Panel Zone and Rigid Offsets

Panel Zone and Rigid Offsets •

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Added by Truly Guzman, last edited by Truly G uzman on May 26, 2011

ETABS has the capability to model both, rigid offsets and Panel zones. In order to illustrate the correct use of both options the following simple examples have been created, all frames have the same steel frame sections and a lateral load of 10kips applied at the top floor, the rigid offsets and panel zone have been modified as follows:

Model 1.1.- Default rigid offsets , rigid zone factor 0, No panel zone

Model 2.2.- Default rigid offsets, rigid zone factor 1, No panel zone

X-Deflection= 0.133in This model is the simplest one, and represents the higher bound for deflections (conservative) but the lower bound for moments.

X-Deflection=0.104in This model assume fully rigid offsets, reports the smallest deflection which is generally unconservative and is not recommended. Results for moments are the highest of the four models (conservative)

Model 3.- Default rigid offsets, rigid zone factor 0, Panel zone from column

X-Deflection=0.152in This model assumes no rigid zone and panel zone modeled based on column properties. Deflections are extremely high due to the fact that flexibility is being duplicated at joints. Moment is the smallest of the four models, and is also not recommended due to unconservative results.

Model 4.- Default rigid offsets, rigid zone factor 1, Panel zone from column

X-Deflection=0.128in This model has fully rigid offsets, and panel zone modeled based on column properties. In this case flexibility of joint is correctly modeled. Results for deflections report a value in between the upper and lower bound. Moments as well indicate a good fit in between extreme bounds. Therefore; this is generally the best modeling assumption.

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If the first model would be modeled using the user defined lengths option as 0,0 for r igid offsets instead of default based on connectivity, results will be identical except for the fact that moments will be reported at end of elements (not a column/beam face) A fully rigid zone should never be used in steel frames, usually for concrete frames a value of 0.5 is recommended. Which I turn indicates that 50% of the actual offset will be consider rigid Rigidity will only affect bending in the 3 axis of the frame, not torsion, not axial properties.