Moment Release in STAAD Pro

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What is STAAD Pro?

STAAD Pro stands for Structural Analysis And Designing Program.  STAAD Pro software is widely used in analyzing and designing structures – buildings, bridges, towers, transportation, industrial and utility structures. Designs can involve building structures including culverts, petrochemical plants, tunnels, bridges, piles; and building materials like steel, concrete, timber, aluminum, and cold-formed steel.

Basically, STAAD helps structural engineers automate their tasks by removing the tedious and long procedures of the manual methods.

I have already write a post regarding What is STAAD Pro and It’s Use in Civil Engineering. Read that post to know more about STAAD Pro.

Why is the moment release given in beams in STAAD Pro?

While designing, there are some members which are meant to take only axial load like bracings and truss members. These members don’t transfer moment at the joints.

So to make a member which shall not transfer moment, in STAAD, we provide moment relies at the end of the members.

How to Release Moment in STAAD Pro

In the given illustration a bay frame with 5 m length, 5 m width & 3 m height is considered & between two primary beams (beams which are supported directly over columns) on secondary beam (beam supported over primary beam) is provided.

The secondary beam is loaded with 20 KN/m UDL, no other weight is considered on any other members.

Bay frame with Loading & beam numbers
Fig.1: Bay frame with Loading & beam numbers

Case-1

when the secondary beam is fixed at joints at primary beam & no release command is given.

Bending Moment

If you notice in the below given figure-2 you can see that hogging moments are coming on the top face of the secondary beam near the joints, this is because when this beam under loading tries to undergo bending in downward direction the rigidity of the joint at secondary & primary beam junction restrains it & thus because of this restraining action hogging moments arise on the top face of the secondary beam.

Bending Moment Diagram
Fig. 2: Bending Moment Diagram

Torsion

Moreover due to rigidity at joint the secondary beam also tries to rotate the primary beams about their own axis in inward direction which will result in arise of torsional moments in the primary member (refer figure-3). Upon analysis in the given condition the joints are treated as rigid offering restraints to all three force directions (Fx, Fy & Fz) & all three moment directions (Mx, My & Mz) & thus torsion or twisting moment about the local axis of primary beams is generated. In the given case 11.51 Knm torsional moment is coming in primary members.

Torsional Moments in Primary Members when supported secondary beam is not released
Fig. 3: Torsional Moments in Primary Members when supported secondary beam is not released
Torsional moments coming in primary members in un-released condition
Fig. 4: Torsional moments coming in primary members in un-released condition

Case-2

When secondary beam is released at joints & is assumed to be simply supported over the primary beams.

In this case secondary beam has been provided with member release command & bending moment about Z i.e. MZ moment is released (you can notice two tiny circles near joints denoting member release command), in our case we have considered 100% release.

Bending Moment

When a member is released at a joint then STAAD will not consider that joint as a rigid joint/node offering restraints in all three force directions (Fx, Fy & Fz) & all three moment directions (Mx, My & Mz) & instead it will release the force or moment restraint as defined in member release command. In this case as we said above we have given member release in Mz, so all the bending moment in Mz is free (without restraint) & due to this the behaivour of the beams has changed & because of that only sagging moments (as in the case of simply supported member) are coming on the bottom face of the beam & no hogging moments will come (refer figure-5).

Bending moment when secondary beam is released
Fig. 5: Bending moment when secondary beam is released

Torsion

Now as the condition of the joint is simply supported & with no restraint offered by primary beams to the secondary beam in bending the secondary beam undergoes bending freely & thus no torsion or twisting of primary members happen, that means zero torsion in primary members.

Zero torsion in primary members supporting secondary beam with member release command
Fig. 6: Zero torsion in primary members supporting secondary beam with member release command

Conclusion

Below given is the comparative analysis of the above two cases:

Moment Release in STAAD Pro
  1. Generally, member release command is given in conventional design practice whenever the primary beam member fails because of torsional moments.
  2. Although it is conventional but it is always advised to make sure that the assumption of member release holds true as per the actual conditions of the structure.
  3. Member release shall be avoided because it causes reduction in nos. of stirrups of the primary member & which in turn reduces the ductility of the member, further the concrete & main reinforcement of beams with less no. of stirrups then required will not be confined properly. Poor ductility & poor confinement is not good for the structure for its resistance to seismic forces. 

I hope this article will help you to understand How to Release Moment in STAAD Pro. You may also want to see my other post on my Blog. If I have missed anything here, please let me know about that in the comment below this post.

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