Issue With My Results In Etabs Model

[abdulqadeer29]

sir,

i have several issues wid result of my model.......it iz giving me a large deflection in x direction, i have few limitations in introducing shear walls , i appled few shear walls bt still rezlts are nt in permissible limits, and secondly i am getting a large %age rft in bottom column , and thn getting ony 1 % after 2nd story,,,,i am confused whether there is sum issue wid model or sth else,

i have checked manually base reactions, and it also comes quite higher in center columns...why, i dont knw....kindly help me out ?

sir i tried to upload my model bt getting above error....

G-15 NEW.EDB

[haro0n]

bro compress this file of edb along with text file then u can upload it.

[abdulqadeer29]

sir check my model....files attached

G-15 NEW.zip

[WR1]

your model looks okay, but following are some of my concerns

apply torsion modifier for beams = 0.35

slab modifier are not applied...make them m11=m12=m22=0.25, this will increase your sawy too

story8 slab load above grid 6 is not applied

story2 and story 8 diaphgram extent misses some external points...look at that

make sure your load combination are okay

​wall meshing is missing in your model....mesh them

your base-shear is equal to about 5% of seismic weight which seems okay with R=5.5 and UBC 2B seismic zone

You dont have to worry about the magnitude as far as you satisify the seismic drift limits of UBC which are 0.025 and 0.020 times story height...based on time period of your building....remember to convert your elastic displacement values to inelastic displacement by mulitplying it to 0.7xR

also check the building separation gap, if necessary

again, make sure your load combinations are okay!

one last thing: incease the column cross section, for 8 storey building in 2B zone, they look smaller...increase the size...to reduce your reinforcement from 3%...make them to 2 or 1.5%....

rest your model is okay apart from above points of concern

good luck..keep us updated

[Ayesha]

sir,

i have several issues wid result of my model.......it iz giving me a large deflection in x direction, i have few limitations in introducing shear walls , i appled few shear walls bt still rezlts are nt in permissible limits, and secondly i am getting a large %age rft in bottom column , and thn getting ony 1 % after 2nd story,,,,i am confused whether there is sum issue wid model or sth else,

i have checked manually base reactions, and it also comes quite higher in center columns...why, i dont knw....kindly help me out ?

sir i tried to upload my model bt getting above error....

G-15 NEW.EDB

You aren't permitted to upload this kind of fil

What do you mean by few limitations introducing shear walls?

[Zoya Hassan]

after analysis of this file it shows these warning

* * * W A R N I N G * * *

THE SOLUTION LOST 6.1 DIGITS OF ACCURACY FOR DOF UZ OF JOINT 10431

LOCATED AT X = 1975.750, Y = 519.000000, Z = 132.000000,

STIFFNESS MATRIX DIAGONAL VALUE = 3.9204E+09

why such errors occur and what to do then

[Badar (BAZ)]

Take a look at this link https://wiki.csiberkeley.com/display/etabs/Lost+digit+of+accuracy .

One thing that you do is to zoom into that point, reported by ETABS, and see if all the members are connected to that point.

[WR1]

there are many reasons for these errors..you should not let it happen

for example

extrude your model and check if your geometry is okay...some times...1000m is put instead of 1000mm in section properties making very stiff elements with flexible

check if out of plane modifiers are applied to membrane

check if base is properly restrained...some times like in springs..vertical restraint is provided but not horizontal

if your struc is 2d make sure to analyze it in 2d not in 3d in analyze>analysis options menu

check if vertical elements are not accidently deleted in mid stories distrubing the load path

check if very thin shells are modelled...

and so on

[abdulqadeer29]

sir rana, i chcked my load combo;s and made few changes according to code, just i have deleted few extra combo;s whch z nt ioncluded in ubc 97, but etab automatically generate them...sir but ma issue is,in this case allowable deflection is 2.64 in so now if ma drifts are okay .but ma deflections are slightly higher than allowable, will it be okay to gao ahead wid that....and sir wether we have to aplly 0.25 for slabs , as in code it z just for the case of flat plate and flat slab..

[WR1]

if it is shell you will apply 0.25 for slabs....as long as you have drifts less than allowable its ok..but what do you mean by deflections slighly more than allowable? where did you find in code which tells about seismic allowable DEFLECTIONS? instead of drifts!

[abdulqadeer29]

sir, in section 1613 of ubc 97, it is mentioned that the deflection of any structural member shall not exceed the value set forth in table 16D. which iz l/360....

and sir i am gng to upload ma revised model...kindly gve a look to ths one as well....

G-15 NEW..fi n.zip

[abdulqadeer29]

and sir Etab generate few load combinations which are not in code...so i deleted them during ma analysis....but i am confused that why Etab generate 26 load combo's but in code they are just 14 for this case....

1.2D + 1.0E (1612.2.1)

so from above equation , i conclude that we have to include only positive cases for earth quake but in etab , -ive sign is also there ....

but in code +ve and -ve is only for this expression.....0.9D + 1.0E and 0.9D -1.0E

need ur comments???

[Ayesha]

and sir Etab generate few load combinations which are not in code...so i deleted them during ma analysis....but i am confused that why Etab generate 26 load combo's but in code they are just 14 for this case....

1.2D + 1.0E (1612.2.1)

so from above equation , i conclude that we have to include only positive cases for earth quake but in etab , -ive sign is also there ....

but in code +ve and -ve is only for this expression.....0.9D + 1.0E and 0.9D -1.0E

need ur comments???

You need to consider for both positive and negative (both directions) for a given X and Y unless your building is symmetrical and you plan to mirror results if you want to use +ve load combo only.

[WR1]

L/360 etc are for which load cases? Read that table again carefully!

[Waqar Saleem]

@raNA

SIR you mentioned above

"apply torsion modifier for beams = 0.35"

is it right? isnt it 0.01?or it depends on the designer to select at his disposal?

[WR1]

update: 19.02.2014

Read about equilibrium nd compatibility torsion. You have to determine which case you have, if it is equilibrium torsion you cannot releaee torsion so your modifier will be 1.0 like in a cantilever slab or hollow core slab supporting beams because if you will release torsion it will redistribute to slab and as in equilibrium torsion there is no redistribution possible so you cant release torsion. But in case of compatibility torsion you can release torsion and it will redistribute. Si you can make its modifier equal to 0.35 but uf you make it 0.01 make sure there is no excess cracking in slab and you design slab for increased moments...thats why code does not explicitly tells u abt torsion modifier because it depends on case to case.

 

Value of equilibrium torsion is not affected by the modifier so always apply 0.1 or 0.35 or whatever for torsion modifier whatever the case is.

[UmarMakhzumi]

sir rana, i chcked my load combo;s and made few changes according to code, just i have deleted few extra combo;s whch z nt ioncluded in ubc 97, but etab automatically generate them...sir but ma issue is,in this case allowable deflection is 2.64 in so now if ma drifts are okay .but ma deflections are slightly higher than allowable, will it be okay to gao ahead wid that....and sir wether we have to aplly 0.25 for slabs , as in code it z just for the case of flat plate and flat slab..

if it is shell you will apply 0.25 for slabs....as long as you have drifts less than allowable its ok..but what do you mean by deflections slighly more than allowable? where did you find in code which tells about seismic allowable DEFLECTIONS? instead of drifts!

For stiffness modifiers to slabs,

If your framing is such that you don't have Stiff Beams (beams normally > 3*Depth of slab), you will see a lot of difference when you apply and don't apply stiffness modifiers. Consider flat plates, where you have thick slabs on periphery beam. Now when you don't apply stiffness modifiers to such a case, you get less moment in you beam as more load is transferred by the slab to the column, and slab is considerably thick and has a reasonable stiffness relative to the beam which is supporting it.

For the same case when you apply a modifier, the slab stiffness is reduced and more load in transferred to your beams which are now stiffer than your slab because of the modifier.

Why should we use a stiffness modifier? for the above example, a flat plate would have some sort of cracking when subject to seismic loading and because of that the stiffness would be reduced, as a result the beams would be carrying higher load than they were actually supposed to take(considering if the original design was based on no stiffness modification for slab). Thus by using modifiers for this case, a structural engineer is able to consider the effect of heavy loaded beams and designs beams to a greater load. Therefore, using stiffness modifiers where there is a reason can help you design robust structures.

What I am trying to say is that for slabs like 4", 5" the difference in moments is very small for the cases with or without modifiers if the beams are deep. Don't worry too much about using modifiers unless you have a condition that demands one. Just, plug your numbers in and do your analysis as for most of the general cases, the difference due to modifiers would be close to nothing as explained above. Every thread I go, people are confused about them. Dont worry that much, just use the default numbers and be cautious of situations like the one explained above where you really need them.

@Rana, why did you say that using stiffness modifiers for slab would result in higher sway? Isnt the lateral distribution (for this case) on rigid diaphragm assumption would result into same story force no matter how thick or thin the slab is, which will determine the story sway(considering frame stiffness as constant)..

Edited February 27, 2013 by Umar Makhzumi

[WR1]

@Rana, why did you say that using stiffness modifiers for slab would result in higher sway? Isnt the lateral distribution (for this case) on rigid diaphragm assumption would result into same story force no matter how thick or thin the slab is, which will determine the story sway(considering frame stiffness as constant)..

i said for "out-of plane" modifiers for slab....not for "in-plane"

[WR1]

apart from strength design...how would you check your serviceability without cracking (without modifiers)?

[UmarMakhzumi]

apart from strength design...how would you check your serviceability without cracking (without modifiers)?

Modifiers should be used, like I said in my post. Checking serviceablity with or without is only a matter of reducing interia or not due to cracking(depending if your stress in a flexural member increase concrete modulus of rupture). I would suggest using modifiers for all flexural members. Beams, column & beam columns . Anyways, Out of plane modifiers se sway pe kaise effect pare ga Rana ? I actually didnt get it. Did you mean in a different way?

[WR1]

OUT OF PLANE stiffness reduction results in more sway because in dynamic analysis we assume

that we have rigid diaphragm supported by flexible columns

this rigid diaphragm is rigid "in-plane" thats why at any point we will get same sway

what i was talking about was not for MEMBRANE (where out of plane stiffness is zero) i was talking about SHELL element which has significant out of plane stiffness....and this out of plane bending causes outrigger action or frame action which causes changes in stiffness and hence the lateral sway

if you include shell in your model it will stiffen the building due to this effect

now if you decrease the shell stiffness (out of plane) ofcouse its lateral sway will increase...

on the other hand if you are talking about membrane only (classical) dynamic approach we dont take this effect...

[UmarMakhzumi]

OUT OF PLANE stiffness reduction results in more sway because in dynamic analysis we assume

that we have rigid diaphragm supported by flexible columns

this rigid diaphragm is rigid "in-plane" thats why at any point we will get same sway

what i was talking about was not for MEMBRANE (where out of plane stiffness is zero) i was talking about SHELL element which has significant out of plane stiffness....and this out of plane bending causes outrigger action or frame action which causes changes in stiffness and hence the lateral sway

if you include shell in your model it will stiffen the building due to this effect

now if you decrease the shell stiffness (out of plane) ofcouse its lateral sway will increase...

on the other hand if you are talking about membrane only (classical) dynamic approach we dont take this effect...

that is a different perspective but "Outrigger action"; is in-plane too & is building mai to koi significance nahin ho ge. Anyhow, I will go through it on weekend to see what potential effects it has!

[abdulqadeer29]

sir, what i have observed is , by using modifiers sway increases....

[UmarMakhzumi]

sir, what i have observed is , by using modifiers sway increases....

Thats because you are using stiffness modifiers for beams and columns too that would obviously increase sway. The discussion above is that using only slab modifiers would increase or not increase sway. I am saying it shouldnt. I don t have a frame analysis software at home so AQ check this too !!