Robot Structural Analysis Forum
Welcome to Autodesk’s Robot Structural Analysis Forums. Share your knowledge, ask questions, and explore popular Robot Structural Analysis topics.
cancel
Showing results for 
Show  only  | Search instead for 
Did you mean: 

Continuous Beam design

40 REPLIES 40
SOLVED
Reply
Message 1 of 41
GordonSharp1992
3922 Views, 40 Replies

Continuous Beam design

how do i model a continouis beam with a roller support at the mid span. i need the beam to be continous.

also how do i model a joint to have a semi rigid/pinned support. with a different stiffness.

thanks

40 REPLIES 40
Message 21 of 41

hello

can you help

i have attached both a paint file to decribe my problem and the autosad file to let u have a look

please can u help

thank you

gordon

Message 22 of 41

What is the question?
Paint file missinig.


Rafal Gaweda
Message 23 of 41

here it sorry

Message 24 of 41
Ken_Marsh
in reply to: GordonSharp1992

I guess I'm not understanding what is so complicated about this problem.  If you need elastic releases at those joints, just use member end release and specify an elastic stiffness in the release for that direction.

The beam should be modeled across both supports and extending out.  It's just one beam.  Apply support conditions and loads then analyze.

 

Where are you getting hung up? 

 

 

Ken Marsh
Owner Marsh API
Message 25 of 41
Ken_Marsh
in reply to: Ken_Marsh

This book http://tiny.cc/ARSAP2014_Book has a good section on member end releases and modeling supports and members. 

Ken Marsh
Owner Marsh API
Message 26 of 41
Rafal.Gaweda
in reply to: Ken_Marsh

You can do it exactly as it is in pdf 

 

rotfr.jpg



Rafal Gaweda
Message 27 of 41

thank u !

i understand this thank you

is there anyway i can design the node with rotational stiffness ?!

i want to make the frame less rigid, so i want more deflection at node 3.

basically the first desgin i want is to be completely rigid so gives litlle deflection . then the second desgin to be semi rigid/ pinned, so elastic at the two joints. do u understand . will i have to increase the stiffness ?  or do i have to degin the acdtual nodes ? as what u have shown me is the members .

but if i model the nodes how do i connect them up with each other as i get nodes not connected to the frame ?!

thanks

 

Message 28 of 41
Ken_Marsh
in reply to: GordonSharp1992

The release definition that Rafal has shown above is introducing rotational stiffness in the connection between the vertical members and the horizontal members.  If you want to first design as rigid, do not add member releases at the ends of the vertical member.  When you are ready to design for semi-rigid, you add the release with elastic stiffness as shown to both ends of the vertical member. 

 

does that help? 

Ken Marsh
Owner Marsh API
Message 29 of 41
GordonSharp1992
in reply to: Ken_Marsh

yes this helps thanks ! just wandering how would i make it less rigid ? increase the stifness or decrease ?! i want the structure to fail more at the end of the cantilever but changinging to elastic doesnt do this when i feel it should as it is less rigid ?!

Message 30 of 41

 increase the stifness or decrease ?! 

 

Guess !

 

elasticrot.jpg



Rafal Gaweda
Message 31 of 41
Ken_Marsh
in reply to: GordonSharp1992

rigid = stiff therefore decreased stiffness = decreased rigidity.
If the support at node 5 in Rafal's model is a roller, I would guess that the stiffness of the connections (nodes 3 and 4) will probably have little impact on the load carrying capacity of the main supporting member (member 5).
Is this something you are experimenting with? The standard procedure would be to determine the loads acting on the structure, apply load combinations per the applicable code and determine the forces in the supporting members and design them appropriately. Maybe you can help us understand what you're trying to achieve.
Ken Marsh
Owner Marsh API
Message 32 of 41

yes i understand this but in a UZ direction !? so a nodal force acting down on node 3 !

i need more deflecstion here can u show me ?!

Message 33 of 41
Ken_Marsh
in reply to: GordonSharp1992

what do you mean that you "need" more deflection? reduce the stiffness of the supporting member number 5 if you really want it to deflect. (that means, make beam 5 smaller)
Ken Marsh
Owner Marsh API
Message 34 of 41

hi ken marsh here is a pdf of my problem ?! hope it makes sense !

appoligies i am inxeperienced with robot !

thank you

 

Message 35 of 41
Ken_Marsh
in reply to: GordonSharp1992

it is very difficult to account for all the factors affecting wood construction. I would not model any elastic stiffness at the joint of the wall to the beam. If you are finding that the measured deflection was less than Robot predicts, my guess is that you have load sharing between members, or some participation of the floor decking in load carrying. If Robot is predicting less deflection, you may have the section or material properties incorrect in Robot.
If you experimented in place with the real elements before the windows were put in, then I would model it without the window wall. Then look at what you have modeled vs. reality: what was present in the real model that is not present in the Robot model? e.g. deadload or walls on the backspan, plywood decking on the beams potentially causing load sharing or participating as part of the section of the beam (i.e., a T beam), support width playing a factor (is it a 1'-0" wide bearing?)
Ken Marsh
Owner Marsh API
Message 36 of 41
GordonSharp1992
in reply to: Ken_Marsh

 

 

so u would recomend that i look at the beam as a single cantilever beam with the self weight and and other loads such as the floor cassette and live load i applied with weights and see if that matched my deflection from live testing. i can do this however . when i go to model the frame how would i be would i be able to model the joints i have been talking about ?! if i was to not make them elastic ?! just keep them rigid. in real life the joints are bolted this is why i thought poassiblty a rotational release with a designeated slip modulus ?! could i possibly change the roller at the top to a fixed of a spring ?

Message 37 of 41
Ken_Marsh
in reply to: GordonSharp1992

If they are truly bolted in a way that induces rigidity in the joint, then by all means you can model them as such. My guess is that, however, the connections, even though bolted, probably aren't intended to provide rotational rigidity to the connection. Without seeing a detail of the connection configuration it's hard to guess.
What I was mostly saying above is that, if you are intent on comparing in-situ testing and measured deflection, you need to be very thorough in accounting for the reality vs the model. All structural analysis is an idealized structure and it will be fairly rare that you will get an exact one-to-one correspondence between an idealized model and the reality. If you want to be exact, you will need to measure the modulus of elasticity of the actual members you have used in the structure, account for the effects of acting loads, the width and compressibility of the support (it's probably sitting on piece of wood loaded radially), the amount of rotational stiffness in the connection (which you would have to measure), the fact that the roof isn't rigidly connected and also has other factors affecting it, etc. etc. It's not a small task. You might want to consider how important it is to go down that road.
Ken Marsh
Owner Marsh API
Message 38 of 41
GordonSharp1992
in reply to: Ken_Marsh

 

 

my project is the logterm deformation of accoya timber as glulam in a live project ! the extenstion u have seen in the picture. i need to find the stresses exerted in the cantilever beam. the structural part i am trying to keep simple if u know what i mean ! but id still want to calabirate the structure with my prelimanary testing !? i think ive just got to play around with the possible loads on the beams and try to get a structure with i similar deflcection to that of my real testing . how would i get the simple structure to deflect more at the end of the continuous beam . so i can say this matches the real structure with a more complex desgin possibly ?! i dont have the experience to desghn somethinmg too complex and im running out of time do you think i shoule look at it as just a contiuous beam or try and model as simple frame and keep changing it till i get a simialr deflection ! the only problem with this is im not sure what to change to get a greater difflection if u know what i mean . like before i tried to make the joints elastic ?!

 

 

 

Message 39 of 41
Ken_Marsh
in reply to: GordonSharp1992

The factor which will have the largest impact on the deflection of the timber at the free end is the modulus of elasticity and the section modulus of the timber. Have you calculated them correctly?
Ken Marsh
Owner Marsh API
Message 40 of 41
GordonSharp1992
in reply to: Ken_Marsh

hello thankzs for the help .

i was wandering when considering the loads that i am going to put on my beams . so self weight of all the beam, the weight of a casstte , the weight of floorboards and furniture and a point load of the windows . how do i consider them all at once and what factors should i give them ?

thanks

Can't find what you're looking for? Ask the community or share your knowledge.

Post to forums  

Autodesk Design & Make Report