What is a Head Restraint and why are Head Restraints required in masonry panels?

What is a Head Restraint?
When are Head Restraints required?
Who makes design decisions about Head Restraints e.g. type, quantity, spacing, and at what stage of the project?
What types of Head Restraints are available?
How are Head Restraints fixed?
What happens if Head Restraints are omitted on site?
What remedial action can be undertaken if Head Restraints have been omitted on site?

What is a Head Restraint?

Head Restraints provide simple support to the top of a masonry panel by transferring lateral load (usually wind or internal pressure load) from the masonry to the primary structure, whilst allowing some vertical deflection (movement) in the frame in relation to the top of the wall.

When are Head Restraints required?

Whilst it is possible to design a wall without the need for restraint at the top, it can be uneconomical to do so as this might require the use of thicker/stronger blockwork and/or the inclusion of bed joint reinforcement to ensure that the panel is sufficiently strong. It is common practice to include head restraints to most types of wall, particularly in external cavity walls where lateral loads are greatest.

Head restraints are typically used on the inner leaf; the outer leaf is usually either tied back to the structural frame separately or tied to the inner leaf of masonry within 225mm of the top of the outer leaf.

They are most commonly used in framed structures where the masonry is non-loadbearing. In cases where the inner leaf is loadbearing, the weight of the floor structure on the blockwork may be enough to provide adequate support against lateral loading however, the Structural Engineer should confirm whether this is the case.

Who makes design decisions about Head Restraints e.g. type, quantity, spacing, and at what stage of the project?

The requirement for head restraint should be identified by the Structural Engineer responsible for the masonry design as early as possible in a project. The geometry and loading of the panel will dictate whether head restraint is required.

Generic “rule of thumb” guidance for preliminary scheme design is provided in literature such as The Structural Engineer’s Pocket Book (Cobb, 2015), which provides an estimate of the maximum permitted area for an external cavity wall panel for various edge support conditions, based upon guidance previously published in BS 5628. This information shows that a panel simply supported at all four edges can typically be 1.7-1.9 times larger in area than a similar panel with no head restraint.

More detailed design is required to specify the final panel design incorporating the exact type and strength of masonry/mortar etc., however information such as that provided by Cobb provides useful guidance for Engineers making early estimates of which panels will require head restraints in a project’s design.

The final masonry design will define the line load to be supported along the top edge which, when compared to the design resistance of the head restraint (refer to manufacturer’s literature), defines the spacing of the restraints. Therefore, the specification of fixing centres should also come from the Structural Engineer.

What types of Head Restraints are available?

Ancon has three types of standard head restraint available: IHR, FHR and SAH.

IHR

This is the most common head restraint provided by Ancon. This product is designed to be built in to the top course of blockwork and remains wholly internal to the wall and comprises a body with a bent leg which is fitted in the bed joint and perp joint, and a sliding head which slots in to the top and is bolted to the soffit.

The Ancon IHR is suitable in conditions where the gap between the top of the blockwork and the underside of the structure is up to 75mm wide to allow for deflection in the frame, or the inclusion of fire protection (e.g. intumescent strips in the joint). The restraint can be supplied with a variety of different heads to suit a range of fixing conditions, and can be fixed to concrete, steel or timber.

This product requires that the structure is directly above the masonry, and by virtue of being fully hidden, may be used in fair-faced blockwork applications where the top of the wall is visible from the interior.

FHR

This is a two-part system which effectively clamps the top of the masonry to hold it in position. The longer part wraps across the top of the masonry and the shorter part then bolts through the longer part into the soffit to clamp the blockwork.

The standard Ancon FHR is suitable for a gap of 25mm between the top of the wall and the soffit, and suits both 100mm and 140mm thick masonry. Specials can also be produced for other block thicknesses or larger gaps – indeed the FHR has the greatest degree of design flexibility in terms of load capacity and joint size.

The fixing for the FHR is offset 30mm from the face of the blockwork meaning that, unless there are finishes (such as a suspended ceiling), the FHR may be seen if the top of the wall is visible. This also means that it is sometimes possible to retrospectively fit the FHR should their inclusion have been omitted originally.

SAH

This is commonly known as a sliding anchor, comprising a stem that bolts to the structure and wall ties that can slide up and down the stem and are built in to the inner and outer leaves. Unlike other alternatives, this product provides head restraint to both leaves of a cavity wall. There are five different Ancon head types to suit different structural arrangements and both one-way and two-way wall ties are available. Stems are available from 340mm – 600mm in length, with the only other geometrical constraints being:

• The first tie is no more than 75mm from the fixing in the vertical direction

• The second tie is no less than 150mm below the first tie

• A minimum of two ties are required

For this reason, the SAH can accommodate a great deal of movement if the geometry of the detail allows. Furthermore, as the restraint is situated within the cavity, it is rare to require any specific fire protection. It is also possible to use the Ancon SAH sliding anchor in single leaf situations, so long as the same geometrical constraints are observed.

How are Head Restraints fixed?

Ancon head restraints are bolted to the structure with the following types of fixing:

• IHR-V, IHR-B, FHR:
- To concrete – M8 FAZII expansion bolt.
- To steel excluding hollow sections (any thickness) – M8 isolated setscrew.
- To timber – M8 fixing to Engineer’s design.

• IHR-S:
- To steel including hollow sections (4mm-12mm thickness) – SDTSS-38-5PT self-drilling self-tapping screw.

• IHR-C:
- To concrete – 28/15 channel, 38/17 channel, 30/20 channel.

• IHR-H:
- To steel (7mm to 25mm thickness) – Hammer on section.

• SAH (all types):
- To concrete – M10 FAZII expansion bolt.
- To steel excluding hollow sections (any thickness) – M10 isolated setscrew.
- To timber – M10 fixing to Engineer’s design.
- Special head types may be supplied for fixing to hollow sections with SDTSS-38-5PT self-drilling self-tapping screws.

As always, please refer to Ancon’s installation guide for specific guidance on each type of head restraint.

What happens if Head Restraints are omitted on site?

The omission of a head restraint essentially changes the way that the wall reacts under load. In most cases, wall panels are supported at all four edges ensuring that the panel is two-way spanning. Removing the support at the top changes how well the wall can span vertically, and also changes how much load is applied to the other supported edges. The failure mode of the wall changes as indicated in Figs. 2 and 3, and can drastically reduce the load capacity of the wall.

In cases where the Engineer has designed the wall with head restraint, the omission of these on-site could potentially lead to unexpected cracking and wall failure. The knock-on effect of increasing load applied to the remaining edges could also lead to restraints at these edges becoming over-loaded.

What remedial action can be undertaken if Head Restraints have been omitted on site?

If head restraints have been omitted and the Engineer is unable to justify the panel without top edge support, Ancon may be able to offer a remedial solution which negates the need for masonry to be taken down. Typically, solutions are bespoke to the individual application, but could include the use of a special FHR, or a simple angle cleat that bolts to the wall and the structure. It is often difficult to provide a remedial alternative to the IHR – indeed it is hard to hide a remedial head restraint unless there are finishes such as a suspended ceiling to conceal the restraint.

Contact Ancon for more information.

Reference
Cobb, F. (2015) Structural Engineer’s Pocket Book: Eurocodes [3rd Edition]. CRC Press.