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Waqas Haider

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Everything posted by Waqas Haider

  1. Assalam o alaikum, For assuring deflection check of flexural members, we compare immediate deflection due to live load with L/180 or L/360. Also we compare total deflection (sum of immediate deflection due live load and long term deflection due to "dead +sustained loads") with L/240 or L/480. These deflections are calculated using corresponding effective moment of inertia "Ie" using equation 9-8 of section 9.5.2.3. To find out immediate deflection due to dead load we find out it using Ie calculated using cracking moment due to dead loads. To find out immediate deflection due to sustained loads we find out it using cracking moment due to sustained loads. To find out immediate deflection due to D+L we find out it using cracking moment due to dead + live loads. But to find out immediate deflection due to live load, we don't find it using cracking moment due to live load. We find out it by the difference of deflection due to dead + live load with the deflection due to dead load only. as shown by following results of manual calculations (Results from example 10.1 of PCA notes of ACI 318) Del.dead = 0.098 in (calculated using Ma due to dead load only and corresponding Ie came out to be 10648 in4 = i.e. Ig because Ma is less than cracking moment) Del.live = 0.0744 (calculated using Ma due to live only and corresponding Ie came out to be 10648 in4 = i.e. Ig because Ma is less than cracking moment) Del.(dead+live) = 0.344 in (calculated using Ma due to dead+live combo and corresponding Ie came out to be 5345 in4 < Ig because Ma is greater than cracking moment) Note that 0.344 is not the simple sum of 0.098(Del.dead) + 0.0744 (Del.live) because here for Del.live, Ie used was based on live only i.e. Ig =10648 in4; for Del.dead, Ie used was based on dead load only i.e. Ig = 10648 in4; and for Del.(dead+live) Ie used was based on (dead+live) load combo i.e. 5345 in4. All three "Ie" are different". My question is, how to cater this in etabs modeling because in etabs it simply super impose deflections due to dead and live for D+L combo which must not be the case. More over, does modifier 0.35 is enough to cater effect of effective moment of inertia? Thanks.
  2. Kindly specify you are talking about gravity loads, or lateral loads?
  3. If beam is stiffer than slab, the slab will bend down more and beam will bend less. In this case slab load will be transferred to beam and beam will be supporting load of wall as well as load of slab on the base of tributary area. But since CONCEALED beam is not thick enough to be stiff enough so it will not support slab. It will just act as part of slab because it has same thickness as slab (please note that stiffness is mainly dependent on cross section and affected very little by reinforcement). If you think wall load can not be born by slab alone, instead of providing concealed beam, either go for inverted beam or convince architect for suspended beam. @baz Brother I m confused whether providing reinforcement below wall in slab is beneficial by any mean (regarding strength or serviceability)?
  4. Thanks brother. I m basically confused that why line stresses on "DEAD LOADS vs LIVE LOADS" ? If live loads are larger as compared to dead ones, will ASD become economical?
  5. Assalam i alikum, Can any one explain me this line please? This is from the book of Z.A SIDDIQUE PRC PART 1 ''Strength design becomes economical as compared to ASD when dead loads are relatively larger compared with live loads'' Thanks.
  6. Till today i have been checking drift only for top story roof. I use to check the displacement of maximum displaced point of the structure at top roof and use to take it Delta.s.... Then i convert it into Delta.M by 0.7xRxDel.S...... and then compare it with Building height from base to top roof x 0.025 or 0.02 depending on T.... This what i was told by my boss at very 1st job. Since then i tried to find out through UBC but could not understand and assumed the same. But i think it should not make much difference to check interstory drift by 0.025 story ht or drift of top with respect to base and 0.025 building height from base.... Whats ur opinion?
  7. Assalam o alaikum, Dear seniors, Recently i had two design a concealed beam for a case where, A slab of Lounge of dimensions 18 ft x 28 ft is to be designed. The slab is resting on walls o all 4 sides. I broke the span of slab in between and converted it into two panels of 14 ft x18 ft with a beam spaning in 18 ft direction. Slab is of 5 inch thickness and 4 inch thickness finishing. So Total debth of concealed beam i can provide is 9 inch ( 5 inch + 4 inch). Total factored load that comes out to be acting at slab area is nearly 0.22 ksf. I designed this concealed beam with dimensions 24 in width and 9 inch depth. The steel i provided is 10 # 8 US bars at bottom and 6 # 8 US bars at top of beam through out. The design philosofy i followed is that i designed for moment wl2/8 for 9 inch depth and steel required came out 8 # 8 bars. a secion of 24 x 9 in2 can take just 4 #8 bars as singly reinforced section ( i.e. p.max). I had to provided 8 # 8 bars so I provided 10 # 8 bars at bottom and 6 # 8 bars at top ( to balance extra steel provided in bottom so that compressive strength of section equally get increased as tensile strength is increased in excess of rho max) and designed ad doubly reinforced beam. The stirups i provided is #4 @ 6" c/c 2 legged...... Can any one of you confirm me this design? Thanx.
  8. Dear seniors, I have a building nearly 35 ft high. I have a crane running at crane level which is at height of 26 ft. I have concrete walls of 200 mm instead of brick walls as exterior walls of the building. Now my question is whether I should model this concrete wall along with columns to find steel in columns and drift etc? Or I should not model it. When I model it obviously wall also take part in bearing loads. What is the right approach. At start of my career i worked with a consultant he used to delete basement wall and then design columns and footings etc. I am attaching two models. Tell me which approach is right. Both of models are not having same loading.But in one model retaining wall is modeled through out and in one model wall is modeled but stopped just before touching columns. Thanks. Thor power house building waqas.EDB HPP(thor Design).EDB
  9. dear shaker, in case you are having just concrete wall as basement and no columns in wall and the length of wall is long then your wall will bend as one way slab. in such case i prefer to model as line element of unit width. But still you need to provide proper consideration to the corner of the wall where this one way action will be restrained. But in case we are having columns with in the wall then our slab will be showing a dishing effect which depends on the stiffness of wall and columns. in this case i prefer to model the real model consisting of columns and wall so that i may see actual deformation and i will read forces from this model. because at interface of column and wall behavior will be changed than mid span. I need opinion of seniors over this. Thanks.
  10. Assalam o alaikum dear friends, It is quite basic level question but I am wondering how steel resists temperature and shrinkage cracks? Co-efficient of Thermal expansion for both steel and concrete is nearly same. Which shows if concrete contracts at an amount steel would also contracts at the same amount. So I m wonder which exactly phenomenon is involved in resisting shrinkage cracks? Thanks in advance.
  11. Thanks brother. Previous confusion is clear. What i m confused now is how to decide when i m to provide temp/shrinkage reinforcement and when to provide for flexural minimum. Our slabs/ mats always going to experience flexural stresses because no strata is ever hard enough. Slab/mat will bend in any case. So what would be the case when temp/shrinkage reinforcement only and when to provide for flexural minimum? Also if u can refer me some study material regarding flexural minimum reinforcement I will be very thankful.
  12. Read this thread http://www.sepakistan.com/topic/1598-need-confirmation-about-modelling-in-safe/
  13. Load size is size of column from which load will transfer to footing. In safe, when you mark a slab and draw a point or you start your model from template, in both case you need to tell about load size i.e. contact area of load with footing slab. This load size dimensions will be used in checking punching shear, cantilever length etc.
  14. You are talking about mats/rafts. I am talking about footing slab and floor slab. But I m very much confused now. Let me put my confusions here. I m confused when our slabs offer this movement and when does not. Because in slabs we cast slabs monolithically with beams and slabs will be offered restraints at ends of slab. Till now what i have been doing is i provide half of minimum reinforcement at top and half of reinforcement at bottom in footing slabs but if i m providing only one layer of steel I provide full 0.002bh or 0.0018bh steel.
  15. Dear Uzair, You mean in slabs or isolated or combined footings, this 0.0018bh is for GROSS AREA i.e. 0.009bh per face because they need minimum reinforcement for temperature control but mat and beams are exception?
  16. Umar bhai, Can i design retaining wall as shear wall? because this concrete retaining wall is also going to resist in plane forces along with out of plane forces? In ETABS ho to cater it?
  17. Thank you Muneeb for sharing your knowledge. It really helped me a lot in clearing my confusion.
  18. But if you have slab on grade in your building, You can put a hair pin around the top of pedestal then the top of pedestal can not bend under the action of horizontal force in such case you will not have to convert this force into moment. In such case, you will have to design that hair pin to dissipate horizontal force.
  19. Hello Goher, You are right most of time wind loads govern but in case of Mezzanines, Seismic forces may govern also. You are absolutely right. You will have to multiply horizontal forces with the depth of footing to get moment which you will have to apply at centroid of footing. If you want to check bearing pressure and strength design of footing, you can use SAFE but according to my knowledge, SAFE does not offer SLIDING, OVERTURNING check and for uplift, you may check pressure and pressure will tell you whether there is contact between your footing or your footing has been lifted. To apply loads in safe, you can either model a footing slab and manually assign loads at centroid (by drawing a point or by meshing footing) or there is a better way also. You can import the whole base with reactions from SAP/ETABS to SAFE. Moreover, I m attaching excel sheets which I have made for simple calculations of isolated Footings. You can use it. If you find any error, kindly report me back. I have tested them many times. These are a little rough but i think usable. footing reinforcement design sheet.xlsx footing sizing sheet.xlsx
  20. But in reality, you r going to construct this beam. What if after making another shear wall, still it gets fail? I have reservations regarding this approach. Kindly explain.
  21. The reinforcement in basement wall is similar to reinforcement in slabs. So design basement wall as a slab. Model an area element resembling your basement wall with suitable edge conditions... apply back fill load and gravity UDL loads. Mess/Divide the wall. Run the model and read moment at critical locations. and Provide reinforcement for these moments. You can also read shear force. Check thickness of wall against this shear force. But if your wall is very long and acts as one way cantilever slab, then you can design it as beam or slab. Basement walls are basically designed as three edge supported slabs. But in high seismic regions, i think your walls will also behave as shear walls so proper consideration should be given to seismic induced forces.
  22. Assalam o alaikum, Can any one provide me with some study material on design of opening in slabs? How to provide reinforcement around opening and how to calculate necessary reinforcement?
  23. Assalam o alaikum, Respected seniors, can any one please provide me with some study material on design of generator/machine foundation?
  24. Thank you very much sir for detailed answer. The above attached document presents calculations for a beam but since minimum steel limit differs for slab and beams, i think maximum steel ratio limit must also differ for beam and slab.
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