How to identify Primary and secondary beams
Today’s structural design training and vlog is about a basic structural analysis and design concept pertaining to stiffness.Though many students learn moment distribution method and many analysis methods like the Kanis method,they fail to practically connect what is increase in stiffness. For example a classic text book example says that Inertia of the column is double by two times to 2I but it never corelates that increase in Inertia means bd3/12 is increasing and it can be even an orientation change of the column and necessarily need not be size increase.Since such gaps exists in education, I thoiught it is relevant to blog on this topic and hence this attempt to simplify this as much as I can.
So as I mentioned,A structural detailing doubt asked by a civil engineer in the civilera structural discussion forum is presented as a structural design tutorial today.
We at civilera have structural design courses and we engage with our students in a community.
This structural community is the place where we engage with the structural design students who have taken up our ETABS courses.
The structural doubts the student had
1. Can we divide beams in ETABS in to pieces and will it still transfer loads across the nodes?
2. How to identify which beam is the main beam or primary beam and which is secondary?
How to cross check structural behaviour
When you have this type of structural doubt, first thing to do is to display the Bending moment diagram and check. Check the Bending moment diagram and Shear Force Diagram and even the deflected profiles of the beams. Do a comparison of values with and without breaking a beam. There will absolutely no difference in the values. I have explained this in today’s structural video. Though I generally recommend to split beams at intersections, it is absolutely ok not to split.This is true for ETABS or even STAAD Pro. However many other software like Staad Pro, it is mandatory to split the beams at intersections.If not it will return an error. In ETABS even intersections need not be essentially split. However I recommend to split due to some other reasons. Any ways it is not going to affect your analysis or design in any way.
In ETABS,By breaking a beam or by introducing a node there is nothing changing in your beam. You can divide a beam in to many parts, nothing will change in the load distribution. You can check by dividing the beam to say 3 or 4 parts. The beams are stiff across nodes.
Can we break and model columns in ETABS?
This can result to another question on columns -Is it ok to break columns. It is a bit complicated to provide a blanket answer since breaking the column will lead to change in effective length and unsupported length of columns. So, the answer depends on how you are dealing the design of columns. The analysis results are not going to affect if you divide the columns in the software.
Which beam is primary and which is secondary? How to identify this?
Again, displaying the BMD will help you to know which is primary and which beam is secondary and taking a support on the other. The BMD and the deflected profile will make it very clear which beam is deflecting more and based on the profile it is easy to understand which is supporting the other.
Stiffer beam will be the main/primary beam. This stiffness is dependent on many factors.
Support stiffness
If the beams are supported on a column, size of column, orientation of column, if it is supported on a beam etc decides the stiffness.
If the beam is resting on another beam, then the support stiffness will be lesser. If the beams are supported on columns, then the support stiffness will be higher. We need to capture all this and that is the reason we model in ETABS or any software like STAAD Pro
Beam size
Point 1 along with size decides which is going to be the primary beam.
The Span of the beam
Span of the beam also decides which is going to be the main beam
Amount of load
Load in the beam also decides the deflection of the beam.
Finally, all these 4 points narrows down to vertical deflection of the beam as well. The stiffer one will act as main and support the other. Sometimes all these 4 conditions will be similar and only minor difference exist and it will be difficult to check this by inspection manually. In such cases you have to check the BMD and deflection patterns.
Please note that when you have 2 beams of almost exact size, span and all the conditions mentioned above, the nearby members will also have a say in the distribution. This is since all members interacts. In olden days and text books, we mostly do a plane frame analysis and now it’s all a 3D space frame analysis and as long as the model is perfect, it’s going to give you accurate results.
Also note that, you are designing and detailing as per the model. So, it will be captured in your detailing any ways and there is no point in knowing which is primary etc. However, a good check of this will ensure you to capture any mistakes in modelling and this is a good way to have an additional check in ensuring the correctness of your Analysis model.
You can read 4th point of this blog which I had written some time back. B2 will be main if span and size is same as B2 support is stiffer.
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Summary
Though software like ETABS and Staad Pro is helping engineers in a big way to analyse structures, it is the responsibility of the design engineers to ensure they validate the designs by good hand checks that establishes structural behaviour and the load take down. You should have these checks identified and implemented in your checking process. In addition to this, there shall be a robust checking of the analysis model too with a check list in place that captures general modelling errors. Both together has worked for me in my work life as well as independent consulting life.
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