Flexible Support Theory Handbook

Last modified by Fredrik Lagerström on 2021/02/11 18:01

The Theory handbook collects all the information used to be able to use and understand the underlying calculations with regards to Flexible Support.

flex 3d.jpg

Figure 1. Excessive schematic representation of a floor slab supported on flexible beams. (F. Lagerström, 2016)

Calculation of stresses in hollowcore cross sections

The aim with calculating these stresses is to avoid any cracking within the hollowcore cross sections. There are several different approaches to this, explained in FIB Bulletin 6 and the Finnish Code Card 18. There is also another approach described by T. Roggendorf in his PhD Thesis Zum Tragverhalten von Spannbeton-Fertigdecken bei biegeweicher Lagerung, Aachen Technische Hochschule (2010).

ApproachSupported by StruSoft Flexible Support
FIB Bulletin 6yes.png
Code Card 18no.png
Roggendorf, 2010no.png

Calculation principles

The calculation uses stiffnesses of the beam, hollowcore, joint concrete, and screed to calculate the deflection of the beam, causing the stresses within the concrete cross section of the hollowcore.


Figure 2. Picture of the shear flow and shear force of the hollowcore cross section (FIB Bulletin 6)

Properties of different regions


Area Beam.png



Area Hollowcore.png


Core fillings

Area Core fillings.png

Core fillings are ignored in the calculation of the stiffness of the composite beam.

They are however considered with the βf-factor.

Slab thickness [mm]200265320400
Filling length < 50 mm1.
Filling length at least equal to the depth of the void (hc). All voids filled0.

Table 3.2 βf-factor (FIB Bulletin 6)


Figure x. Distance hc.

Joint concrete

Area Joint concrete.png


Topping on beam

Beam web is lower than hollowcoreArea topping over Beam.png 
Beam web is higher than hollowcore, but not higher than hollowcore and topping1613032475294-805.png 
Beam web is higher than hollowcore and topping1613032557329-497.png 

Table x. Properties of different cases of topping


Figure x. Definition of bweb, hweb, bjoint, hhc, htopping and bsupport.

Topping on hollowcore

Area topping on HC.png