Hi is there any way of setting defined lengths for the LT buckling lengths in the LRFD steel design module? The only options that appear show these to be relative lengths as multiples of the bar lengths.
Hi Rafal,
Yes that was what I was looking for, but it appeared not to be working entirely as hoped. I started setting the LT buckling length, but this was not implented when running the design.
Thanks
Could you be so kind and either send model or show it on your model (real connections, element , behavior) to let me understand why do you need this coeff or LTB length grater then member lenght?
Only one situation comes to my mind: you are trying to check longer member but consisting of several shorter ones without creating supermember.
Yes, we got a cranked truss with the chord made up by several members between restraints.
So replace serie of elements by one or create supermember for design:
Hi Rafal,
Aplogoies for not reply to this one sooner. I've finally got around to doing some screencaps that show my the problems I've been having with superbars.
In short, when trying to define lateral restraints for superbars, it looks like the prgramme is only able to identify members that are perpendicular to the z-axis of the members.
As you can hopefully see from the illustration below, the restraints from the internal members of the truss are correctly identified for the major axis bukcling and LT buckling. For the minor axis buckling on the other hand, it does not appear to 'see' te slightly angled bracing members that connect to ever second or third chord member.
I was hoping there was something wrong with my appraoch, but not been able to find anythign so far.
regards,
Even
Points for buckling lenghts should be automatically recognized, but not for LTB.
Adjust position for LTB (if you need for buckling also in similar way) as shown below:
It is not recognizing anything that is not perfectly in plane with the buckling plane in questions.
I made a similar model to yours, and the fist picture shows the beams horizontally from the superbeam correctly picked up as buckling restraint points. The second shows the support point lowered 0.1m out of the plane, and the members are no longer recognised as buckling support points.
Yes, I have forgotten to move them out ouf plane.
You are right, from help:
"Use the option to automatically calculate buckling coefficients for the member segments between bracings based on the analysis of rigidity of bracings adjoining the analyzed member in the corresponding buckling plane. Values of calculated buckling coefficients for successive segments are displayed in the field under the bar diagrams."
http://docs.autodesk.com/RSA/2012/ENU/filesROBOT/GUID-1177E678-E14E-4C8E-8AB2-0A76B44DF34-955.htm
That option appears only to calculate proposed effective lengths of segments based on relative stiffness. It requires the actual bracing points to be predefine, and does not help detecting the non-horizontal bracing members.
First picture shows ky<1.0 as calculated, and second picture shows kz = 100 as calculated, probably because no restraints were found.
On a separate note, it appears RSA calculates the buckling lengths as <1.0 where there is a crank. What is this based on? As one could expect the effective buckling length to be >1.0 for cranked members. The help link you included also states "For intermediate segments 1.0 is always proposed.", which does not appear to be the case here.
Yes you are write. It is written in help.
"Buckling coefficients of component segments - Defines values of buckling (lateral buckling) coefficients for the member segments between bracings. After selecting an icon representing the analytical model (bar diagram) of the analyzed member,Robot proposes values of coefficients for extreme segments. For intermediate segments 1.0 is always proposed."
I found description of algorithm:
A. GENERAL INFORMATION
Calculations of the column buckling length according to the automatic procedure (commonly called Automatic Buckling Length – ABL) in the ROBOT program consist in analyzing geometry of bars located in the column’s neighborhood and attempting to adjust it to code formulas for regular rectangular frames.
The option is obtainable only for 2D structures.
B. ALGORITHM OF OPERATION
The program analyzes separately both column end nodes, calculating for each of them their stiffness values according to code regulations. In order to apply code formulas, a user should know stiffness of the analyzed column (which is known from definition), stiffness values of transversal beams that meet in a node as well as stiffness of the adjoining column. The last two ones, called further - by convention - ”beam” stiffness and ”column” stiffness, are calculated in the following manner.
The column direction indicates direction contained within the area of ±15° from the direction determined by the initial analyzed column.
The beam direction indicates direction contained within the area of ±15° from the direction perpendicular to the initial analyzed column.
All bars that are not included in the above-listed classification, belong to the ”intermediate” group.
The division presented is illustrated in figure 1.
a) a node in which at least 3 bars meet
b) support
c) release in node or element (hinge).
d) change of direction by the angle greater than ±30° with respect to the initial position
e) too large number of changes in bar stiffness (more than 10)
A change in stiffness reaching the order of 1.0e-12 is considered insignificant and is not included in calculations. Since version 12.5, the substitute stiffness is calculated based on the following formula (J1*L1+J2*L2)/(L1+L2).
Column and beam stiffness values (calculated as a relation of moment of inertia to length) for individual chains of bars are added up, which enables determining the ultimate beam stiffness and column stiffness of a node after analyzing all the bars that meet in a node. These values are substituted in the appropriate code formulas.
If there is a support or hinge in a node, analysis of a bar chain is not performed and the support pattern implies the appropriate equivalent stiffness of the node. If both nodes are supported, then the buckling length coefficients corresponding to those known from the material strength theory are adopted.
C. COMPARISON WITH CALCULATIONS ACCORDING TO MODELS WITH ADJOINING BARS
For simple rectangular frames, results obtained on the basis of ABL correspond to results obtained due to applying the appropriate model with adjoining bars. In case of composed bars of different stiffness values (e.g. bars with brackets), the results will be identical only when ”superbars” have been used in the model (only then the method of calculating averaged branch stiffness will be identical as for ABL). Application of the list of adjoining bars provides minimally different results.
Another situation in which differences between the methods may occur, concerns the inclined spandrel beams exceeding the area of ±15° (see fig.1). Then it is necessary to calculate manually equivalent length values of columns and beams following the rules specified in point.
Figure 1.
Thanks. That goes a long way towards clarifying the details of how it is set up.
Back to the original issue, it looks like this limitation in the program prevents superbars from being an option, and that all buckilng lenghts will have to be manually calculated and entered.