zero-rods in truss

[roibpro125]

Hello, first of all I am new on forum, so correct me if this folder is not ok for such question
my problem is about zero stress rods at truss

this is prabably right solution, bolded rods are not zero-stessed. As I start to match zero-stressed I understand, 1, 2, 3, but why 4th rod would be zero stressed, could someone explain it to me? Thank you in advance

 

[roibpro125]

here in attachment is this truss, cause it didn't load

 

[Agent666]

It's acting like a tied arch if the loads are symmetric.

Remove the symmetry in the loading and you will see loads in the other members.

 

[BAretired]

Sorry Agent666, but I disagree. The members would still have zero stress if the loading is made unsymmetrical.

Members 1 and 3 have zero stress because there is no applied vertical load at either Joint 1L or Joint 3L where L denotes the lower end of the member. A vertical load applied at either joint will produce a force in the member.

Members 2 and 4 have zero stress because there is no applied load at Joint 2U or 4U where U denotes the upper end of the member. An applied load at Joint 2U having a component normal to the chord member will produce a force in member 2.

Similarly, an applied load at Joint 4U having a component normal to the main diagonal member will produce a force in member 4.



BA

 

[roibpro125]

thanks BAretired, all clear

 

[paddingtongreen]

A word of warning, if you run this with computer program, you may get small forces in those zero members. This is because strain in the loaded members may change the geometry of the zero load members and thus cause strain in them.

Michael.
"Science adjusts its views based on what's observed. Faith is the denial of observation so that belief can be preserved." ~ Tim Minchin

 

[BAretired]

paddington,

That may be true if some members are entered as continuous, but using the traditional assumption for truss design, namely that all joints are deemed pinned, there should be no force in any of members 1 through 4.

BA