Moment Release 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.
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.
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.
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.
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.
Fig. 3 Torsional Moments in Primary Members when supported secondary beam is not released
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).
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.
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.
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;
S.no.
|
Point
Observed
|
Joint
conditions
|
Remarks
|
|
Without
Member Release command
|
With
member release command (100% release in Mz)
|
|||
1.
|
Rigidity at the joint & support condition
|
Fixed Support condition.
STAAD Pro treats the Joint/node as rigid &
thus upon analysis restraints is provided in all three force directions (Fx,
Fy & Fz) & all three moment directions (Mx, My & Mz).
|
Simply Supported condition.
With release command the joint is released in the
specified force or moment & thus no restraint is offered for the same, in
our case Mz release is used.
|
|
2.
|
Hogging Moment in Secondary Beam at Top face near
supports.
|
23.02 KNm
|
0.0 Knm
|
Reinforcement at top will be more in without
member release command.
|
3.
|
Sagging moment at bottom mid face of the
secondary beam.
|
-39.48 knm
|
-62.50 knm
|
Reinforcement at bottom face of the beam will be
higher in member released case due to concentration of moments at bottom
face.
|
4.
|
Torsion In primary beams.
|
11.51 Knm
|
0.0 knm
|
In without member release case the nos. of the
stirrups or transverse reinforcement will be increased in primary beams i.e.
stirrups will be required at closer spacing.
|
5.
|
Bending moment & shear force in primary
beams.
|
Same
|
Same
|
|
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.
Any further queries or suggestion, kindly drop in the comment section below.
Regards
Alok Dixit
Any further queries or suggestion, kindly drop in the comment section below.
Regards
Alok Dixit
