Skelly

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About Skelly

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  • Birthday 04/07/80

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  1. Hi, i was wondering could i have some of your thoughts on the following in relation to the analysis of a horizontal rigid floor diaphragm over a transfer structure or maybe someone has come across it before. If you have a shear wall layout at different floor levels on upper floors on a structure which the location of the bracing walls mirrors each floor level i.e. shear walls are in the same location the entire height of the building. However when you get to lower level the walls become blade column, thicker walls and some are located in different areas i.e. thus the horizontal load distribution on the lower floor level where the wall has change would be different to the walls above. For the lateral load distribution analysis of the lower wall level diaphragm, to find out how much load is allocated to each wall depending on its stiffness, would you apply the total horizontal load for the upper levels at the centre of rigidity of the upper walls, at the centre of rigidity location on the lower level diaphragm as one force in each direction, then just analyse the lateral loads at the lower level which are applied directly at that level separately. I have previously always placed the total lateral load over transfer levels at the centre of rigidity of the upper wall layout., however now I am not too sure? Has anyone ever come across this in the past, analysing using hand calculations rather than software. As a really simplistic example basically if You had say a 30m x 30m square area on plan forming a floor diaphragm on upper levels of a building. At each one of this those levels You then have one bracing element only in the right hand corner of the 30m x 30m square. From this You apply say evenly distributed horizontal wind load across the face of the level applied at the centre of pressure I.e. Say 15m in from each side, the bracing member in the corner then takes all the loading based on its stiffness as per any general shear wall layout. The horizontal load acts at the bottom of this wall, and this diaphragm has a shear centre or centre of rigidity (say it works out at 3m in from right hand side as an example) at which the overall load acts away from the centre of pressure which is where eccentricity causes any torsion etc in the diaphragm.So when you get to the bottom transfer level which is still a 30m x 30m square, however on this level you suddenly have 2 bracing elements one on the right hand side and one on the left hand side this time, thus the shear centre or centre of rigidity changes of the wall stiffness of the diaphragm changes, so my issue is that the loads applied directly at that level to the face of the transfer level are placed at the centre of pressure at that level say again 15m in from either side. However are the loads from the levels above which are also transferred through this level applied at shear centre of the above wall system (in this case say 3m in from right hand side as above) or at the centre of pressure on the wall system above which would be 15m in from either side? I would have thought that the total horizontal load above are applied at the shear centre of wall system above on the plan of the transfer level wall system below? And that's what I have assumed in the past when we construct buildings with transfer levels. Thanks in advance for any reolys on this one. S.
  2. Thanks for the reply. However i have found some guidance in the new concrete society design examples manual. They have used the section 6.2 of ec2 to calculate the horizontal shear capacity of the wall, taking allowance for the axial load acting down on the wall as per any shear capacity calculation, thus i think this is correct.
  3. Hi, I was wondering could anybody help me, in relation to RC Shear walls, when shear walls are subject to lateral loads such as wind and notional loads in a tall structure, is there a specific check for horizontal shear in plane/or traverse of the wall which should be checked? or is it just based on section 6.2 of EC2 as per beam and slabs etc. Section 12 gives a shear check for plain walls however this would be based on the section remaining plain, i.e. no tension/uplift in the wall, thus section is un-cracked. However if you have a tall structures with shear walls there is usually tension, thus does section 6.2. apply in this case? I have checked literature etc. on RC wall designs and there doesn't seem to be any examples or commentary on checking for shear in walls in accordance with the Eurocodes. if anyone has any comment or already done this I would like hear your thoughts? Thanks, S
  4. I could not find any guidance in the eurocodes on using one central layer of reinforcement in a wall above or below the minimum requirement for it to be designed as an RC wall, unlike the british standard which did mention one layer of reinforcement. However, thats not to say it does not exist in the eurocode, as i may have missed it somewhere!
  5. Thanks for your thoughts on this guys, its been a big help.
  6. Hi, has anyone any thoughts on the following as i would be interested in hearing them? In relation to minimum cover to reinforcement in an rc concrete beam or column, with regards its environment and fire resistance, if say all bars within a beam have satisfied the cover requirement in relation to there specified exposure classes (bs8500) and fire resistance, and say 1 bar or link within the beam or column does not conform to this cover requirement, is there an average cover which can be used or when does engineering common sense/judgement kick in? Or is this beam then classed as rejected/non-conforming to spec.? Thus it should be recast or remedial work carried out? BS EN 1992 - 1 - 2 for concrete fire design states an average axis distance may be taken based on bars placed in layers, as fire is based on the axis distance to the centreline of the main bar. However when cover to the reinforcement is governed by its exposure class in its environment, Cmin is the minimum cover to an individual bar, thus overall cover specified on the drawings would be Cmin + delta c for deviations, however the cover would be as measured on site i.e. Cast, thus delta c would become irrelevant? If any bar on that beam falls below Cmin minimum cover is this to be taken out or a remedial action undertaken? I understand that in theory and practically all bars should have correct cover, however with good intentions and all the will in the world, from time to time, the odd bar may not achieve this due to construction practice, tolerances etc. Where does common sense prevail? Is there any clause in any code which speaks about this? If anyone has any thoughts i would appreciate hearing them. Thanks
  7. Danzi, I see where you coming from now, its lateral bursting tensile forces in the slab in accordance with cl. 6.7 (4) of ec2, which has referred you back to cl. 6.5.3 (3). However from looking at fig. 6.25 ( i don't think it applies? As this is generally set up for column heads, as most of the stresses under the column head would be in direct compression and pass straight through, its around the perimeter where the lateral tensile forces will build up. Thus if, you let a = h (as max a<h), a in your case would be 250/2 = 125mm and h = H/2 the formula for full discontinuity would become T = 1/4 * (1-0.7) * F , thus T = 0.075 * F, however the issue is how much load is allocated to F? I don't think you could assume the full axial load on the column as the majority of the load should be passing straight through? If you look at fig. 6.25 ( it states that beff = 0.5H + 0.65a thus in your case it would be 206.25mm and a = 125mm, thus if you base F on a percentage of the axial load passing through the slab i.e. 125mm/206.25mm thus 61% of axial load F passing through slab leaving 39% bulging at the sides. Thus F axial force to be used in the formula above would be T = 0.075 * 0.39 * F , thus if your axial load were say 5000kN , As required in both directions over the column width would be 147kN/(0.87 * 500) = 336mm2 in each direction over the column width. By the way i am only thinking out loud above and i am not 100% sure on the way i have allocated load to bursting? As you may need to take the full axial load. What is your ult. axial load on the columns? To be honest when designing to bs8110 with this method of construction we never considered bursting forces in the slab, well at least i didn't. There is some literature which discusses the topic in depth which i haven't had a chance to go through just yet, namely the designers guide to en 1992 - 2 concrete bridges by hendy and smith and also in the fib precast concrete connections manual. In the designers guide to en 1992 -2 concrete bridges they seem to indicate that this lateral bursting forces could be resisted by the max tensile stress in the concrete alone in some instances, so it may be beneficial to have a read of this one. If you do find something new on this one let me know as i would interested to find out on how this progresses. Skelly
  8. Dik, Thanks for the comments, I understand that the eurocode allows a tension capacity of pure concrete, however this tension capacity is fairly limited, and what i have found in the UK is that when dealing with lets say more experienced engineers that have been using the British standards and cp codes a long time, that they would be extremely hesitant/reluctant to except such a design approach. I think it would be there inexperience with using the eurocode more than anything else. I have recently finished designing 3 no. 14 storey accommodation blocks in London designed as precast concrete cross wall frames, and from the 5th floor up on all blocks i designed all the wall panels as plain concrete, and there were no issues, i think once you can keep shrinkage cracking under control there would no issues.
  9. Ernest, I think the key to designing unreinforced concrete is checking the section on whether tension exists, and if not it may be design as plain concrete, however i think a check on shrinkage requirements would be prudent if pouring mass concrete and designing in this manner, as the engineer who is telling you to provide the reinforcement may be set in there ways and worried about the effects of drying shrinkage, temperature effects and nominal hogging moments at simple supports i.e. min reinforcement contents.
  10. Danzi, I am currently designing a similar project consisting of a PT slab and precast column construction. The tensile force you mention i am assuming is due to hogging moment over the column head? If this is the case and your reinforcement is coming out to high try taking some of the moment into the column on top and and bottom below, as you should have vertical dowel reinforcement there anyhow going from the bottom column through to the top column to satisfy progressive collapse requirements, this would ease the moment in the slab somewhat but introduce further moment in the columns. Generally with this type of construction in the UK from previous projects I have worked on, you would assume the column to be pinned top and bottom taking for nominal moment at the joint in accordance with section 5.2 of ec2. Which generally depends on the axial load at that point, thus the maximum moment is taken at the top of the column and varies linear to zero at the joint i.e. pinned, as when you allow moment from the slab into the column this just complicates matters. Also, in the design of the slab itself, other than punching shear around the column, shear in the slab and designing for hogging reinforcement, you do know that you need to design for triaxial compressive forces in accordance with section 6.7 and you also need to cater for the bearing stresses above and below the column head in accordance with section 10.9.5.2 (2). Hope this helps at all.
  11. There is a good example of an annular design in the Reynolds Steedman book, reinforced concrete designers handbook 10th edition. Not sure if its in the 11th edition to Eurocode, would have to look. As Andy above says its just back to first principles and analyse it that way.
  12. In relation to plain concrete design, design of plain concrete walls and and columns has been included in British standards for years? Both in sections cl. 3.8.1.4 and cl. 3.9.4 of BS 8110 part 1. You just need to know find if there has any tension developed within the section and then add reinforcement accordingly if tension does exist. Designing a concrete beam as plain concrete in accordance with ec2 section 12 would not be practical or allowed, as what the section should be applied to is listed in cl. 12.1 (2).
  13. Lee, the IStructE publication, manual for the design of concrete building structures to EC2 recommends you adopt the same approach as BS8110, its in section 5.3.2.1 of the IStructE EC2 manual. Also the Eurocode for precast concrete hollow core flooring also gives a method of load distribution for a hollow core slab.
  14. Dik, Yes generally when designing a wall, I would design the wall as reinforced if the rebar content is above the minimum specified for a reinforced concrete wall, in Eurocode this is stated as 0.002Ac, so whether it is a singe central layer or between two faces once the rebar content is above the 0.002 Ac I would consider this to be designed as a reinforced wall, however say for simplicity and speed when designing a multistorey precast wall frame, I like to just check if there is tension in the walls and if not, design all the walls as plain walls as this fairly speeds up the design process, I would always provide the min amount for reinforced wall as a precaution for anti cracking and fire anyhow. Thanks for the replies Andy, The reason I ask in relation to the limit between stocky and slender plain walls is there seems to be fair difference in load carrying capacity when worked out.
  15. Many thanks for the reply Andy. The only thing is that section 5.6.2 of the IStructE manual deals directly with reinforced concrete and the formulas for slenderness contain areas of reinforcement to be accounted for, however in theory with a plain wall we have a wall with no reinforcement (ignoring min. cracking requirements and fire etc.). Thus this lead me to believe that these formulas for slenderness are based on a uncracked and cracked section, where as with a plain wall we are more like a timber column section as it is a section of one material, where generally Eulers theory would be applied or the like? This is topic is not addressed in BS 8110 either?