Analysis result of Section Offset

July 4, 2022

Analysis model of section offset in Beams


Generally, the beam should be installed below the floor, so the analysis is performed considering the section offset as shown below.
IIn order to consider the effect of section offset, a rigid body element is created internally as much as the section offset distance.
The analysis results are calculated similarly both cases



Figure 1. Analysis result of section offset in beams





Why is it necessary to consider section offset? 


However, if the section offset is considered when analyzed with a slab with mesh as shown below, you can see a moment diagram like saw-tooth shape.
This is due to the influence of the rigid element generated internally.



Figure 2. Analysis result of section offset with meshed slab


Not consider ‘Section offset’ : Since the slab is centered on the beam, a larger moment is generated in the beam.

Consider ‘Section offset’ : Since the slab is located on the top of the beam, the beam with T-shape is reflected in the analysis, and the moment of the beam is relatively reduced.



Figure 3. Comparison with consideration 'Section offset' -1


Not consider ‘Section offset’ :Since the slab is located at the center of the beam, the moment is distributed according to the stiffness of slab and beam.

Consider ‘Section offset’ : Since the slab is located at the top of the beam, T-shape beam is reflected in the analysis result.An axial force is generated in the slab, and the reinforcement for the axial force can be considered.



Figure 4. Comparison with consideration 'Section offset' -2


Not consider ‘Section offset’ : The moment equilibrium value can be obtained.

Consider ‘Section offset’ :The moment equilibrium value cannot be obtained. This is because it does not show the magnitude of the deflection that occurs on an invisible rigid element. Internally, moment equilibrium is established as shown below.


Figure 5. Comparison with consideration 'Section offset' -3


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Young-il Seo
Young-il Seo | Principal Structural Engineer

Young-il has over 13+ years of experience in building design, especially high-rise buildings with column reduction analysis, plant structures, pushover analysis, health monitoring, and vibration control projects. Since 2016, he is planning and providing technical supports for midas building products such as midas Gen, nGen, and Design+.

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