Joints - Construction Aid for Planners

Each component and each structure ist more or less on the move during their production, as well as during their action period.

These movements will be caused through:

• deformations (temperature, evanescence, creeping)
• uneven settlements
• uneven pollution

In case this movements will be hindered, strains arises which lead to a split in the construction if it exceeds a special limit.

The arising splits are typically not straight-lined and because of this, it is very difficult to control them.

Splits can lead to damages of the reinforcement in extensive concrete components if the split wide exceeds 0,3 – 0,4 mm, also by sufficient concrete cover.

It will become more critical if wide splits occur ar the effect of, e.g. dew salt.

Because of this, straight-lined splits – the so called weakening – are given to avoid rough crack initiations.

Joint = the scheduled straight-lined split

We distinguish between

• dummy joint or
• expansion joint

Joints are always a weak point of a construction.

You have to aware of something:

Without joints, the construction would be even weaker in many cases.

The function of a joint is

• to avoid rough splits /remedy through dummy and movement joints)
• to equalize the variation of length (movement joint)
• to facilitate working steps (construction joint)

Joints should leave enough clearance for unrestrained movementas for each construction stage.

The movement joint severed the whole component. The joint width – b – is depended on the expected variation of length (see calculation example in the supplement, as well as referance values of the DIN 18540).

Rule of thumb (for exterior joints) b[mm] = 3xL[m] + 10

b = joint width [mm], L = length of the component [m]

The dummy joint is only a notch of 0,25 – 0,30 x density. She represents a weakening of cross-section, on which the load bases the biggest tension. The cross-section will be tear apart on the weakened part if it exceeds the maximun of the contained concrete tension.

The upper notch keeps straight-lined and because of this it is easy to fill in/controllable.

Dummy joints can transmit compressive forces.

Due to the complex connection, the are no rigrid, irrevocable information about the maximun joint distances but only referance values.

Entirely for buildings with an increased fire risk, the DIN 18540, Abs. 14.4.2 refers to a ceiling of the joint distances, which should ideally below 30 m. The width of the joints should be al least to 1/1.200.

In the general construction building, the DIN 18540 indicates the reference values for the joint width. The joint width considers already the practical elasticity of the jointing compound of 25 %.