Design as a Column

For each strength load case contained in the job for that footing, the pedestal is designed at 2 locations - top and bottom.

Top is defined as the point where the column ends and the pedestal begins.

Bottom is defined as the bottom of the footing.

Thus, the pedestal is treated as a column whose height is equal to the distance from the bottom of the column to the bottom of the footing.

At the top of the pedestal, the forces and moments used in the design are the column reactions which can be viewed in the Loads & Factors table

The forces and moments acting at the bottom of the pedestal are derived from those at the top by (a) adding the selfweight of the pedestal to the FY force at the top (b) adding the moments due to the lateral forces FX and FZ acting at the top of the pedestal. To these moments, additional moments as described next, if applicable, are added.

So, for each strength load case, the aggregate moments along with the axial force for that load case are used to check the suitability of the cross section at both ends (top and bottom) using the interaction method, and, the appropriate amount of reinforcement required for that load case is determined. The highest area of steel from all the strength load cases is deemed the steel required for the pedestal, and the corresponding load case is identified as the critical load case.

Design for Shear

The shear force carrying capacity of the section without shear reinforcement is determined for both directions from Clause 6.2.2 of EC2. If the applied shear force exceeds the capacity, the stirrup requirement is computed.

A design summary that is presented in the calculation sheet is shown below.

The following are some of the terms that appear in the output and their meaning:

For capacity ratios:

Flexural capacity ratio

= Mres / Mcap where

Mres	=	resultant moment = √(Ma2 + Mb2) calculated from final moments Ma and Mb about the two principal axes (including slenderness and other criteria)
Mcap	=	Moment capacity of the section from the P-M curve

Axial capacity ratio

= Pmax / Pu where

Pmax	=	maximum axial force on the column from all load combination
Pu	=	Axial capacity of the column calculated using below formula = [(0.4 × fck × Ac) + (0.8 × Fy - 0.4 × fck) * Ast]
fck	=	concrete grade
fy	=	steel grade
Ac	=	area of concrete
Ast	=	area of steel

For shear design:

VRD,c

Design value of shear resistance as per Clause 6.2.2 of EC2

As Tension

the term, A_sl, described in Clause 6.2.2, BS EN 1992-1-1:2004

CRdc

Intermediate parameter for shear calculation, (See Note at the bottom of Cl. 6.2.2, BS EN 1992-1-1:2004)

deff

effective depth (distance from cg of tensile reinforcement to extreme compression fiber

k

= Min(1+√(200/deff), 2) as per Clause 6.2.2 of EC2

ρ1

= As Tension /(b × deff)

k1

See Note at the bottom of Cl. 6.2.2, BS EN 1992-1-1:2004

σcp

Ned/(B×D) where

Ned	=	Axial force in the section
B and D	=	the smaller and larger dimensions of the cross section respectively

Axial Load and Biaxial Bending Design

λD

slenderness ratio about the side D which is the side whose length is the longer dimension of the cross section

λB

slenderness ratio about the side B which is the side whose length is the shorter dimension of the cross section

eD(top)

eccentricity at top of pedestal along the direction parallel to the "D" side

eD(bottom)

eccentricity at bottom of pedestal along the direction parallel to the "D" side

eB(top)

eccentricity at top of pedestal along the direction parallel to the "B" side

eB(bottom)

eccentricity at bottom of pedestal along the direction parallel to the "B" side

Biaxial bending ratio

= (M_D/M_capD) + (M_B /M_capB) where

MD	=	moment about the axis parallel to the longer side
McapD	=	moment capacity about the axis parallel to the longer side
MB	=	moment about the axis parallel to the shorter side
McapB	=	moment capacity about the axis parallel to the shorter side