Question 1

What is a spreader plate?

Accepted Answer

A spreader plate is a steel plate welded or bolted to the end of a steel beam where it bears onto masonry, or placed under a beam on the masonry surface, to distribute the concentrated point load from the beam over a larger area. Masonry (brick or block) has a relatively low compressive strength - a steel beam bearing directly on brickwork over just its flange width can cause local crushing of the brick or mortar joint. The spreader plate increases the bearing area, reducing the stress intensity on the masonry to within its permissible bearing stress. Spreader plates are typically 6-10mm thick mild steel plate, sized by structural calculation.

Question 2

What is the difference between a spreader plate and a padstone?

Accepted Answer

Both spreader plates and padstones serve the same function - distributing a concentrated beam load over a larger bearing area to prevent crushing the supporting masonry. The difference is material and form: a padstone is a solid block of high-strength concrete, engineering brick, or natural stone, bedded in mortar on top of the masonry at the beam bearing position. A spreader plate is a steel plate, typically thinner than a padstone, either welded to the beam end or placed on the masonry and the beam bearing onto it. Padstones are more common for heavy-duty bearing on new masonry (they can be built into the wall as the masonry is constructed); steel spreader plates are more convenient for retrofitting to existing steel beams or where space is very limited.

Question 3

When is a spreader plate required?

Accepted Answer

A spreader plate (or padstone) is required at any steel beam bearing where the beam reaction load exceeds the permissible bearing stress of the supporting masonry. Whether one is needed depends on: the magnitude of the beam reaction (the load the beam transfers to its support); the bearing length (how much of the beam sits on the masonry); the masonry unit strength (engineering brick has higher bearing strength than standard facing brick; dense concrete block higher than aircrete block); and the mortar type and condition. For most domestic RSJ installations (new steel over an opening), a structural engineer's calculation will determine whether a padstone or spreader plate is needed and will specify the minimum size. Building Control should be consulted when structural steelwork is installed.

Question 4

How is a spreader plate sized?

Accepted Answer

A spreader plate is sized to ensure that the bearing stress on the masonry below the plate does not exceed the permissible bearing stress for the masonry type. The calculation is: bearing stress = beam reaction force / (plate length x plate width). This must be less than the design bearing strength of the masonry. For example, if a beam has an end reaction of 40 kN and the masonry has a permissible bearing stress of 1.0 N/mm2, the required bearing area is 40,000 N / 1.0 N/mm2 = 40,000 mm2 (200mm x 200mm). The plate also needs sufficient thickness to spread the load without bending - this depends on the plate overhang beyond the beam flange. A structural engineer will calculate the required plate dimensions and thickness for each specific case.

Spreader Plate

Just so you know...