis as done above, which showed that the dimension of 450mm x 1260mm is in excess as such will amount to wastage of material, in as much as the dimension of 450mm x 325mm is enough and could comfortably support the beam. If not, then what do we do in order not to run afoul of the code’s provision?”
Regards
Teddy
If you should take a look at this structure, you will notice that the ultimate limit state condition or strength will not govern the design. The dimensions of the frame components will be determined, rather by serviceability condition, particularly deflection.
If we should adopt the span/effective depth ratio of 26 (BS 8110) for continuous beam and concentrate at the platform (30m span), then required effective depth = 30000/26 = 1154mm; add cover and the dimension of the link plus diameter of 25mm diameter main reinforcement (say a total of 100mm), we have 1254mm which is practically 1260mm. if we should follow strictly the provision of the BS 8110, we will have to modify this further for the fact that 30m is outside the base length of 10m (actually, it should be between 1.5 and 3 times the figure calculated above if we should proportion the beam following to the letter, the stipulation in the standard). If we are to follow the weak beam strong column doctrine, what will be the right dimension for the column, if we cannot hide the strength of the column in the reinforcement? In that case, the column should be at least as thick as the beam i.e., if we have a beam of say 450mm x 1260mm (as estimated for the platform as above), then the column should be of dimension at least of 450mm x 1260mm. Considering the span of the column of 6.5m (which could also be treated as another span on a continuous beam with the central platform), we will notice that it will amount to the waste of material if we should provide this dimension, when a column of dimension 450mm x 325mm could have served our purpose (if we had used the slenderness ratio of 20 for the column i.e. 6500/20 = 325mm). Note that this is in the absence of the provision that Mr.col should be greater than or equal to 1.2 Mr.bm as stipulated in the code. (Mr.col = capacity of the column and Mr.bm= capacity of the beam). So if we are to put this fact into consideration, we will have to further increase the dimension of the column, thus the moment of inertia as to have an Mr.col greater than or equal to 1.2 Mr.bm. Though this may serve the provision of the code, but my question is:- “Is it necessary to meet up with this requirement even in the light of the analysis as done above, which showed that the dimension of 450mm x 1260mm is in excess as such will amount to wastage of material, in as much as the dimension of 450mm x 325mm is enough and could comfortably support the beam. If not, then what do we do in order not to run afoul of the code’s provision?”
Regards
Teddy