Seismic expansion joint covers: how they move in every direction, and how to choose one

A seismic expansion joint cover has a harder job than a thermal one. A thermal joint opens and closes a little every day as the building warms and cools. A seismic joint can be pushed wide open, squeezed shut, sheared sideways along its length and lifted out of level, all in a few seconds, and the cover is expected to be in place and usable afterwards. This guide explains what changes when a joint is seismic, how Genotek seismic covers take up the movement, and how to read our movement drawings.

What makes a joint seismic

Every structural expansion joint separates two parts of a building so that they can move independently. The difference between joint types is in the direction and size of the movement the cover has to follow:

• Thermal movement is mostly opening and closing across the joint. It is slow, repeated daily and seasonally, and usually small compared with the gap.
• Settlement is a slow change of level between the two sides as the structure and the ground below it settle.
• Seismic or multi-directional movement combines all three directions at once: opening and closing across the joint, shear along it, and vertical displacement between the two slabs. It can be large compared with the gap.

On a Genotek datasheet these appear under Suitable movement as thermal, seismic and settlement. A cover whose datasheet lists thermal and settlement only is not the choice for a seismic joint, however wide its gap range is. Start in the seismic and large span range and check the movement line of each datasheet.

Four ways a seismic cover takes up movement

A rigid plate cannot simply bridge a seismic joint: if it is fixed on both sides, the first strong movement tears it out. Genotek seismic covers solve this in four different ways. Each one suits a different kind of floor, wall or traffic.

1. Centre bar and self-centring cover plate

The Z4000 (PROFLEX RM) is the classic arrangement. Extruded aluminium support profiles sit on both sides of the joint, and a metal cover plate spans the gap. Below the plate, a galvanised steel turnbar assembly pulls the plate back to the centre of the joint after it moves, so the plate is self-centring. The cover plate is available in aluminium or stainless steel. The max/min drawing of the Z-4000-100 shows ±50 mm of opening and closing (±50 % at a 100 mm joint) and ±25 mm of vertical movement (±25 %).

The same principle is used for surface-mounted floors in the OF4000 (PROFLEX SM), which its datasheet recommends for renovation and retrofit work, for walls and ceilings in the D4000 (PROFLEX WC), whose cover plate can be made in aluminium, bronze or stainless steel and powder coated to match the wall, and for roofs in the R2001 (PROROOF), which adds an elastomeric rubber water barrier and rubber gaskets between the frames and the plate to stop metal-to-metal noise.

2. Glide plate

In a glide-plate cover, a free-floating centre plate slides between recessed support frames and cover frames on both sides of the joint. Nothing is fixed across the gap, so the plate can move in any direction within the frames. The Z3000 (ECOGLIDE FL) is a floor system of this type with elastomeric rubber gaskets that reduce noise and protect the profile; its datasheet recommends it for low-maintenance interior areas where a tamper-proof installation is wanted, with several floor finishes. The max/min drawing of the Z-3000-100 shows the joint closing by 29 mm and opening by 50 mm (−29 % / +50 % at a 100 mm joint).

The Z3100 (PROGLIDE FL) works the same way with extruded aluminium cover frames that can be removed to replace the centre plate. For walls and ceilings, the D3200 (ECOGLIDE) rests directly on the wall substrate, so the cover is flush, and can be powder coated to match the wall; its datasheet lists drywall applications among its uses.

3. Pan cover with the floor finish inside

Where the cover should almost disappear, a pan-type cover holds the floor finish in a deep central recess. The Z5000 (PROFINISH) and Z5500 (LUXFINISH) are full aluminium systems of this kind: the tile or stone in the pan matches the floor around it, so only a thin line of aluminium shows. Their datasheets state that they take thermal movement without stress on the metal components and multi-directional movement including lateral shear, and recommend them for hospitality, retail and commercial areas where appearance matters. The max/min drawings of the Z-5000-200 and Z-5500-200 both show ±100 mm of opening and closing (±50 % at a 200 mm joint) and ±50 mm of vertical movement (±25 %).

4. Flush plate that slides over the finish

Some covers stay flush with the floor during everyday thermal movement and accept that a seismic event will move them out of position for a while. In the Z4100 (PROFLOOR RM), Z4100A (SUPERFLOOR) and Z7100A (LUXPARK), the top plate remains flush during thermal movement and, during seismic movement, temporarily displaces and slides over the finishing surface. The Z7100A can be supplied with PTFE tape so that the plate slides with little friction. Read the traffic line carefully in this group: the Z4100 datasheet states that it is not recommended where forklifts or other heavy concentrated wheel loads may cross it.

Heavy traffic and wide seismic joints

A seismic joint in a car park, a warehouse or a service road has to carry vehicles as well as move. The Z8000 (ULTRAPARK) is built for that combination: its datasheet recommends it for areas with regular passage of forklifts, trucks and other heavy vehicles, and for wider joints where multi-directional movement is required. It uses a steel spring, and a damper system stops the cover from clunking from side to side under moving traffic. Its design allows vertical displacement between the slabs. The max/min drawing of the Z-8000-200 shows the joint closing fully and opening by half its width (−200 mm / +100 mm, −100 % / +50 % at a 200 mm joint) with ±50 mm of vertical movement. Our guide to car park expansion joints covers the traffic, water and noise questions in more detail.

Reading the movement figures on a Genotek drawing

Every model’s document pack includes maximum and minimum movement drawings: the profile drawn at its normal position, fully closed, fully open and, for multi-directional systems, at its vertical limit. Four points help when you compare them with your engineer’s figures:

1. Movement is a percentage of the nominal joint width. The nominal width is the gap as designed at the time of installation. A Z-5000-200 is drawn for a 200 mm joint, so ±50 % means the gap may close to 100 mm and open to 300 mm.
2. Opening and closing are not always equal. The Z-3000-100 and Z-8000-200 figures above are asymmetric. Compare each direction separately.
3. Vertical movement is listed separately where the system takes it, as on the Z4000, Z5000, Z5500 and Z8000 drawings.
4. Each size has its own drawing, named by model and joint width: Z-4000-100 is the Z4000 for a 100 mm joint.

Ask your structural engineer for the expected movement in each direction, not only the gap width, and check it against the drawing for the size you need. Our guide on how to choose an expansion joint cover goes through the other selection questions.

Floors, walls, ceilings and roofs

A seismic joint runs through the whole building, so the floor cover has to meet a wall cover, the wall cover a ceiling or roof cover, and each change of plane is a place where movement concentrates. Plan the whole joint line, not only the floor:

• Floors: centre bar, glide plate, pan or flush-plate covers as above, recessed or surface mounted.
• Walls and ceilings: the D4000 and D3200, which can be powder coated to match the wall or ceiling (wall joints, ceiling joints).
• Roofs: the R2001, which its datasheet recommends for roof areas with little foot traffic and for transitions to exterior wall joint covers (roof joints).

Where the joint also has to keep water out, many of these systems take an optional vapour barrier of EPDM or TPV below the cover; our guide to watertight joint covers explains how that layer must connect to the slab waterproofing. Where the joint passes through a fire-separating floor or wall, see our page on expansion joint covers with a fire barrier and tell us what the wall or floor requires.

Before you order

Recessed seismic covers sit in a blockout in the slab. Their datasheets ask for enough clearance from the reinforcement for the anchors, a surface level to within ±2 mm and a gap level to within ±5 mm, with the largest gap along the run matching the size ordered. That has to be planned before the screed or finish goes down. Then follow the model’s installation instructions, which are part of its document pack.

Send us the joint schedule or the drawings, with the movement figures for each direction, through the contact page or at support@genotek.global, and we will propose a system for each joint.

Frequently asked questions

What is the difference between a thermal and a seismic expansion joint cover?

A thermal cover follows the slow opening and closing of a joint as the building warms and cools. A seismic expansion joint cover also follows shear along the joint and vertical displacement between the slabs, and returns to position afterwards. On a Genotek datasheet the difference is in the Suitable movement line: seismic covers list thermal, seismic and settlement.

How much movement can a Genotek seismic cover take?

It depends on the model and size, and each size has its own maximum and minimum movement drawing. Examples from those drawings: the Z-4000-100 takes ±50 mm across the joint (±50 % at a 100 mm joint) and ±25 mm vertically; the Z-5000-200 takes ±100 mm across (±50 % at a 200 mm joint) and ±50 mm vertically; the Z-8000-200 takes −200 mm / +100 mm across and ±50 mm vertically.

Which seismic cover suits a car park or a forklift route?

Check the traffic line of the datasheet. The Z8000 (ULTRAPARK) datasheet recommends it for areas with regular passage of forklifts, trucks and other heavy vehicles. The Z4100 datasheet states that it is not recommended where forklifts or other heavy concentrated wheel loads may cross it.

What do I need to send for a seismic joint proposal?

The joint schedule or drawings with the nominal joint width, the expected movement in each direction (across, along and vertical), the traffic, the floor or wall finish, and whether the joint must be watertight or carry a fire barrier. Send them through the contact page or to support@genotek.global.