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WR1

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Posts posted by WR1

  1. On 1/11/2020 at 12:45 AM, Talha Rasul said:

    I am attaching the modal participation of mass of building 
    first mode and second mode time period is quite apart that is good but first mode seems to be torsional 
    help me out if ETabs modal results are placed as first torsion then followed by x and y .

    I want to interpret the results if these are good that I can lock my structure and go into designing?

    824369733_F9E4218E-284E-49D6-B61B-77F239E06B1B.png.thumb.jpg.a2094e1acc83cec014633538e43209d0.jpg

    Moreover these are my center of mass and rigidity they are also 7 to 8% apart so I want to know if my building first mode is torsion.?

     

    image.thumb.png.19f1381eb2277d5c188a16f37dedf772.png

    Thanks

    Yes, 1st mode is torsion and it is good that translation and torsional modes are well separated (1.23s and 0.48s). However, I can see the sum of mass participation is still not 90%. Bring it up by including more modes, and then check results. There is no problem having 1st mode as torsion as long as you design for it. However, the building framing will be working very hard and is not very efficient against lateral loads but no is stopping you to not have 1st torsional mode.

    On 1/13/2020 at 7:08 PM, Asif Mostafa said:

    Yes your first mode is in torsion. To avoid torsion in the first mode you need to keep the mass participation ratio for Rz below 5% (.05) and below 10% (.10) in second mode. Also how many mode should be evaluated depends on the number of storey.  

    In addition to this also check that whether the maximum deflection at corners are less than 1.2 times the average of corners.

    Hi Asif, I am very much interested to see if there is any reference for keeping Rz below 5% and 10% for 2nd mode?

  2. On 11/18/2019 at 4:50 PM, Fatima Khalid said:

    Asalamualaikum,

    We have so many industrial buildings which have concrete moment resisting frame system. In Pakistan, I have seen some industrial buildings having a storey constructed at roof with steel structure. Like RC structure is constructed and after few years, they have constructed a storey with steel roof.

    Sometime the concrete columns at periphery are extended upto a level and then steel I section is provided till steel roof. Steel roof made of rafters and purlins.

    In case of analysis/seismic assessment, Do we need to consider this storey as a storey while distributing base shear. OR should we consider only RC structure as a storey and take load of steel roof on the existing columns?

    Like will it be considered as G+1 structure or only ground storey structure?

    Thank you

     

    I'd do it separately due to following reasons:

    1. You will get weird dynamic and mode shapes due to a very rigid concrete structure and very flexible steel structure above. So difficult to interpret time periods etc by most users (will tend to believe software's values are correct when they will not be).

    2. It is usually not very straightforward to combine R factor for two systems with variable stiffness in a single model.

    I'd analyze the steel structure and apply it's worst loads on concrete structure (taking caution about horizontal loads and directions) on top of concrete columns as point loads (and include these point loads in seismic weight for concrete structure).

  3. 6 hours ago, Fawad said:

    @Rana Please explain the Pattern Live Load Factor effect on design of beam bottom reinforcement.

    wouldn't the bottom design reinforcement for a continuous beam be very high/conservative than that required based on analysis, if calculated based on wl2/8 ???

     

    1. Pattern loading (continuous beam assumed as simply supported) is used for live load only.

    2. It is necessary in a real building to account for variability in loading conditions.

    3. If you are trying to do just a simple beam, make pattern live load = 0

  4. On 9/25/2019 at 4:14 AM, abbaskhan2294 said:

    Etabs only gives you minimum area of reinforcement incase of slabs and columns provided that the designed reinforcement is less than the minimum area of reinforcement as per design code. However as far as design of beams are concerned, you must always calculate the area of steel manually and check it with the designed reinforcement by etabs. Etabs always give the designed reinforcement for beams whether its less or more than the minimum area of steel. 

    ETABS does give correct values of reo (it does take into account the lower/upper limits of reo for a particular code).

    On 9/22/2019 at 7:25 PM, BAZ said:

    Check following:

    • Design code assigned in the software
    • RCC design manual of the software and find how it calculates this for the design code of your interest
    • If you still reach at the same conclusion, ignore these results. The software can do that because of some bug.

    Totally agree and recommend first 2 points. However, I do not think, ETABS in 2019 has bugs for such simple things. Yes, it does have few bugs like any other package but for sure not related to these basic things.

    On 9/21/2019 at 11:12 PM, SMAQ said:

    Hello Fellow Engineers.

    Well I do Know that, Etabs Checks all the members for there minimum area of Reinforcement as per code. But I came across one design of mine where I see for a beam size of 228.6 mm wide and 304.8 mm deep. Etabs gives Bottom reinforcement less than what I calculated manually. I am attaching a few pics for the problem. I request to please put some light on this. The beam is designed for M20 and Fe 415 steel.

     

    astmin.JPG

    b-1.JPG

    bmd.JPG

    Please check what code says when you provide As = 1.33 x Asreq ???

    And why ETABS giving less moment and reinforcement than what you getting manually is totally a different question and depends if you are modelling it right or if you are manually calculating it right.

  5. On 10/16/2019 at 2:35 AM, Rifat said:

    Dear,

    As salamu alikum.

    I have a query. I designed a beam (12"x 24") which required bottom longitudinal rebar area is 2.00 in2  & top longitudinal rebar area is 1.50 in2. Required combined with flexural and axial reinforcement (torsion) area is 1.2 in2.

    My question is how to provide the torsional rebar in this beam how its arrangements.Though my bottom longitudinal rebar area is more than torsional reinforcement then is  it necessary to provide more 1.2 in rebar? What is rules?

    Thanks in advanced

    Also, take care that the torsional shear reinforcement = area of only outermost 1 closed stirrup. In other words, make sure the outermost continuous big stirrup (have 4 legs all around) is sufficient to carry all torsional shear. You can't distribute in among internal stirrups.

    However, you can provide multiple stirrups (verticals legs only effective) for flexural shear.

    Also, if one stirrup (flexural or torsion) is not enough, provide 2 bundled stirrups side by side along beam length or reduce spacing.

  6. Maximum reinforcement in beams (flexure only members) are controlled by maximum net tensile strain. The above-mentioned limit corresponds to 8% max reo (4% on each side), similar to column.

    Beams usually have reo less than 2%. If you are trying to put more reo in a beam using ETABS, change that section property from beam to column. I always love the idea of putting beams as columns in ETABS.

    A frame section in ETABS defined as a 'column' will be designed for flexure in both direction + axial loads,  a true interaction diagram. If defined as beam, ETABS wil ignore any axial load (if present) in beam design.

  7. I can discern this is due to the moment as your base is fixed and probably the governing combination include lateral loads.

    Few checks that you need to perform:

    1. Are columns sway special in ETABS? If yes then why?

    2. Can you tie these columns with tie beams at PARKING or BASE level to reduce critical length of column?

    3. If PARKING to BASE height is 1m then from scale of the attached image, PARKING to 1ST FLOOR is just 2.5m. Is it right?

    Base on the grid spacing of 2.5m, I do not see any problem having 300x300 columns with 2 to 3% reo giving 1800 to 2800 square mm. Or you are not concerned about sizing and just curious why the increase in reo? 

    If so, check the bending moment on governing load combination and you will get a fair idea what's happening here.

  8. I would not do that for a cantilever structure. Even if the post installed epoxy bars are okay strength wise, they would be carrying all the loads alone and deflect too much. Imagine the full balcony resting from these steel bars at one end?? The section at junction of old to new concrete is essentially fully cracked.

  9. On 6/25/2019 at 4:02 AM, abbaskhan2294 said:

    Rana what if i have 4 columns in a row at corner of mat footing then what should I do for option 2?

    I would be designing them as corner columns with load (or moment) = sum of all column loads and geometric properties of 1 composite section (it also depend of the distances between the columns) so just rationalize it with any appropriate assumption.

  10. 8 hours ago, BAZ said:

    I have not understood that why are you thinking that there is some discrepancy.

    As far as the ultimate design is concerned, I believe you mean: strength based design. This is the only procedure that we follow. No body uses working stress design procedure.

    As far as your wall is concerned, it is not necessary that every structural component that you will encounter in actual structures will be subjected to a moment which is at-least grater than the cracking moment.  If the demand is less than what is require to cause stresses-inelastic-section-cracked situation, the strength design is not applicable on the cross-section. However, the section must have minimum reinforcement to initiate ductile failure as per applicable code.

    This is it. So in short, if Mcr has not reached, strength based design equation does not apply.

  11. 22 hours ago, Beenay Beenay said:

    Actually there is nothing much to elaborate. I have designed many buildings in etabs. I came across this problem in two buildings and I cant find the cause. Cantilever beams just fail in shear no matter how big of a section I assign or how little load I apply. Its just a regular building. Nothing exceptional about it. I have remodeled it few times in Etabs 2015 and 2016 to see if there is some issue with the version of etabs that I am using. But I came across the same problem in both versions. But cantilever beams don't fail when I remove earthquake load pattern. Design summary show very very high unrealistic shear force. I don't know the reason why. thanks for showing interest.

    Never came across such an issue. Share the results may be?

  12. 7 hours ago, Sagar said:

    Will modelling the top floor with cantilever columns give me an approximate result?

    If yes, should I input dead load and live loads of truss to the model?

    Thank you :)

    Yes, put the dead and live loads manually to the columns to keep things simple and if resulting column reo is not that much.

  13. 1 hour ago, abbaskhan2294 said:

     incase of short columns, area of reinforcement primarily depends upon the cross sectional area provided and not the length of the columns.

    True, and that's where my question kicks in. How did you determine these columns are not slender? By the way I am talking about effective lengths and cantilever column effective length = 2 x L

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