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what are the advantages of working stress method

 
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raghavendrabaroda
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PostPosted: Wed Nov 09, 2011 11:30 am    Post subject: what are the advantages of working stress method Reply with quote

dear sefians,
      what are the main advantages of working stress method over limit state method. please clarify
thanks and regards
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suresh_sharma
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PostPosted: Wed Nov 09, 2011 12:21 pm    Post subject: Reply with quote

According to me working stress will give less steel and more concrete.
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Dr. N. Subramanian
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PostPosted: Wed Nov 09, 2011 6:26 pm    Post subject: Reply with quote

Hi Er Ragahavendra,

Working stress method is simple to use. but does not give consistent values of factor of safety. That is the reason Limit states methods were developed. But Working stress method is still in use for serviceability limit states like deflection, cracking, vibration etc.

Best wishes
NS

suresh_sharma wrote:
According to me working stress will give less steel and more concrete.
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P.K.Mallick
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PostPosted: Thu Nov 10, 2011 1:05 am    Post subject: Reply with quote

IS:456:2000 allows design by Working stress method except for column.
The clause B-4.3 of IS:456-2000 stipulates that "Members subjected to combined direct load and flexure  shall be designed bu LIMIT STATE METHOD.

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raghavendrabaroda
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PostPosted: Thu Nov 10, 2011 5:05 am    Post subject: Reply with quote

thanks a lot sirs
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rajancivildesign@gmail.co
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PostPosted: Fri Nov 11, 2011 4:10 am    Post subject: Advantages of Working stress method Reply with quote

Even though the working stress method is simple as in case of long column design of RCC member,the ultimate loads cannot be accurately predicted. Also the results give
uneconomical section. For time being provision is made in the code, practically all uses LSM only for RCC design.

V.M.RAJAN.
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vikram.jeet
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PostPosted: Sat Nov 12, 2011 4:55 am    Post subject: what are the advantages of working stress method Reply with quote

Rather, limit state method has more advantages over working stress design and  
LSM is economical too.

However working stress method is not outdated and will remain in practice  
for structures subject to fatigue , water retaining structures, steel structures
and structures subject to loadings (if any) without prescribed load factor

However lately LSM is also extended to Steel structures, Water retaining structures
and bridges( which are subject to fatigue) but WSM is still the popular method in
these areas of design .

best regds

vikramjeet



dear sefians,
what are the main advantages of working stress method over limit state method. please clarify
thanks and regards
-- ญญ

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suresh_sharma
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PostPosted: Sun Nov 13, 2011 3:11 am    Post subject: Reply with quote

Dear Vikram,
I do not think limit state method is economical. The consumption of steel is about 20 % higher. I am of the view that it is rational but not economical. Please comment.
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Dr. N. Subramanian
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PostPosted: Sun Nov 13, 2011 4:43 am    Post subject: Reply with quote

Dear Er Suresh sharma,

It depends. In some cases it seems to be economical than Working stress method. May be in those cases the WSM may not give reliable safety! You can not compare the results of one refined method and another app. method, and say the app. method is economical!

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NS
suresh_sharma wrote:
Dear Vikram,
I do not think limit state method is economical. The consumption of steel is about 20 % higher. I am of the view that it is rational but not economical. Please comment.
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vikram.jeet
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PostPosted: Mon Nov 14, 2011 7:16 am    Post subject: what are the advantages of working stress method Reply with quote

Dear Suresh ji

If we compare the two methods , the consumption of concrete is less in LSM w.rt WSM
For same working moment , WSM will require higher concrete section than LSM .

In case section is kept same the requirement of comp reinf (as per WSM) will also add
to the steel consumption


For example A beam 23cm X 45cm (M-20) and Fe415
(effective depth 40cm)
Comparing the two methods

A] Moment of resistance of section as singly reinfd section

WSM
Working Moment of resistance of beam = 8.985 *23*40^2 =330648Kgcm =3.31tm
Beyond this, compression steel will be required
LSM
Limiting Moment of resistance of beam = 27.6 *23*40^2 =1015680Kgcm =10.16tm
Working MR as calculated based on LF= 10.16/1.5 =6.77 Tm

It is seen that MR is almost double in case of LSM.i.e. higher moment carrying capacity

B] Steel consumption for a working moment of 6.77tm
WSM
Moment of resistance of beam = =3.31tm
Beyond this, compression steel will be required
Balance moment = 6.77-3.31 =3.46tm

Comp ASc = 3.46*100000/(723*35)=13.7cm2

(723 kg/cm2 is the comp stress in comp steel based on concrete comp
stress around it multiplied with (1.5*m-1) and assuming cover to  
Asc center =5cm)

Tensile Ast
Ast1 = 3.31*100000/(2300*0.905*40) =3.98 cm2
Ast2 =3.46*100000/(2300*35) =4.29 cm2
Tension Ast = 3.98+4.29 =8.27 cm2

Overall steel consumption = 13.7+8.27=21.97 cm2

LSM
Ast = 0.955 *23*40/100 =8.79 cm2
Asc =NIL

Thus overall, WSM will be costly on both fronts but it may definitely assure higher  
safety factor for concrete in comparison to LSM., whereas for tension steel  
both methods will have almost same FOS .Probably due to this inconsistency of the
safety margins of two constituents, LSM has been evolved which ensure consistent
margin of safery for both . Yes - - -concrete is produced at site whereas steel is  
manufactured in factory, the two had different safety margins as per WSM philosophy.


However if section is oversized, the two methods may have only SLIGHT
difference in tension steel consumption .

Beam size 23X 65cm

WSM
MR = 8.985*23*60^2=743958 Kg cm= 7.44 Tm
Ast = 7.44*100000/(2300*0.905*60) = 5.96cm2

LSM
Limiting Mo Mu = 1.5*7.44 =11.16tm

Mu/bd^2 =11.16*100000/(23*60^2*10.2) =1.32 N/mm2
As t =0.401*23*60/100= 5.54 cm2

So the tension steel requirement is marginally different only.

May kindly bear with the long posting  

best regds

vikramjeet


Dear Vikram,
I do not think limit state method is economical. The consumption of steel is about 20 % higher. I am of the view that it is rational but not economical. Please comment.
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