Column anchorage, anchor rods and base plates

A steel column standing alone on its foundation, before any beams are attached, is held up by four things: its base plate, the anchor rods cast into the concrete, the nuts on those rods, and whatever is under the base plate carrying its weight. If any of those is wrong, the column can lean, twist or fall. A falling column can kill the connector at the top and anyone it lands on, and it can pull the beams already attached to it down with it. That is why the first steel work at the Prairie Gate Community Centre was not lifting at all. It was Bill Okafor and Rosa Delgado walking every column footing with a tape, a level and the anchor rod layout drawing.

This lesson covers how columns are anchored: anchor rods, base plates, leveling methods, grouting, checking the rods before the column arrives, repairs, and deciding whether a standing column needs guying or bracing.

Why four anchor rods

Older buildings sometimes had columns anchored with only two rods. A two-rod column can rotate about the line between the rods with very little resistance; a connector's weight at the top, a bump from a beam or a gust of wind can tip it. Four rods arranged in a rectangle give the column resistance to tipping in every direction.

CSA S16 and the engineer of record's design set the number, size and arrangement of anchor rods for each column. Tallgrass, like most Canadian erectors, requires at least four anchor rods on every column that will stand on its own during erection, with the base plate, the column-to-base-plate weld and the foundation designed to resist an eccentric load at the top of the column during erection. Tallgrass uses the same minimum as OSHA: an eccentric gravity load of 136 kg (300 lb) applied 460 mm (18 in) from the extreme outer face of the column in each direction, which roughly represents a connector and tools hanging off one side.

# Tipping check for a lone column under the erection eccentric load (illustrative).
# The 300 lb load 18 in from the column face makes a moment about the far rods.
column_depth_in = 12.0          # W12 column
ecc_in = column_depth_in / 2 + 18.0   # from column centreline to the load
load_lb = 300.0
moment_lb_in = load_lb * ecc_in
rod_spacing_in = 8.0            # distance between the two rows of anchor rods
uplift_per_row = moment_lb_in / rod_spacing_in
print(f"moment at the base: {moment_lb_in:,.0f} lb-in ({moment_lb_in / 12:,.0f} lb-ft)")
print(f"tension on the near row of rods: {uplift_per_row:,.0f} lb ({uplift_per_row / 2:,.0f} lb per rod)")

moment at the base: 7,200 lb-in (600 lb-ft)
tension on the near row of rods: 900 lb (450 lb per rod)

The forces are not large for a properly anchored column. The point is that two rods on a line cannot resist them at all in one direction, while four rods resist them easily in every direction, provided the rods are where they should be, are cast to the right depth and have nuts tightened on them.

Getting the rods in the right place

Anchor rods are cast into the concrete by the foundation contractor, usually using a plywood or steel template with holes drilled to match the base plate. The template holds the rods at the right spacing, projection and plumb while the concrete is placed. Rods that move during the pour end up out of position, bent, too short or too long.

Before any column arrives, the prime contractor surveys the rods. At the Prairie Gate Community Centre, the surveyor checked every group of rods for position against the grid lines, spacing within the group, projection above the concrete, plumbness and thread condition. Rods with damaged threads were cleaned and chased with a die. Rods with too little projection, which would not allow a full nut and washer, or rods out of tolerance, were reported to Helen Kowal, the engineer of record.

Repairs and modifications

If an anchor rod is in the wrong place, too short, bent or broken, nobody on site may cut, bend, drill, weld or replace it without the engineer of record's written approval. Field fixes such as heating and bending rods, welding extensions, or drilling and epoxying new rods change how the rod carries load and may make it brittle or weak. At the Prairie Gate Community Centre, the two misplaced rods at grid line C4 were dealt with by a base plate with slotted holes and heavy plate washers, designed by Helen Kowal. Tallgrass listed the repair in its written notice to Northern Iron Erectors Ltd. before the column was delivered, so that Bill's crew knew to expect the modified plate.

Leveling the column

The base plate must sit at the right elevation and be level so that the column is plumb and the beams arrive at the right height. There are three common ways to support a base plate before grouting.

• Leveling nuts. A nut and washer on each rod below the base plate are set to the right elevation, the column is set on them, and the top nuts are tightened. Leveling nuts are quick and adjustable, but the rods then carry the whole column load in compression and bending until the grout is placed, so the erection plan limits how much load the column may carry on leveling nuts alone.
• Shim packs. Steel shims under the base plate carry the load directly to the concrete. They must be large enough and placed so that the column does not rock.
• Leveling plates. A thin steel plate is set and grouted level on the foundation before the column arrives, and the column base plate bears on it.

The columns at the Prairie Gate Community Centre were set on leveling nuts, with the top nuts tightened and the base plates grouted within the time and number of floors set by Sanjay Mehta's erection procedure. Whatever method is used, it must be able to carry the construction loads until the grout is placed and has reached strength.

Washers and oversized holes

Base plate holes are often made larger than the rods to allow for small placement errors. A standard washer may then be too small to bear on the plate around the hole, so heavy plate washers, often welded to the base plate after the column is plumbed, are used. The erection drawings show the washer size. Without the right washer, a nut can pull into an oversized hole under load and the anchorage loses its grip.

Grouting

Non-shrink grout fills the space between the base plate and the concrete, so that the column load is carried evenly into the foundation rather than through the leveling nuts or shims alone. Grout is placed after the column, or the group of columns, has been plumbed and the first level of beams has been connected and adjusted. In cold weather, as at the start of the Prairie Gate Community Centre, grout must be protected from freezing until it gains strength, following the grout manufacturer's instructions. The erection plan said how many levels could be erected before grouting, and Rosa Delgado scheduled the grouting crew accordingly.

Does a standing column need guying or bracing?

Even with four rods, a tall column standing alone can be pushed over by wind or by an impact before beams tie it to its neighbours. The erection plan must say, for each column, whether it can stand alone, how long and in what wind, or whether it needs temporary guy cables or bracing. At the Prairie Gate Community Centre, a competent person, Bill Okafor, assessed each column against Sanjay Mehta's erection procedure, and the 12 m (40 ft) columns at the gymnasium were guyed in two directions until the roof girders were connected.

# Rough wind check for a lone gymnasium column (illustrative values, SI units).
import math
height_m = 12.0
width_m = 0.31           # flange width of a W310 column
drag = 2.0               # shape factor for an I-shape, illustrative
rho = 1.2                # air density, kg/m^3
for wind_kmh in (30, 50, 70):
    v = wind_kmh / 3.6
    q = 0.5 * rho * v ** 2               # dynamic pressure, Pa
    force = q * drag * width_m * height_m
    moment = force * height_m / 2
    print(f"{wind_kmh:3d} km/h: force {force:5.0f} N, base moment {moment / 1000:5.1f} kN·m")

 30 km/h: force   310 N, base moment   1.9 kN·m
 50 km/h: force   861 N, base moment   5.2 kN·m
 70 km/h: force  1688 N, base moment  10.1 kN·m

The moments rise with the square of the wind speed. A column that is fine in a light breeze can put several times as much load on its anchor rods in a strong gust. Sanjay compared these moments with the capacity of the anchorage on leveling nuts and set a wind speed above which lone columns had to be guyed or not left standing overnight.

Before the column is released

The connectors may not let go of a column, and the crane may not release it, until the column is on its rods, the top nuts are on and drawn up, and any guying the plan requires is attached. At the Prairie Gate Community Centre, Bill also checked that the column was roughly plumb and that its piece mark matched the location on the drawing, because a column set in the wrong place or turned the wrong way may not take its beams.

Records for every column

For each column at the Prairie Gate Community Centre, the file held the survey of its anchor rods, the engineer's approval of any repair, the date the column was set, the competent person's decision on guying, and the date it was grouted. If a question came up later, such as a crack in the grout or a column out of plumb, the records showed exactly what had been done and by whom.