Masonry walls: limited access zones and bracing

A finished concrete block wall is strong and stable, held in place by the floors and roof it connects to and by mortar that has had weeks to cure. A wall under construction is neither. Fresh mortar has almost no strength in its first hours and days, the wall may stand alone for weeks before the structure that will support it is built, and a tall, thin wall presents a large surface to the wind. Masonry walls under construction collapse more often than most people expect, usually in wind, and when they fall they bury whatever is beside them. This lesson covers why walls under construction collapse, the restricted zone that keeps people out of the fall area, and how walls are braced until the structure supports them.

The west wall at Cottonwood Place

Along the west property line, Reyes Masonry Ltd. is building a reinforced concrete block wall for the ground-floor service corridor. It is 190 mm (8 in) thick, 6.0 m (20 ft) high and about 30 m (100 ft) long, with vertical reinforcing bars grouted into some cores. It will eventually be tied to the level 2 slab and the steel roof of the corridor, but for several weeks it will stand on its own. On one side is the masons' scaffold. On the other is a 3 m (10 ft) wide lane and then the occupied office building next door.

Why walls under construction collapse

• Wind. Wind is the main cause. A 6 m (20 ft) wall 30 m (100 ft) long has a face of 180 m² (2,000 ft²). Even a moderate gust pushes on it with many tonnes of force, trying to overturn it at its base, where the mortar may be only hours old.
• Fresh mortar. Mortar gains strength over days. In its first day, especially in cold weather, it can resist almost no tension, so the wall relies almost entirely on its own weight and any bracing to stay up.
• Height and slenderness. The taller and thinner a wall, the easier it is to overturn. Grout and reinforcing help, but only once the grout has gained strength.
• Unbalanced loads. Backfilling soil against one side of a basement or retaining wall before the wall is braced or supported can push it over. So can material stacked against a wall, or a scaffold tied to a wall that cannot yet resist the load.
• Impact. A forklift, telehandler or swinging crane load striking a fresh wall can start a collapse.

Collapses happen fast. A wall that has stood for days can fall in seconds in a gust, and workers and passers-by on the downwind side have no time to get out of the way.

The limited access zone

In Manitoba, the employer's general duty under the Act requires workers to be protected from a wall that could collapse onto them, and the regulation's requirements for temporary support of structures under construction apply. Many contractors in Canada, including Tallgrass, use a restricted zone like the limited access zone in the US rules. Tallgrass's site rule at Cottonwood Place is:

• the zone is set up before construction of the wall begins;
• it is equal to the height of the wall to be built plus 1.2 m (4 ft), measured out from the wall;
• it runs the entire length of the wall;
• it is on the side of the wall that will not be scaffolded;
• only workers actively engaged in building the wall may enter it;
• it stays in place until the wall is adequately supported to prevent overturning and collapse, or, for walls over 2.4 m (8 ft), until the wall is braced as described below.

The zone is the area the wall would land on if it fell outward. Because the planned height is used, not the current height, the zone is set up once at its full size before the first course is laid. At Cottonwood Place, the zone covers the whole lane and reaches into the office building's back entrance, which is why lesson 1's coordination meeting closed the lane and moved the tenants' access. The zone is fenced, signed, and checked every morning by the masonry foreman.

Bracing walls under construction

Walls that will stand alone for any length of time need temporary bracing until the permanent structure, such as the floors, roof and cross walls, supports them. Bracing is designed for the wind the wall could experience and the strength of the masonry at its age. For tall walls like the west wall, the bracing design is done by an engineer. CSA A371, the Canadian standard for masonry construction, covers how walls are built, and bracing guides published by masonry industry associations set out how temporary bracing is designed and installed.

Common bracing methods are:

• Diagonal braces of wood or steel, fixed to the wall at a set height and to anchors, deadmen or the slab at the base, usually on the scaffold side;
• Braces built into the scaffold, using scaffold systems designed to brace the wall;
• Internal reinforcing and grout that make the wall itself stronger, though only once the grout has gained strength;
• Early connection to the structure, such as building cross walls or connecting to columns or slabs as soon as possible.

Braces are installed as the wall rises, at the heights and spacing shown on the bracing plan, and inspected daily and after high winds. Braces are part of the structure of the wall until the permanent supports are in place: they are not to be removed to make room for deliveries, used to hang material, or loosened by other trades.

Wind: watching, stopping and evacuating

Because wind is the main cause of collapses, masonry bracing plans and site rules set wind speeds at which work stops and the restricted zone is cleared. Industry bracing practice uses lower wind limits for walls in their first day, when the mortar is weakest, and higher limits for walls that are braced and have had time to cure. The masonry foreman watches the forecast every morning and an anemometer on the scaffold during the day.

At Cottonwood Place, the bracing plan and Tallgrass's site rules set three wind levels:

• below the first limit, work continues normally;
• above the first limit, no one, including the masons, enters the restricted zone, and work on any wall laid that day stops;
• above the second limit, masons come off the scaffold and the restricted zone and any other area within reach of a falling wall are cleared.

The program below applies those rules, which are invented site values for this example, to wind readings taken on the scaffold during a day.

# Restricted zone and wind rules for the west wall (invented site values; the bracing plan sets real ones).
wall_height = 6.0                    # m, planned height
zone = wall_height + 1.2             # m from the unscaffolded face
print(f"Restricted zone: {zone:.1f} m out from the wall, full length")

FIRST_LIMIT = 32     # km/h gust: stop work on walls laid today, nobody in the zone
SECOND_LIMIT = 56    # km/h gust: masons off the scaffold, clear the fall area
readings = [("07:00", 14), ("09:00", 22), ("11:00", 35), ("13:00", 41), ("15:00", 60)]
for time, gust in readings:
    if gust >= SECOND_LIMIT:
        action = "CLEAR the scaffold and the fall area"
    elif gust >= FIRST_LIMIT:
        action = "stop new work, nobody in the restricted zone"
    else:
        action = "work normally"
    print(f"{time} gust {gust:3} km/h -> {action}")

Restricted zone: 7.2 m out from the wall, full length
07:00 gust  14 km/h -> work normally
09:00 gust  22 km/h -> work normally
11:00 gust  35 km/h -> stop new work, nobody in the restricted zone
13:00 gust  41 km/h -> stop new work, nobody in the restricted zone
15:00 gust  60 km/h -> CLEAR the scaffold and the fall area

By late morning the gusts pass the first limit and new work stops, and by mid-afternoon the scaffold is cleared. On a day like this, the masons lose half a day of work. That cost is the reason the limits are written down in advance: a decision made on the scaffold, with half a course still to lay, is too easily bent.

Daily checks on walls and bracing

The masonry foreman walks every wall under construction at the start of each shift and after any high wind. The check covers:

• the restricted zone fence, signs and gate, with nothing parked or stored inside;
• every brace present at the spacing on the bracing plan, with connections tight and anchors or deadmen undisturbed;
• the wall plumb and showing no cracks at the base or along bed joints, which could mean it has started to move;
• the scaffold ties and planks, and whether any scaffold ties load a wall that cannot yet carry them;
• material stored on the scaffold within its rated load and not stacked against the wall;
• the forecast for the day and the anemometer working.

Anything wrong is fixed before work starts. A wall that has cracked or moved is not worked on until the engineer who designed the bracing has looked at it. The check is recorded, because if a wall is found leaning after a storm, the record shows what condition it was left in.

Neighbours and the public

On a tight urban site, the fall area of a wall often reaches past the site fence, onto a sidewalk, a lane or a neighbour's property, as it does along the west side of Cottonwood Place. Tallgrass handles this at the planning stage: the restricted zone is drawn on the site logistics plan, the city permit covers any sidewalk or lane closure, and neighbours are told in writing when and why access will change. If the full zone cannot be closed, the wall is built in shorter lifts, braced from both sides, or redesigned with the engineer so the fall area stays inside the site.

Other walls and other hazards

The same principles apply to every masonry wall on the site, not just tall ones. Low walls can still fall onto a worker crouched beside them. Basement and retaining walls are not backfilled until the engineer confirms the wall is supported, by the floor above or by bracing, and the backfill is placed in layers on both sides where the design calls for it. Masonry built on top of a slab adds load to that slab, and pallets of block and mortar stacked on a young slab are construction loads that must be allowed for, as lesson 3 explained. Forklifts and telehandlers moving block near a fresh wall have marked travel routes that keep them out of the wall's fall area.