Padstones for steel beams: what they do and how they are sized

Why a beam cannot sit straight on the wall

A steel beam gathers the weight of a wall, a floor and sometimes a roof, and delivers it to two small patches of masonry. The end of a 133 mm wide beam resting 100 mm onto a wall presses on an area smaller than a paperback.

Steel shrugs that off. The wall may not. Common bricks, lightweight blocks and old lime mortar crush at pressures a beam can easily produce. The padstone sits between the two and widens the patch, so that by the time the load reaches ordinary masonry it is spread thinly enough to be safe.

The types you will see

Precast concrete padstones. Dense concrete blocks sold by builders’ merchants in standard sizes, such as 440 x 100 x 215 mm, 440 x 140 x 215 mm and 440 x 215 x 215 mm. Quick to fit and the default on most jobs.

Cast in place. A timber box is formed in the wall and filled with strong concrete, often with a couple of reinforcing bars. Used where the load is high or the space is an awkward shape. It needs several days to gain strength before the beam goes on.

Engineering bricks. Two or three courses of Class A or B engineering brick in strong mortar do the same job for light loads, and suit the coursing of an old wall.

Steel spreader plates. A thick plate welded to the beam or bedded beneath it. Useful when there is no room for a deep block, though it spreads load less far than concrete.

How the size is worked out

The engineer takes the reaction at the end of the beam, in kilonewtons, and divides it by the strength of the wall beneath. That gives the area the padstone must cover. The length along the wall does most of the spreading, which is why padstones are long and thin.

Depth matters too. Load spreads through concrete at roughly 45 degrees, so a longer padstone needs to be deeper or its ends do nothing.

The answer depends on the wall. A beam end that needs a 440 mm padstone on a dense concrete block wall may need 600 mm or more on soft Victorian brick, and on aerated blockwork the engineer may want the blocks beneath replaced altogether. This is why a padstone size copied from another job proves nothing.

Bearing length

Separately from the padstone, the drawing gives the bearing: how far the beam sits onto its support. 100 mm is a common minimum and 150 mm is typical. Less than that and the beam end is perched on the edge of the padstone, where concrete is weakest.

In a cavity wall the beam normally bears on the inner leaf only. If it is to carry the outer leaf as well, a plate is welded to its bottom flange, and the engineer will show how the steel is kept from bridging the cavity and letting damp across.

Fitting

The padstone is bedded level on a full bed of strong mortar, with no voids. The beam is then set on a thin bed of mortar or on steel shims, never on slate, timber or pieces of brick. Once it is level, the gap between the top of the beam and the masonry above is packed tight with a stiff, almost dry mortar rammed in from both sides.

Props stay in until that packing has hardened. Taking them out early is how cracks appear above new openings.

What the inspector looks for

Building control will usually ask to see the bearings before plaster goes on. The checks are simple:

• the padstone is the size and material shown on the engineer’s drawing;
• it sits on sound masonry, not on a cut block or a loose pier;
• the beam has its full bearing length at both ends;
• the packing above is solid.

Take photographs with a tape measure in shot before anything is covered. If the inspector cannot attend that day, the photographs usually satisfy them.

Where the pier is too small

If the opening leaves too little wall to carry a padstone, the engineer replaces the masonry with steel: a column under each end of the beam, standing on a new concrete pad in the ground. That goalpost arrangement is the usual way a wide opening is made into a steel frame extension, and it removes the padstone question altogether.