Timber Member Design Check

[simjack]

Hi,

I am carrying an assessment of a timber truss. 

I assigned each member a "Member type". For example, member 8 is a "top Chord" member type. The effective length for buckling above major axis has been defined to coincide with points where the member meets adjoining elements and for buckling along minor axis every 400mm. I tried to define bracing for "lateral buckling" at every 400mm as well, however, when I run the analysis the effective length comes as 7.09m Do you know why is that?

Also I was wondering, does the software automatically set up the material parameters for an appropriate load duration? And the deflection values, are those final deflection values including creep? 

I have attached the model and was wondering whether you could have a look and advise on the issue or point out any other issues connected to the model. 

Thank you

[Artur.Kosakowski]

Hi @simjack

You missed the model.

[simjack]

Sorry, I try again.

[simjack]

Any ideas anyone?

[Simau]

Hi @simjack

You haven't created a "top chord" type of element, but only a name "top chord".

You need to create a new type of bar as shown in the screenshot below

[simjack]

Hi Simau,

I think that is what I have done. 

If you have a look you highlighted "Elelement de construction" in properties box but "type de barre" in a pop out window?

[Simau]

@simjack

Sorry, my Robot is now busy with a huge lodel, so i can't make a new screenshot.

You have to create a new type.

LMK if this modified screnn is understandable, if not i'll make a new one asap.

[Artur.Kosakowski]

Hi @simjack

You may find the recording of the 3rd Robot webinar helpful as the principles of the timber design in Robot are exactly the same as the ones for steel design.

See: https://forums.autodesk.com/t5/robot-structural-analysis-forum/robot-structural-analysis-webinars/td...

@Simau Thank you for your help.

If one or more of these posts answered your question, please click Accept as Solution on the posts that helped you so others in the community can find them easily.

[simjack]

Thank you for your quick reply. I am going to do what you said, create a new type. I dont understand however how is it different to what I have there now?

[simjack]

I feel like i've done what the video says. I've defined the internal bracing for lateral buckling at every 400mm but yet I am still finding the effective length to be 7.33m (screenshot2) I cant figure out why?

[Simau]

Hi

Your model and results are corrects

[simjack]

however, why is the lateral buckling length being 7.39m I tried to define it as 400mm?

[Simau]

@simjack

You must check this option:

See attached picture

[simjack]

that one is for buckling, not LTB and this one seems to work?

[Simau]

@simjack

it seems to me that you are not convinced.
what do you mean by LTB
There are 2 possibilities of buckling, one in Y direction, that is to say in the plane of the truss, and one in Z direction, that is to say out of the plane of the truss.
In the out-of-plane direction L and Lf = 40cm
In the plane of the truss L  (bar 8) = 7.94 and Lf = 2.33m 

See  attached Pics

[simjack]

Im not convinced at all. LTB = lateral torsional buckling for members imposed to flexure 

[Simau]

@simjack

Sorry, my mistake du to a mismatch between french and english.

Yes you are rigth.

By defining Top chord type, use relative length option ( 0.4/7.95 = 0.05)

See pics

[simjack]

Hi Arthur, 

Could you please have another look at the model and comment on your thoughts? I seem to be going in circles here. I defined the lateral restraints for all buckling in Y and Z axis, for LTB (for the top flange only as I dont think there's a requirement to apply any to the bottom flange since it should be in compression?) and yet getting strange effective lengths for lateral buckling. Could you please comment as at the moment I have no confidence in my model and results I am getting. 

Also is the deflection taking creep into account?

Thank you

[Simau]

@simjack

for LTB (for the top flange only as I dont think there's a requirement to apply any to the bottom flange since it should be in compression?)

Yes you need also to defined lateral restraints for bottom flanges, as they may be in compression.

 

[simjack]

You're right. However, I did apply it in my latest version (r2) and it still didnt make any difference.