Let’s decode another enigmatic word that the more senior guys in the office use very often: release.
If you are a graduate engineer, you might have heard at some point conversations just like these:
“No, the girder will not work with that. Release that connected beam.”
“Release this one…”
And then you felt intimidated and frustrated hearing it again and again. “Release WHAT?!”
Well you will know what exactly today so keep your cool.
The Gist
So what is exactly meant by release? Release is removing the capacity of a certain structural member to support forces like bending moment, torsion (more of torsion in the next posts) axial and shear forces. This can be done in almost all structural analysis and design programs such as Staad, SAFE, and ETABS.
But most often than not, what the senior guys mean is to release the bending moment. Another simplistic explanation is to try to imagine a fully restrained or a fixed-fixed single span beam. Once you “release” the beam, there will no longer be negative bending moments at the supports such that it will approximate that of a simply supported beam.
But first, let us establish some facts you might want to internalize before we continue. After which the succeeding discussions will no longer be alien to you.
- In a monolithic concrete structure, simply supported beams do not really exist. That is, it can still carry a bending moment based on the minimum flexural capacity of the section, that is if you provided the minimum requirement based from the code. The right term would be “fixed, but only to a certain degree.
- Aside from the geometry of a structural member, the reinforcement that is not present in a structural analysis and design software such as ETABS, will determine if the end condition will be either pinned or fixed. In a beam for example, it will behave more fixed if you provide two layers of reinforcement on top at the support than if you only provide two top bars.
Why Do We Need to Release?
Yes there are valid reasons beyond your perceived whims and power-tripping of your officemates and these are:
- In order not to transfer the end moments on purpose. Stiffness dictates that the stiffer it is, whether it’s a structural member or a connection, the more forces it will attract. So less reinforcement means it will not attract anything larger than what the capacity can support.
- The development length is not enough. In order for the connection to qualify as a fixed connection, the bars should be able to fully develop in tension. Imagine a 1000×3000 beam 12 meters long supported at the weak axis of a 200×1000 column. Do you think the column can really provide a fixed connection?
- The support cannot carry the end moments coming from the beam. Like the column described above, I am very certain that you will get an O/S when you design it in ETABS.
Where Do Releases Usually Occur?
On beams supported by girders. This is to avoid introducing torsion (again, more on torsion in the succeeding posts)
- On beams to walls and blade columns on their weak axis. In columns, additional bending on the weak axis guarantees increased moment magnifiers that can render the column overstressed or with unusually high reinforcement ratios. That is why they are often released.
The strength of walls is in its strong axis so instead of transferring out-of-plane bending moments to the wall, releasing will transfer only vertical and horizontal end reactions as a result of the pin-end condition.
- On planted columns. The effect is also to minimize if not remove the effect of torsion to supporting beams.
But how exactly do you apply frame releases in say ETABS?
Just follow the procedure below. But before that, you need to select first the member you intend to apply releases.
A Few Words of Caution
There are special instances where releases are required which I will no longer discuss. And be careful in assigning releases because you might already be messing with the stability of the whole system. READ: an instability notice in ETABS and/or SAFE or in other softwares for that matter.
The post To Release or Not to Release: That is the Question appeared first on Civil Engineering Community.
source https://www.civilax.com/to-release-or-not-to-release-that-is-the-question/
