Regarding unity bending code check? 1

Could you be using a different design code than AISC 360-05? If so, then the program will not give bending checks for single angles. That's currently only availaible with that version of AISC.

Also, it is a very new feature (version 9.1.1, I think). Therefore, if you are running a slightly older version of the program then you would not have access to that new feature and you would get that standard "code check based on zz axial only" message.

Thanks for the reply JoshPlum. Actually i was using AISC 360-05 code, but i am working with RISA 3D Version 9.0.1. This version might be the reason.

Also, regarding the calculation of compressive axial stress. how does RISA handle KL/r ratios? I went through the calculations for typical member and this is what i understood: Software used the conservative K value of 1, L as node to node distance, and r as rZ value to calculate KL/r ratio. Using the equations in section E3 or E7 it calculated the compressive stress. But, as per section E5, for single angle members it is required to increase KL/r ratio to account for eccentricities in connecting the leg member. Should I consider this increase in KL/r? Please forgive me if i am saying something wrong here.

Thanks

Yes, this would be one of the differences between version 9.0 and the latest version 9.1 of RISA-3D.

There should be a good amount of information in the help files concerning K and L values. K values (when not directly set by the user) default to a value of 1.0. The unbraced lengths (when not set by the user) default to the full length of the member. You can set the unbraced length to a specific value, or you can use the term "segment". The term segment means that the distance between nodes will be used as the unbraced length. This can be very useful for "physical members".

The program doesn't make any attempt to account for eccentric loading. So, we are not adjusting the KL/r values per section E5a and E5b. We leave that up to the user.

JoshPlum,

Thanks for the information and suggestions. I upgraded my version to 9.1 and this helped resolve my previous questions.

Also, I have a question about modeling column and base plate interaction. Here is the scenario:

I have a beam supported on two columns and these columns are supported on plate and in turn plate is supported on 4 angles forming a box frame and this frame is connected on top to the tower.

Can you suggest me the best way of modeling this accurately in RISA 3D.

I appreciate your help in advance.

Thanks and Regards,

Sagan26

Sagan -

Can I assume that each column attaches to a separate plate / 4 angle box?

If so, I tend to prefer simplicity in modeling. So, my preference is to model the the connection of the columns to the tower in a simplified way.... either perfectly pinned or perfectly fixed end releases. In reality, you know that the connection has to be between pinned and fixed, but we usually just make an engineering decision that idealizes the connection.

In cases where it is not clear whether the connection is pinned or fixed, you can solve the model both ways. Run it once using one assumption then solve it again using the other assumption. That way, you'd be enveloping the design....

Alternatively, there are ways to model in connection flexibility. But, this involves a good deal of modeling effort. I usually start by adding in a short "link" member. Then I can adjust the stiffness (by modifying A, I and J) of this member to give me whatever level of connection flexibility that I desire. It's tricky, but it works.

Hi Josh,

What is the significance of Section Type in RISA 3D?

Regards,

You're referring to Member Type (Beam, Column, HBrace or VBrace), right?

1) The distinction between column and beam is very importan for concrete members.

2) The assignment of the members is important for the Steel Seismic Detailing provisions if you are using them.

3) When you assign load using the "Member Area Load" feature, the H and Vbraces do not get load attributed to them.

4) The stiffness adjustments used in the AISC "Direct Analysis Method" will change slightly based on the Member Type.... Braces only get their axial stiffness adjusted. Beams only get their flexural stiffness adjusted. Columns get both stiffnesses adjusted.

Thank you very much again Josh.

I am applying area wind load to my tower model. I used open structure option and defined leg members as columns, X Diagonals as VBraces, Horizontal struts as HBraces. Is that the right way of defining the section types? Because, as you said, when i use area load with open structure option, RISA is applying the transient loads only to the leg members. But in reality, all the HBraces and VBraces should also see the wind load right?

Can you guide me with this in modeling my tower accurately in RISA 3D.

Regards,

Sagan26.

The rational for excluding H and V braces from member area loads is really only valid for buildings. You've got some form of cladding or decking that spans between beams or columns or such.

For open structure area loads that assumption sort of falls apart. In your case, I'd probably just call everything a column and then use the open structure area load. For a tower, you're probably not thinking about using the seismic detailing provisions or such, are you? If so, then those member type designations become less important (outside of area load attribution).