Torsional stiffness of beams

The structural frame
The construction
The reinforcement I
The reinforcement II
Quantity/Cost estimation
Detailing drawings
Introduction >

Wind and Seismic Forces >
Structural model and Analysis
Slabs
Seismic behavour of frames
Appendix A
Appendix B
Appendix C
Appendix D
Introduction >
Modelling slabs

Materials
To be continued >
Introduction

Torsional moments of beams of torsional stiffness 1% Triangular finite element model

The effect of torsional stiffness of beams was studied parametrically as a percentage οf full elastic torsional stiffness of beams (100%, 10%, 1%).

Ιn the case of full elastic torsional stiffness, both side beams are stressed by high torsion, caused by the moments on the supports of the main joists sitting on those beams.

Torsional moments of beams of torsional stiffness 10% Member model

For 10% the torsion of the two beams is of moderate intensity.

Torsional moments of beams of torsional stiffness 1% Member model

For 1% both beams are practically not stressed by torsion.

Torsional moments of beams of torsional stiffness 100% Triangular finite element model

Similar results obtained from both models (members, two-dimensional finite elements).

Torsional moments of beams of torsional stiffness 10% Triangular finite element model

For 10%, the torsion of the two beams is of moderate intensity.

Torsional moments of beams of torsional stiffness 1% Triangular finite element model

For 1% the torsion of the two beams is lower but not zero.

Text/HTML

The regulations refer to members, in both analysis and (mainly) reinforcement design. No matter how ‘accurate’ static and dynamic analysis methods may be, or models of two-dimensional or three-dimensional finite elements may be used, the results (moments-shear forces) should refer always to the appropriate members, allowing the design of slabs subjected to bending. A slab strip can also be considered as an appropriate member. In the centroidal axis of this strip, moments and shear forces are being determined by integrating the appropriate stresses along the respective finite elements. The assumption of a structure with beams of adequate depth, providing significant stiffness to the structural frames, results in a pure space frame model consisting of members which is reliable for both analysis and design.

In conclusion, the assumption of the members for modelling structural frames and the approach of independent functionality of slabs are both acceptable and, under certain circumstances, form an adequately reliable solution. In finite element analysis, the common assumptions adopted for slab supports are: zero torsional stiffness, pinned supports on beams and columns, and monolithic supports on walls. Regardless of the calculation method, the minimum structural provisions of the regulations regarding the minimum (secondary, negative) reinforcement of slabs should be followed diligently.