Construcții din profile ușoare curs 2-6

EFFECTS OF COLD-FORMING

Strain-hardening

cold-rolling vs. press braking or folding

Cold-rolling provides an increase of strength that is important in the corners and still appreciable in the flanges.

Press braking or folding provides an increase of strength that is appreciable only in the corners.

Necking

Forming corners of small radius can have the effect of producing “thinning” of the corners and this can have an effect on the section properties, but this effect is generally small since the corners usually are just a small proportion of the overall cross-sectional area.

If it is desired to take corner-thinning effects, this can be achieved approximately by the following expression:

where:

tr is the reduced thickness

t is the material thickness

r is the inside radius of the corner

k is a coefficient function of the r/t ratio:

k=0.3 for r/t=1

k=0.35 for r/t=1.5

CROSS-SECTIONAL PROPERTIES

CF members generally have a nominally constant thickness and consist of a series of plane elements connected by rounded corners.

If the internal radius of the corners is small, its influence on the cross-section properties may be neglected and the cross-section may be assumed to consist of plane elements with sharp corners.

Eurocode 3 (EN1993-1-3) allow this simplified approach provided that proper limits of the radius are respected.

Each plane element of the cross-section may be considered as a line element, concentrated along the midline of the actual element.

The accuracy of this method for computing the properties of a given section depends on the thickness of metallic sheets and the shape of the section.

For the thickness of the metallic sheets generally used in CF metallic constructions the error in the evaluation of the cross-sectional area and the other properties is usually negligible.

BASIC MODES OF BUCKLING

CLASSIFICATION

The basic modes of buckling of a generic member are:

Local buckling mode

Global buckling mode

Distortional buckling mode

GEOMETRICAL DEFINITIONS

Referring to a CF lipped C-section the following elements can be defined:

plate elements

fold lines: intersection lines of adjacent plate elements