Frame member details, comparison with ETABS

Results.

Problem. End zones, pins and semi-rigid connections are solved separately with MindFEM and with ETABS in two buildings where they are used in practice. The rigid end zones of beams and columns and the secondary beams pinned to the main beams are compared in a concrete frame, and the rotational spring of an end-plate connection in a steel frame. Vertical and lateral load cases, load combinations and modes are given together.

Reference document. The reference is the same buildings modelled in ETABS 20.1.0 with its own objects. The end zones are given as ETABS end offsets with a rigid-zone factor, the pins as end releases and the semi-rigid connections as partial-fixity springs.

Numerical model. The concrete frame has 3 × 6 m bays in X and 2 × 5 m bays in Y and three storeys of 3.2 m; the columns are 50/50, the main beams 30/60, the secondary beams 25/50 and the concrete C30/37; the end zones are 0.25 m at beam ends and 0.30 m at column ends, and the loads are G = self weight + 6.25 kPa, Q = 2 kPa and a lateral load growing with the storey level (EX). The steel frame has 2 × 6 m bays in both directions and two storeys of 3.5 m; the columns are HEA 300, the X beams IPE 400 with a 40 000 kNm/rad rotational spring at both ends, the Y beams IPE 300 and pinned, the Y direction braced with tubes, and the loads are G = self weight + 4 kPa and EX.

Steel frame, semi-rigid connections#

Independent calculation. ETABS 20.1.0, the same building modelled with ETABS's own objects

Note. In the semi-rigid connection the beam end moment is the moment carried by the rotational spring. The mass is the members' own mass. ETABS lumps it at the two ends of each member, MindFEM distributes it along the member. When the MindFEM stiffness is solved with the ETABS mass rule, the periods agree to four digits.

Steel frame, rigid connections#

Independent calculation. ETABS 20.1.0, the same building modelled with ETABS's own objects

Note. In the semi-rigid connection the beam end moment is the moment carried by the rotational spring. The mass is the members' own mass. ETABS lumps it at the two ends of each member, MindFEM distributes it along the member. When the MindFEM stiffness is solved with the ETABS mass rule, the periods agree to four digits.

Concrete frame, rigid-zone factor 0.5#

Independent calculation. ETABS 20.1.0, the same building modelled with ETABS's own objects

Note. ETABS gives the beam-end internal force at the column face, and the MindFEM value is read at the same point. The weight of a beam's end zone is not counted in either program, the column's is. ETABS treats the end zone as rigid in bending and shear only and computes axial and torsional deformation over the full length. In MindFEM the end zone is rigid in every deformation. The difference in the interior column axial force and in the Y beam moment comes from this difference in the torsional stiffness of the edge beam. When the axial and torsional stiffness of the members with end zones is taken over the full length in the MindFEM model, every static row agrees with ETABS to five digits. The mass is the members' own mass. ETABS lumps it at the two ends of each member, MindFEM distributes it along the member. When the MindFEM stiffness is solved with the ETABS mass rule, the periods agree to four digits.

Concrete frame, rigid-zone factor 1.0#

Independent calculation. ETABS 20.1.0, the same building modelled with ETABS's own objects

Note. ETABS gives the beam-end internal force at the column face, and the MindFEM value is read at the same point. The weight of a beam's end zone is not counted in either program, the column's is. ETABS treats the end zone as rigid in bending and shear only and computes axial and torsional deformation over the full length. In MindFEM the end zone is rigid in every deformation. The difference in the interior column axial force and in the Y beam moment comes from this difference in the torsional stiffness of the edge beam. When the axial and torsional stiffness of the members with end zones is taken over the full length in the MindFEM model, every static row agrees with ETABS to five digits. The mass is the members' own mass. ETABS lumps it at the two ends of each member, MindFEM distributes it along the member. When the MindFEM stiffness is solved with the ETABS mass rule, the periods agree to four digits.

Model files#

The ETABS models that produced the results in the tables can be downloaded. They open in ETABS 20 and later with File > Import > ETABS .e2k File.

• Concrete frame, rigid-zone factor 0.5 (.e2k)
• Concrete frame, rigid-zone factor 1.0 (.e2k)
• Steel frame, semi-rigid connections (.e2k)
• Steel frame, rigid connections (.e2k)