# Constraint

The expansion joint of a bridge is to prevent the coercion in the longitudinal direction indicated by temperature expansion could arise.

Restraint or restraint is the tensioning of components in a structure . The term is used to denote forces that act on a component from the outside. To restrict its free movement (approximately opposite spreading and expansion movements or cuts the establishment ) and cause residual stresses (compressive, tensile or shear stresses ) in the component. It can be both external constraints (see also: Section reaction ) and internal constraints (e.g. tensile stresses on the surfaces and compressive stresses in the core). When used consciously, compulsions are often calledCalled restraint .

In a broader sense, the stresses that arise without the action of external forces (through clamping), but rather through deformations and elongations within a component, can also be referred to as constraint . Constraints in this sense can occur on a nanoscopic , microscopic or macroscopic scale.

Constraints are critical when they assume an order of magnitude that leads to undesired deformation of components or, in the case of brittle building materials, to cracking and breakage . To avoid damage caused by coercion are regularly in construction movement and expansion joints and movement possibilities for structures provided. The right degree of freedom of movement is given when a system is statically determined .

## Examples

If you make the floor plan of a bridge curved, you can reduce more than 90% of the constrained stresses with supports held firmly on both sides

Constraint often occurs when changes in temperature create stresses from obstruction to thermal expansion . This is taken into account u. a. by

Moisture absorption and release causes in certain building materials , a swelling and shrinkage , leading to deformations of the components produced therefrom. This is taken into account, especially with wooden components such as windows and doors, by providing sufficiently wide joints in the rebate between the sash and the frame .

In statically indeterminate systems , the yielding of a bearing , like thermal expansion, generally leads to constrained stresses. Static systems are often preferred for problematic substrates or large temperature fluctuations. (The advantage of statically indeterminate systems is that if individual components fail, in some cases the load can be redistributed to neighboring components to prevent failure of the overall structure.)

When combining different materials with different thermal expansion ( sandwich construction ), internal residual stresses ( thermal stresses ) occur when the temperature changes . When reinforced concrete few Zwängsspannungen occur because the thermal expansion of concrete and steel is not all that different.

Concrete shrinks when it sets , which can lead to constraints. The resulting constrained stresses must be taken into account, especially in the case of waterproof concrete in white tubs and reservoirs , in order to estimate the width of the crack and guarantee water impermeability.

In mechanical engineering, one makes use of constraint by joining two parts with an interference fit . In timber and formwork construction , components are often fixed by being wedged .

In the case of crack width limitation in concrete construction (proof of usability according to Eurocode ), restraint is often the decisive dimensioning criterion in bridge construction.

## Individual evidence

1. Michael Pötzl, Jürgen Maisel: Design parameters for seamless concrete bridges with a curved floor plan . In: Concrete and reinforced concrete construction . tape 100 , no. 12 , December 1, 2005, ISSN  1437-1006 , p. 985-990 , doi : 10.1002 / best.200590343 ( wiley.com [accessed December 5, 2019]).
2. A. Klinkenberg, B. Jäger, H. Saal: Investigations into the statically optimal holder position for point-supported glass panels . In: Steel construction . tape 67 , no. 4 , April 1, 1998, ISSN  1437-1049 , pp. 275–280 , doi : 10.1002 / stab.199800940 ( wiley.com [accessed December 5, 2019]).
3. a b A. Klinkenberg, B. Jäger, H. Saal: Investigations into the statically optimal holder position for point-supported glass panels . In: Steel construction . tape 67 , no. 4 , April 1, 1998, ISSN  1437-1049 , pp. 275–280 , doi : 10.1002 / stab.199800940 ( wiley.com [accessed December 5, 2019]).
4. ^ Institute for Structural Analysis and Design ETH Zurich: 9th Research Colloquium of the German Committee for Reinforced Concrete (DAfSt): Summary of the research reports . Springer-Verlag, 2013, ISBN 978-3-0348-5291-3 , pp. 26 ( google.at [accessed December 5, 2019]).