The paper presents the issue of unbraced and semi-rigid steel frames stability with special attention paid to the determination problem of columns buckling length Lcr in these frames. The paper discusses ways of buckling length determination in frames columns with the use of well known, European and American standard procedures, as well as numerical method of stability analysis based on the Finite Element Method (FEM). The presented procedures and analysis methods in calculations of certain steel frames with semi-rigid joints were used. On the basis of obtained results, it has been shown that in many practical cases, the simplified standard procedures of columns buckling length determination can give the results burdened with errors. These errors can have a significant influence on accuracy of columns resistance calculations. The issues presented in the paper are very important from the practical point of view, and according to the author, they can be used in the practical design of unbraced steel frames.

Experimental tests of steel unstiffened double side bolted end-plate joints have been presented. The main aim of the conducted tests was to check the behavior of joints in an accidental situation and possibility of creating secondary mechanism, i.e. catenary action in the scenario of column loss. Two types of end plate joints were tested: flush end-plate (FP) and extended end-plate (EP) with different thickness and different number of bolt rows in each. The tests were carried out on an isolated cross beam-column-beam type system until joint failure. During tests the available moment resistance and rotation capacity of bending joints and also values of tension forces in the beam were determined. The joints with extended end-plate have demonstrated higher bending and rotational capacity than flush end-plate. Significant deformation of column flanges, web and end plate were observed. The fracture of bolts was the failure mode of joints. Obtained results of axial force values in beam exceeded standard requirement what confirmed that the joints with unstiffened web column, flush or extended end-plate possess the ability of development the catenary action.

The problem of uniqueness and representativeness of steel frame fire resistance assessment is considered in this paper. The thesis, that the selection of analysis method determines the result in both qualitative and quantitative terms is given scrutiny. It is also shown, that the differences between computed values may be significant. The selection of an appropriate computational model for an analysis of this type seems to be especially important, as the possible overestimation of the fire resistance determined during computation is equivalent to an unjustified optimism of the user with respect to the safety level warranted. In the considerations presented here the critical temperature determined for the whole bearing structure is considered as the measure of sought resistance. The determined temperature is associated with the bearing structure reaching the bearing capacity limit state subject to fire conditions, treated as accidental design situation. Two alternative computational methods have been applied during calculations: the first one – classical, based on 1st order statics and using the buckling length concept for members of the considered frame, and the second one – taking account of 2nd order phenomena via simple amplification of the horizontal loads applied to the frame. Special attention has been paid to the influence exerted on the final fire resistance of the considered structure by the real joint rigidity, decreasing with increasing temperature of the structural members. The obtained results differ not only in the value of determined temperature but also in the indicated location of the weakest frame component, determining its safety.

The article draws attention to certain aspects of calculating the width of cracks and stresses in composite elements under bending, in which the slab is located in the tension zone. If semi-rigid joints are used in the element, in which the beam is attached to the column by bolts, two types of areas should be distinct in which the reinforcement stresses will be calculated in a different way. The method of calculating stresses in reinforcement will depend on the type of a used joint or on the distance of the considered cross-section from the semi-rigid joint. In order to distinguish the method of calculating stresses in the paper, two areas were introduced: specifically area B and area D. Area B will be the area where the principle of flat sections can be applied, and stresses in the reinforcement are determined using the classical theory by adding the component responsible for the tension stiffening phenomenon. Area D is the area in the vicinity of the semi-rigid joint, where the principle of flat sections cannot be applied. To calculate stresses, consider the balance of joints using the available models of the semi-rigid joint, in particular the spring model. The paper presents the formulas for calculating stresses in the D area for two types semi-rigid joints: joint with a flush end-plate with 2 rows of bolts are used and joint with an extended end-plate with 3 rows of bolts are used.