Fixed Baseplate Design
The scope of this calculation module is to design fixed base plate for uniaxial moments and axial loads transferred from steel column to foundation concrete with or without stiffener plates. Axial load can either be tensile or compressive in nature. The tensile force is dispersed to the foundation concrete with the help of holding down bolts and anchor plates.
The resultant moment is computed form the major and minor axis moment as follows,
The compression out-stand of the section, c = √ ((pyb * tpl2) / (3 * 0.6 * fcu)) + 0.8 * Sf
Effective length, Le = D + (Sb - D) / 2 + min(c, L1)
Effective breadth, Be = B + (Sg - B) / 2 + min(c, B1)
Resultant moment, M = Ma + Mi * (Le / Be)
The compression block is considered to be rectangular in shape if it lies in the area between the column flange and when a combination of normal force and bending moment acts then the compression block extends into the flanges, hence the shape of compression block changes to ‘T’ shape for I section and ‘U’ shape for rectangular or square hollow section. Similarly, when only bending moment acts then the compression block is present in a rectangular shape near the bottom flange of the I section
Reaction forces – compressive force (cf) are considered to be acting at the centroid of the compression block and tension force (tf) are considered to be acting at the holding down bolts and are computed as follows,
Compression force, Cf = 0.6 * fcu * B * X
Tension force, Tf = Cf - N
fcu = Concrete cube strength
B = Width of base plate
X = Depth of compression block
N = Axial load
- An option is given for the user to choose the steel sections of their choice such as,
- I section
- Universal beam
- Universal column
- Rectangular hollow section
- Square hollow section
- Welded section (user defined)
- Bolt anchorage.
- Bolt with Plates
- The application calculates the anchor capacity based on the bolt and anchor plate properties defined where, anchor plate dimensions can also be edited by the user.
- Other Bolts
- The application allows the user to directly input the anchorage capacity of bolts.
- Stiffener plates may be used in the calculation if required by the user.
- To know the critical case and to design the base plate for the best suited requirement load combinations are given for the user to edit the loads as per their design/site requirements.
- The column is assumed to be concentric to the base plate.
- All welds are fillet welds and the weld strength relate to the minimum strength of the connected parts and the class of weld used.
- The bolts are doubly symmetrical and are spaced along the width of the base plate.
- The bolts cannot lie beyond the line drawn at 45° outwards from the edge of the column flange for no stiffener case.
- The minimum size of welds shall not be less than 3 mm and not be greater than the thickness of the flange in case of I sections and section thickness in case of rectangular hollow or square hollow section.
- The calculation involves determining the depth of compression block for the applied load combination. Static equilibrium condition is used to determine the compression block depth.
- In stiffened base plate, the stiffeners are checked for moment and shear capacities. Additionally, weld forces acting on 'stiffener - column' weld and 'stiffener - base plate' weld are calculated in addition with the member - base plate weld.
- In unstiffened base plate, weld forces are determined for 'web - plate' and 'flange - plate' and are checked against the strength of fillet weld.
- Holding down bolts are designed either by punching shear approach or by determining the pull- out capacity of cone of the foundation concrete.
- The punching shear approach used assumes a reinforced base, whereas the cone pull-out approach is appropriate for unreinforced base.
- The overall moment capacity and shear capacity is also calculated for the base plate and for shear capacity, the shear strength of bolts is also taken in account along with frictional shear resistance.
National Standards Available
- BS 5950-1:2000 Code of practise for design - Rolled and Welded connections.
- BCSA publication “Joints in Steel construction - Simple Connections”.
- BCSA publication “Joints in Steel construction - Moment Connections”.
- Ver 1.0 - Original version