A standard H-shaped steel column presents a relatively small area to the substrate on which it stands. A base plate is used to spread the loading forces. For all but the most massive columns, a prefabricated column base plate assembly is hoisted into position and shims are used to level it; then grout is injected underneath. The grout forms a uniform contact between the entire underside of the base plate and the substrate to adjust for any variation in the substrate's slope or level.
Manufacturing Column Base Plate Combinations
Except in the case of extremely large columns that necessitate equally large -- and therefore heavy -- base plates, base plates are welded to columns by the manufacturer. This ensures that the cut at the bottom of the column is absolutely square and the base plate meets that cut with pinpoint accuracy. This, in turn, ensures that the base plates are absolutely perpendicular so that uniform spreading of the column load onto the substrate is possible.
Siting Separate Components
When erecting extremely large columns, the base plate is typically sited and grouted before the columns are attached. Leveling bolts are driven through the perimeter of three of the base plate’s four sides; then grout is used beneath the base plate to lock it in place before the column is welded on. Vent holes must be drilled through the base plate to allow any air trapped below the plate to escape as the grout is injected; the vent holes are typically the same diameter as those made for the leveling bolts to minimize the amount of drill bits required on-site.
Grouting prefabricated column base plate assemblies takes place as soon as practicable after the steel components have been aligned and plumbed. Individual columns can only be grouted, after setting the adjacent column in place, to allow for the uninhibited installation of the beams, but they must be grouted before additional construction loads are placed on the columns, for instance, a second tier or a poured concrete floor. For larger applications, where base plates are installed before the columns are welded in place, the grouting must be timed so that it is set and cured before the columns are welded in place. According to code, the base plates or column base plate combinations must be inspected by the appropriate agency before grouting takes place.
The grout used for setting base plates is manufactured by numerous chemical mortar companies across the globe, most of which service the iron-working industry. All grout types have slightly different qualities that suit various applications; for this reason it is not possible to give a definitive statement with regard to what type of grout should be used. That said, a common set of qualities is desirable and can be enumerated. The compressive forces exerted by the load, through the column to the base plate, must remain evenly distributed, so the grout must be non-shrinking and non-expanding. It must also be impervious to water, abrasion and the ingress of any contaminants during the course of construction and throughout the life of the building. The grout should be non-metallic to reduce any risk of galvanic reaction between itself and the underside of the base plate and with the leveling bolts and shims. It should set up as quickly as the installation process allows and should cure to most of its compressive strength within a few hours.
- Modern Steel: Base Plate Grouting
- Modern Steel: Grouting Base Plates
- Beard Design: Practical Design and Detailing of Steel Column Base Plates
- "The Construction of Buildings: Column Bases and Foundations"; Robin Barry; 2001
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