0The Column Base-Plate is welded tp the base of the column and in turn, fastened to the foundation by means of specially designed Anchor Bolts.
They have to deal with a variety of load conditions including, Compression, Shear and Rotational Moments. All column bases will have to deal with a combination of these loads to a greater or lesser extent depending on the height, width, and intended use of the structure.
We'll take a look at each of these load cases:
Columns subject to Axial loads only are usually designed to have nominally-pinned bases, and in such cases the shear component is very low and largely ignored. Common practice is to use simple, unstiffened base-plates welded to the base of the column by means of fillet-welds. The end of the column must be cut square and flat (usually saw-cut) in order to make full contact with the surface of the base-plate so that the column can effectively transmit the compressive load.
The distribution of pressure between the base-plate and the concrete foundation (through the grout) is assumed to be uniform
Portal-Frames and ‘Braced-Bays’ are particularly subject to shear forces, but if the forces are modest (up to 10% of the axial load) they will most likely be transmitted by friction and can be effectively ignored. Above that figure, it may be necessary to make some provision. The most common solution is to use the properties of the Holding-Down bolts to resist the shear, but it must be specified that the grout must effectively surround, and be properly compacted around the bolt in the grout-pocket to prevent lateral ‘give’
For more substantial shear forces, it may be necessary to employ a ‘Shear-Key’ under the base-plate. The shear-key may be either an angle or channel section welded to the underside of the plate, alternatively it may be a round or square bar.
In such cases it will be necessary to provide ‘pockets’ in the concrete base to accommodate the key, thus complicating its construction. Like the HD bolts, the shear-key must be effectively bonded to the concrete with the grout.
Most column bases will be required to transmit end-moments as well as the axial loads and shear. When a moment is applied to a column base-plate, one side of the plate (in the direction of rotation) will be in compression, which is transmitted by the contact surfaces between the base-plate and concrete on the compression side. The other side will be in tension which will be transmitted by the holding-down bolts, which must be securely anchored to the sub-structure. To counter these affects:
- The holding-down bolts must be able to counter the ‘uplift’ by being of sufficient length and diameter to provide sufficient bond between the concrete (and grout) and the bolt, together with the anchor-plate (See Holding-Down Bolts)
- The concrete must be of sufficient strength and quality to resist the compressive forces acting upon it without crushing. Likewise, the packing grout must be similarly resistant.
- The base-plate must be of sufficient thickness to resist bending, alternatively, the base-plate will need to be stiffened. (See Fig’s 3 and 4)
Base Plate Stiffening
Base Plate Thickness and Bolt-Hole Dimensions
|Unstiffened Base Plates|
The above table and recommended dimensions are intended for guidance only - the base-plate design must be approved by the responsible engineer.
However, the following rules-of-thumb have proved pretty reliable and are useful for estimating purposes.
- The Base Plate thickness should not be less than the column flange thickness.
- The Outstand -A-(Distance from the edges of the column to the outside of the base plate) should be between 100 and 150 mm depending on the bolt diameter. The base plate size should then be rounded-up to the nearest round figure (usually in increments of 50 mm). This will generally be found to work for base plates with moderate moment resistance (See Fig. 1)
For columns not required to resist moments, such as internal posts or columns which are restrained in moth the X and Y axes the bolts may be placed inside the column section disregarding the Outstand. In such cases, the Anchor Bolt spacing -B-should be between 120 and 180 depending on the bolt diameter (See Fig. 2)
- The Edge-Distance -e- should not be less than 2 x the bolt diameter (Refer to Table above)
- The bolt-hole diameter should be 5-6 mm greater than the bolt diameter