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stiffness modifiers for beams and columns

 
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jagan0819
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Joined: 30 Mar 2016
Posts: 51

PostPosted: Thu Jul 09, 2020 12:27 pm    Post subject: stiffness modifiers for beams and columns Reply with quote

hi sefians

i have analysed a framed structure with stiffness modifier-1 and for the same i have given stiffness modifier(0.35 for beams & 0.7 for columns) and when i compare results ,,

1) the frame with stiffness modifier-1 ( moment in beam =58.6kn-m sagging and moment in column=45.7kn-m for upper storey)

2)the frame with stiffness modifier (0.35 for beam & 0.7 for column), the moment in beam sagging =49 kn-m and moment in column=48 kn-m for upper storey)

my doubt is that when stiffness is decreased for beam and column the moment also should get decreased but from above observation moment in beam got decreased(in sagging) and moment in column is increased (for upper storey)


could you please put some light on it



image was attached

regards



with stiffness modifier 1.JPG
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with stiffness modifier 1.JPG



with stiffness modifier 0.3 & 0.7.JPG
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with stiffness modifier 0.3 & 0.7.JPG


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eswar_reddy
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Joined: 01 Jul 2020
Posts: 3

PostPosted: Mon Jul 27, 2020 9:39 am    Post subject: Reply with quote

Dear Jagan,

I understand your point. I will try to answer as clearly as possible.

Stiffness, by definition, it is the load per unit deformation. And at any frame joint, the moment distribution happens based on the relative stiffness of all the connecting members to that joint.

It is said that, the stiffness is decreased by you for both the column and beam. Let us go with an example:
Initial beam stiffness = 2 units
Initial column stiffness = 3 units
          here the relative stiffness factor for beam = 2/(2+3) =0.4
               relative stiffness factor for column = 3/(2+3) = 0.6
now I am reducing the stiffness of both by 1 unit as below

Final beam stiffness = 1 units
Final column stiffness = 2 units

          here the relative stiffness factor for beam = 1/(2+1) =0.33
               relative stiffness factor for column = 2/(2+1) = 0.66


here you notice that the relative stiffness of column has increased and beam got decreased, though i decreased the stiffness of both the column and beam. This emphasizes us that, the column attracts more moment than that of the beam.

At this situation, you may notice, the beam sagging BM decreased as the fixity is comparatively increased at the joint.

Regards,
Eswar Reddy Rokalla
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jagan0819
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Joined: 30 Mar 2016
Posts: 51

PostPosted: Wed Jul 29, 2020 5:31 am    Post subject: thank u sir for ur response Reply with quote

eswar_reddy wrote:
Dear Jagan,

I understand your point. I will try to answer as clearly as possible.

Stiffness, by definition, it is the load per unit deformation. And at any frame joint, the moment distribution happens based on the relative stiffness of all the connecting members to that joint.

It is said that, the stiffness is decreased by you for both the column and beam. Let us go with an example:
Initial beam stiffness = 2 units
Initial column stiffness = 3 units
          here the relative stiffness factor for beam = 2/(2+3) =0.4
               relative stiffness factor for column = 3/(2+3) = 0.6
now I am reducing the stiffness of both by 1 unit as below

Final beam stiffness = 1 units
Final column stiffness = 2 units

          here the relative stiffness factor for beam = 1/(2+1) =0.33
               relative stiffness factor for column = 2/(2+1) = 0.66


here you notice that the relative stiffness of column has increased and beam got decreased, though i decreased the stiffness of both the column and beam. This emphasizes us that, the column attracts more moment than that of the beam.

At this situation, you may notice, the beam sagging BM decreased as the fixity is comparatively increased at the joint.

Regards,
Eswar Reddy Rokalla
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