In concrete , timber and steel construction, the upper and lower straps of a girder with an "I-shaped" ("double T-shaped") cross-section are called straps and are held at a distance by a continuous web. In the case of the truss , the web is replaced by obliquely arranged bars .
Beams with an I or double T cross-section consist of two flat or squat ( compact ) straps and a comparatively thin web that connects the two straps. Analogous to the double-T girders, the sides of box girder girders (in bridge construction) can also be called chords, which curve in a concave or convex manner when they are bent.
Depending on the position of a belt, a distinction is made between the upper and lower belt . Depending on the load, we speak of a tension or compression belt .
Structural behavior and dimensioning
Above all, the belts take on the load from bending . When a girder is loaded onto two supports from above, the lower chord is stretched by tensile forces and is therefore called the tension chord. Similarly, the top chord is compressed by compressive forces and in this case is a compression chord.
For the dimensioning of a profile, the tensile and compressive forces in the chords are calculated with the help of the internal variables bending moment , shear force and normal force over the distance of the chords from the center of gravity of the profile. If normal force is present, the belts also absorb this force proportionately.
The tensile or compressive stress at the outermost edge of the profile (the so-called “edge fiber”) is decisive for the load-bearing capacity of a beam subjected to bending. The larger the cross-sectional area of the profile at this point, the lower the tension, as the forces that occur are then distributed over a larger area.
The cross-section of the web plays a comparatively minor role for the level of material stresses during deflection. The web can therefore be made very thin to save weight. He must be just dimensioned so strong that he was not under stress to buckle or bulge .
The cross-sectional area in relation to their distance from the profile center of gravity is the section modulus expressed. The further apart the straps are, the more stable a carrier is. However, if the thickness of the belts and web is too thin in relation to the height and width of the profile, there is an increased risk of the beam being deformed by torsional torsional buckling and failure.
- Ulrich Smoltczyk: Grundbau-Taschenbuch. Part 3, 6th edition, Wilhelm Ernst & Sohn, Berlin 2001, ISBN 3-433-01447-7 .
- Gottfried CO Lohmeyer: Reinforced concrete construction. Dimensioning - Construction - Execution, 5th edition, BG Teubner Verlag, Wiesbaden 1994, ISBN 978-3-663-01540-6 .
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