GGU-UNDERPIN: Simplified analysis concept
DIN 4093:2012:08 stipulates that, in approximation, separate analyses may be performed during design for the acceptable compressive and shear stresses. Under design loads it must be demonstrated that the design values of the normal stresses σE,d do not exceed 0.7 · fm,d and the design values of the shear stresses τE,d do not exceed 0.2 · fm,d.
The normal stresses σE,d are determined assuming a linear distribution.
σE,d = ME,d/W + NE,d/A ≤ 0.7 · fm,d (no foundation gap)
ME,d = design value of the moment
W = section modulus
NE,d = design value of normal force
A = cross-sectional area
σE,d = 4 · NE,d/(3 · b – 6 · e) ≤ 0.7 · fm,d (foundation gap)
NE,d = design value of normal force
b = width of cross-section
e = eccentricity
If analysis results in a foundation gap, the area of the gap may not be adopted in an analysis of the shear stresses.
The design value of the shear stress τE,d is given by:
τE,d = 1.5 · QE,d/Av ≤ 0.2 · fm,d
QE,d = design value of the shear force
Av = cross-sectional area under shear loading
The characteristic value of the compressive strength of the underpinning body fm,k is determined from load tests. The corresponding design value of the compressive strength fm,d is given by:
fm,d = α · fm,k/γm
α = coefficient for considering a long-term effect = 0.85
γm = partial safety factor for the compressive strength
(= 1.5 for DS-P and DS-T
= 1.3 for DS-A)