Design and Detailing of Windposts to Masonry

A windpost is a vertical element offering lateral support to cladding element typically made from masonry. Historically masonry elements were required to sustain only vertical forces and designed as principal load-bearing elements. They were thick and able to withstand even lateral forces due to their very “stocky” geometry. However, today masonry walls are quite slender, even to the point where they require additional horizontal support to make them stable. One element that provides this lateral support to masonry panels is known as “windposts”. Its principal function is to provide lateral support against destabilising horizontal forces that typically originate from wind pressure, hence the name.

Wind posts do not not provide vertical support and are normally restrained at the head. This requires a horizontal structure at the top of a wind post to achieve the restraint. They are typically made of steel elements, either open steel section such as channel or angle or closed steel section such as RHS or CHS.

Design Guidance

A wind post is effectively a simply supported beam subjected to uniformly distributed action applied to it. However, sometimes the wind post might need to be modelled as a propped cantilever, with the base having a moment connection (Figure 1). This is usually intended to reduce the size of the wind post but will result in the base connection becoming onerous than a simple connection.

Shows the idealized model of a wind post
Figure 1: Idealized Models of Wind Posts

The frequency and geometry of a wind post depends on the geometry of the wall it supports as well as the magintude of the applied actions. It is important to know that windpost are installed as a last resort as they are typically expensive and difficult to install. However, where returns, piers and vertical elements of the primary structure cannot be used as a point of restraint due to space restrictions, and the wall is required to span a long distance or contain large openings, a windpost is the only viable solution.

Just like beams supporting brick walls,windposts are subject to quite stringent movement criteria. Masonry is sensitive to any form of excessive displacement and is likely to crack if movement due to variable actions exceeds span/360 or ┬▒5mm. whichever is the lesser outside of datum┬╣. With such stringent consdition in place, it is typical for stiffness to govern the design of windposts.

Typically, windposts are considered to be fully laterally restrained due to the connectivity between the post and the wall it is supporting via the wall ties. This simplifies the design of windposts considerably, as buckling typically does not need to be considered. The only exception to this is when an angle is used as an externally mounted windpost. In this case, the outer leg would be prone to buckling as it is unrestrained.

Detailing Guidance

Windposts are typically located in semi- exposed or exposed locations where they are exposed to moisture on a regular basis as they are in contact with external facing masonry. They therefore require a high grade of corrosion protection. Historically, galvanising has been used to provide corrosion protection to a mild steel windpost. However, where stainless steel wall ties are used to fix a windpost to a wall, the post itself must also be made of stainless steel, to avoid bimetallic (or galvanic) corrosion.

Wall ties need to be designed with sufficient stiffness to transfer the lateral load from the wall into the post. There is a large number of proprietary products that serve this purpose. The technical specifications of a particular product should be consulted before it is used in a design solution. There are also proprietary products that include both the windpost itself and the means by which it is connected to the masonry via wall ties.

Windposts must also meet other criteria, including acoustic performance, thermal expansion of the wall they are supporting, and their behaviour when exposed to fire. When a windpost is supporting a wall that forms part of a fire compartmentation strategy, it must comply with the fire rating of the area to ensure that the wall remain standing in the case of a fire.

As windposts are designed to resist only lateral forces, it is important to effectively restrained the head of the windpost to avoid axial forces being transmitted into it. To achieve this, the fixing detail between the windpost and the primary structural element is allowed to articulate vertically relative to the connection, typically through the slotted bolt holes. Figure 2 shows a selection of typical head details for windposts that allow this vertical movement to occur.

Shows Typical head restraint details
Figure 2: Typical head restraint details

Wind posts are typically fixed to a wall via a series of wall ties that are placed at every other bed course within the masonry. The ties are post-fixed to the wind post as the wall is constructed to allow for vertical tolerances. The fixing method must take into consideration the anti-corrosion methods applied to the windpost.

Another method is to install the windpost within the wall itself (Figure 3). In this method, the windpost is installed, masonry units are threaded over the top of it, and the post is then grouted into place within the wall. This method provides excellent corrosion protection and much stronger connectivity between the windpost and the wall.

shows Detail of windpost within masonry wall
Figure 3 Detail of windpost within masonry wall

However, the complexity of the detailing and construction is much greater. In addition, as the post is embedded, it is more difficult to replace should a problem occur during its lifespan. Embedded windposts also cause problems when surface-fix items, such as shelving, are
installed on the wall, as they form part of the fabric of the masonry.

Also see: Designing a Laterally Restrained Steel Beam

To conclude this post, it should be noted that although wind posts do not form part of the primary structure, it is not uncommon for them to be designed and specified by the structural engineer.

Worked Example

A blockwork wall that is 3.2m in height is to be supported by RHS wind posts placed at 2.5m centres. The wall will serve as a barrier at a train station and therefore has a line action of 1.5kN/m acting 1.1m from the finish floor level. A wind action of 0.4kPa is also applied to the wall. Design a wind post to ensure it does not deflect any more than 5mm from datum

{ w }_{ k }\quad =0.4\times 2.5\quad =1kN/m
{ w }_{ line }=1.5\times 2.5\quad =3.8kN/m

Since the wind action is occurring at the same time with the variable action, the appropriate combination factors must be applied. In this case, the line load is taking as the leading variable action and wind as the accompanying variable action.

{ w }_{ ult,k }\quad =1.5\times 0.5\times 1\quad =0.75kN/m
{ W }_{ ult,line }=1.5\times 3.8\quad =5.7kN
Bending Resistance
{ M }_{ Ed }=\frac { 0.75\times { 3.2 }^{ 2 } }{ 8 } +\frac { 5.1\times 1.1\times 2.1 }{ 3.2 } =5.1kN.m
{ M }_{ Rd }=\frac { { W }_{ ply }{ f }_{ y } }{ { \gamma }_{ m0 } } =\frac { 92.7\times 355 }{ 1.0 } \times { 10 }^{ -6 }\quad =\quad \\32.9kN.m>5.1kN.m\quad o.k
Deflection Verification
{ \delta }_{ a }=\frac { 5{ w }_{ ult,k }{ l }^{ 4 } }{ 384EI } +\frac { { W }_{ ult,line }{ l }^{ 3 } }{ 48EI } \quad =\quad 4mm<5mm\quad 0.k

Where Iyy = 425cm3

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