04.09 Connections and fixings

Introduction
Brackets and their fixings form the link between the curtain wall and the structure. They are of critical importance to the safety and serviceability of the wall and also have a profound effect on its buildability. Bracket design is normally undertaken by the system fabricator. Bespoke connections can account for around 20 per cent of the cost of a curtain walling system, when full account is taken of the design costs, or the same proportion as the framing members themselves.

The information included in this Section is applicable to brackets for all types of curtain walling but the examples discussed relate to stick curtain walling where the primary load bearing elements that must be supported are mullions. SCI publication 101 describes curtain wall connections in more detail with greater emphasis on panellised systems.

Performance criteria
Brackets for fixing curtain walling are required to fulfil some or all of the following functions;

Transfer loads from the curtain wall to the structure;
Accommodate induced deviations (tolerances);
Accommodate inherent deviations (movements);
Resist corrosion;
Resist fire;
Be quick and simple to fix, adjust, inspect and maintain (buildable).

Loads
Vertical forces due to dead and live loads and horizontal forces due to live loads are transferred to the structure by the brackets.

Dead load
The precise weight of the cladding will be determined as the design is developed, but early estimates need to be realistic to prevent lengthy re-design of the support members. This requires a knowledge of the type of cladding system, materials, wind load and grid dimensions.

The curtain wall is normally supported in front of the supporting structure with a buffer zone to accommodate tolerances. The line of action of the load will therefore be in front of that of the support and bending and/or torsional stresses will be induced in the connecting bracket.

Live load
Wind loads in the form of negative (suction) or positive pressures are usually the dominant load case, with negative pressures at, for example, corners twice the magnitude of positive pressure at the centre of the windward face. Wind loads are determined by the site location and surrounding terrain, the shape of the building, local effects (e.g. sharp corners) and the size and location of openings.

Live loads resulting from building occupancy and maintenance are usually less significant than wind loads but still need to be considered.

To transfer these loads two types of fixings are required:

Supports
Support fixings are required to carry dead loads and these fixings will prevent vertical movement of a mullion relative to the supporting structure. Only one support fixing is necessary for each length of mullion and provision of additional support fixings is undesirable as movement will be restricted (see discussion of inherent deviations below).

For panellised systems two support brackets at the same level are normally required to provide stability.

Restraints
Restraint fixings are required at both top and bottom of mullions to resist wind loads.

Two possible arrangements of supports and restraints are possible for a single storey height mullion;

Top hung with a support at the top and restraint at the bottom, image
Bottom supported with the support at the lower end, image

For a mullion of a stick curtain wall the bottom restraint of a top hung mulliuon may be provided by means of a spigot connecting to the mullion below, image

For panels the support brackets are generally larger than the restraints and connect to the primary structural frame, image

Induced deviations
Deviations are differences between specified nominal dimensions and the actual measured dimensions. Induced deviations are permanent deviations, which arise due to variations and errors in the manufacturing and construction process. Tolerances are agreed limits to these deviations, which the design should be able to accommodate.

Deviations are considered in greater detail in Section 03.07 and Section 03.08.

BS 5606 provides guidance on the accuracy that can be achieved in masonry, concrete, steel and timber members and structures based on measured and estimated survey data. This reflects the standards of construction/erection and manufacture achieved by industry in 1979 and 1990 respectively. BS5606 indicates that the edge of a suspended concrete floor slab may vary from its intended position by +/-15 mm in plan and +/-25mm in level. These values have a probability of 1 in 22 of being exceeded and to reduce this probability to 1 in 80 the deviation would be increased to +/-19mm and +/-31mm respectively. For a probability of 1 in 370 the tolerances would be 22.5mm and 37.5mm.

All brackets should provide adjustment in three directions, image, to overcome the induced deviations. Means of adjustment include:

Slotted holes for fixings - These may need to be combined with serrated surfaces to prevent further movement after adjustment or low friction surfaces to allow for inherent deviations after installation;
Site-drilling or welding after positioning of components - This may be used for final fixing to mullions after initial fixing with slotted holes. It is likely to be less successful for fixings into concrete as the required hole positions may coincide with reinforcement;
Shims, packing pieces or washers - If excessive thicknesses are used nuts may not engage fully with bolt threads and bending stresses may be induced in bolts. Packing pieces may also reduce the contact area between components increasing stresses and inducing additional bending;
Sliding connections;
Threaded rods which pass through brackets and can be secured by nuts on both sides of the bracket;
Channel fixings - Comments for slotted holes apply.

Inherent deviations
Inherent deviations are changes in dimensions arising as a result of inherent material properties. They may be permanent or reversible and include:

Deflections due to applied loads;
Thermal movements;
Shrinkage;
Moisture movement;
Creep;
Settlement.

Design of brackets needs to take account of differences in the inherent deviations of the curtain wall and structure to avoid:

Imposing loads on the curtain wall for which it has not been designed;
Breakdown of seals due to large movements being transferred from the frame to the curtain wall.

Movement and dimensional changes described above can be accommodated at fixings by:

Slotted holes in brackets and low friction washers;
Interlocking joints which allow sliding in one direction but allow load transfer in the other two directions;
Design to minimise movement, for example by avoiding connections at mid-span where deflections are greater.

A number of different bracket geometries can be viewed here.

Wind restraint brackets that incorporate vertically slotted holes and low friction washers allow the curtain wall and frame to move independently of one another in the vertical direction.

Thermal movement of the curtain wall is accommodated in the splices between mullions.

Sufficient clearance should be provided within the rest of the curtain wall framework to allow the other components to move freely and still remain secure, and the system as a whole weathertight.

Although vertical movements will normally be greater than horizontal movements, horizontal movements must also be considered.

Resistance to corrosion
Two forms of corrosion warrant consideration - general corrosion of individual components including brackets, fixing bolts and curtain walling and bi-metallic corrosion resulting from contact between components made from different metals. Corrosion mechanisms are described in Section 12.01.

CWCT Guide to good practice for facades requires steel components to be protected by hot dip galvanising or coated with an approved treatment in accordance with BS 5493. BS 5493 has been superseded by BS EN ISO 12944. Not all coatings provide the same degree of protection and an appropriate level of protection must be provided.

If different metals come into contact in the presence of moisture a corrosion cell can be set up. PD 6484 gives guidance on the risk of corrosion with different combinations of metal. Bimetallic corrosion can be prevented by using gaskets, bushes or coatings of PTFE, neoprene or nylon to electrically isolate different components.

Resistance to fire
The performance of brackets will be affected by fire. Brackets fixed to soffits will be more vulnerable than those on the top surfaces of floors as they will generally be subject to higher temperatures during a fire. The bracket material will also affect performance, as steel will retain its integrity at higher temperatures than aluminium.

Aluminium and glass curtain walls do not have significant resistance to fire hence providing fire protection to the brackets will give limited benefit unless the whole wall construction is modified. With cladding materials with greater inherent fire resistance such as precast concrete, more extensive fire protection of the brackets may be required. The consequences of failure of the brackets may also be more significant if large elements can fall.

Fire protection to the cladding supports or firestops must not interfere with other aspects of the performance of the cladding fixings.

Buildability
Cladding is often erected at height in inclement conditions, Fixing details should therefore be simple to construct, to improve safety and reduce the risk of poor workmanship. Design of the interface between the frame and cladding must be appreciated and discussed between the structural engineer, architect and cladding contractor at an early stage for it to be effective. Too often fixing details are decided upon late in the design process and as a result are poorly resolved. Building elements are poorly co-ordinated, and possibly costly and difficult to construct.

When panellised curtain walling is used it will be important to ensure that the cladding can be fixed quickly to minimise the time that a crane is required. This will not be so important when stick systems, which can be manhandled, are used.

Fixings to concrete present problems and there are differing views as to the best solution. Cast in channel fixings appear to provide a simple means of providing a connection which allows for adjustment however if they are misplaced they can be unusable and difficult to replace by drilled fixings due to the area taken up by the channel. Drilled fixings are more time consuming to install and may be difficult to get in the right place due to the presence of reinforcement. It is probably best to use channel fixings where they can be fixed to the formwork reducing the risk of displacement and to use drilled fixings in the top surfaces of slabs where there is generally less risk of hitting reinforcement.

Brackets which are capable of being lined and levelled in advance of the cladding operation can produce overall cost benefits.

Installation
Bolts should be tightened to a specific torque (advised by the bolt manufacturer) and checked before being concealed. To ensure that all bolts are checked it is good practice to clearly mark bolts after checking (for example by spraying with paint). If bolts are subsequently undone for any reason they should then be re-marked (for example by spraying with a different colour paint) to indicate the need for rechecking. Torquemeters used for tightening and checking bolts should be calibrated. When bolted connections are made to hollow aluminium sections care is necessary to ensure that the section is not distorted by over-tightening bolts.

Connections should ideally be accessible for inspection after fixing and tightening; this is not always possible but can be achieved by the use of removable internal linings.

The flexibility designed into the cladding connection system must not be abused to overcome fixing problems arising from, say, an out-of-tolerance frame. For example, the final position of a bolt in a slotted hole may need to be specified so as not to restrict post-installation movement. Ill-conceived remedial measures, to overcome problems of fit can compromise safety.

Where excessive induced deviations are found, the participants should discuss whether to modify the structure, drill some additional fixing positions or supply one-off bespoke brackets. The latter course of action would involve the cladding engineer, structural engineer (the solution alters the loads on the structure), and the architect if the solution generates a different aesthetic effect.

Fixing arrangement
A typical support arrangement for stick curtain walling is shown in here. In this case the brackets are fixed to the steel edge beam but they could also be connected to the floor slab.

Note that the splice joints between mullions can accommodate live load movement of the structure and thermal expansion of the mullions. Structurally, the splice is designed to transfer horizontal shear.

Splice joints should be located above the points of connection in a suspended curtain wall and below them in bottom-supported walls. For a continuous curtain wall, the most efficient structural use of the mullion occurs when the joint is at about one fifth of the span from the support.

Types of brackets
This section shows a number of examples of brackets for curtain walls.

A pair of angle brackets used to fix top of mullion to face of floor slab: adjustment is provided by positioning of drilled fixing holes in the slab and shims the between brackets and concrete. The spigot in top of mullion provides restraint to foot of next section, image.
A composite bracket formed from three pieces of aluminium angle bolted together. The slotted holes provide adjustment, image.
Close-up of part of bracket for panellised curtain wall showing slotted fixing holes to allow adjustment and serrated surfaces to prevent slip after fixing, image.
A panellised curtain wall bracket showing keyways to provide connection to the wall section The bolts may be used to adjust the height of the support bar at the bottom of the keyways and hence the height of the panel, image.

Structural performance
Design of brackets and fixings requires an understanding of how the loads are transferred from the wall to the structure and the following modes of failure should be checked when designing the connections and fixings:

Shear, bearing and tension failure of bolts;
Ability of metal components to carry stresses from bolts, particularly the mullion which is likely to be a thin aluminium section. Bearing stresses can be reduced by increasing hole size and using bushes
Shear failure or pull-out of connections to concrete;
Bending, axial force and shear in brackets.

Design
A typical bracket detail at the head of a mullion is shown in this image. The head of the mullion is fixed to a shoe bracket, which is connected to an angle cleat, which itself is bolted to a channel cast into the top of the concrete floor slab. Alternative arrangements include fixing the shoe bracket to a channel cast into the outside face of the slab and using drilled fixings rather than cast in channels for the connection to the concrete slab.

A single slotted hole and two circular holes have been provided in the shoe-bracket. The single slotted hole will serve to carry a bolt to be used for initial fixing and adjustment of the mullion. However this type of connection is likely to slip in service as the hollow section of the mullion will prevent the bolt being tightened sufficiently to mobilise sufficient friction between the surfaces of the bracket and mullion. The two circular holes in the bracket will therefore be used as a template to site-drill holes in the mullion to carry the fixing bolts and complete the connection. These two bolts will share the loads transmitted from the mullion to the bracket and prevent movement taking place after fixing. This fixing arrangement restrains the connected mullion against horizontal and vertical movement but can be assumed to allow rotation due to bending under live load if the bolts are in clearance holes.

The intermediate wind load restraint bracket is of a similar design however as it provides restraint against lateral movement only, the connection to the mullion is by the vertically slotted hole thus allowing movement in the vertical direction.

The base of each mullion is restrained by being sleeved over a spigot inserted into the mullion below or, at the ground floor slab, the base bracket. This sliding spigot joint restrains the mullion against lateral movement but allows movement in the vertical direction. The mullion must be secured on one side of the joint only, leaving the mullion on the other side free to move and expand. The sleeve joint maintains alignment and visual continuity of the mullions and its integrity must be maintained to prevent water leakage.

Brackets are normally made of aluminium, galvanised steel or stainless steel. Bolts and screws into the aluminium should be stainless steel, whereas bolts into the structure may be galvanised, sherardised and dichromate passivated or zinc plated and passivated.

Load paths
Design of brackets and fixings requires an understanding of how the loads are transferred from the wall to the structure and in particular the mechanisms involved in load transfer between components.

The load transfer in a normal bolted connection is achieved by shear in the bolt and bearing of the bolt against the sides of the hole. The holes require some clearance to allow the bolts to be inserted but if this is excessive the capacity of the connection will be reduced. The normal permitted clearance in BS 8118 for bolts up to 13mm in diameter is 0.4mm. When the bolt is tightened and subject to low loads the load may be transferred by friction between the contact surfaces of the joined components however as the load is increased the joint will slip to allow the bolt to bear against the sides of the hole. The design procedures for normal bolted connections are shown further down this page.

High strength friction grip bolts are made from high strength steel and are tightened to produce a controlled clamping force across the joint that allows transfer of load by friction. They cannot be used to join hollow sections, as it would not be possible to develop the required clamping force. They may be used to join components with slotted holes provided suitable load spreading washers are used. They are not commonly used in conjunction with stick curtain walling and guidance on their use is given in BS 8118 and BS 5950 according to the material being joined.

When joining components with slotted holes to allow adjustment, it is necessary to incorporate a mechanism to prevent subsequent movement. The development of friction by the clamping force of the fixing, as in friction grip bolts, only gives reliable results when the clamping force is accurately controlled and the contact surfaces are carefully prepared. This is rarely possible and the normal method adopted is the use of serrated surfaces. In the example shown in this image a slotted hole would normally be provided in the angle bracket where it is connected to the cast in channel in the slab to allow adjustment of the gap between the face of the slab and the curtain wall. To prevent movement after fixing, the top surface of the angle would have a serrated surface and a plate with a similar surface would be used under the nut on the fixing bolt. If aluminium components are used serrated surfaces can readily be formed by extrusion.

The design of connections may be considered in three stages:

Transfer of load from the mullion to the bracket,
Transfer of load through the bracket,
Transfer of load from the bracket to the structure.

This is illustrated in the following calculations, which relate to the design of the brackets for the mullion and bracket shown. The loads on the connections are those for the mullion given in Section 04.05, structural design of stick curtain walling-sample calculations which are as follows:

dead load – 2380N, which will be entirely taken by the bracket at the head of the mullion (calculated as the maximum tensile load on the mullion in Section 04.05).
wind load – 6144N per span (calculated in Section 04.05). For the arrangement shown here, 5/8 of the wind load on each span will be taken on the intermediate support giving a total load of:

6144 x 5/8 x 2 = 7680N

The load on the bracket at the head of the mullion will be:

6144 x 3/8 x 2 = 4608N

assuming that there is another, similar section of mullion above that being considered. If there is no further curtain walling above the mullion under consideration, the horizontal load on the upper bracket will be half that calculated above.

The design formulae in the following sections are taken from BS8118, which is the Code of Practice for the design of aluminium structures. Similar formulae are given in BS5950 for the design of steel structures but the values adopted for safety factors are different. In BS5950 the safety factors applied to the loads are higher whereas the safety factors applied to material strengths are lower. Calculating the load effect in accordance with one code and the strength of the section in accordance with another can therefore give unsafe results.

Mullion to shoe connection
The transfer of load from the mullion to the shoe is achieved by shear in the connecting bolt. The bracket at the head of the mullion will be subject to both horizontal and vertical shear due to wind and dead loads respectively. The bracket at the intermediate floor will only be subject to horizontal shear but the horizontal load will be greater than for the bracket at the head of the mullion. It can be seen from the loads above that the load on the intermediate bracket will be more severe and the following calculations will be carried out for this case.

In accordance with BS 8118 the shear capacity of a bolt (V_RS) is given by:

V_RS = a_s x p_f x A_es x K₁ / g_m

where:

a_s is 0.7 for steel
p_f is the limiting stress which is the yield stress in the case of mild steel. For stainless steel it is the lesser of 0.5 (f_0.2+f_u) and 1.2 f_0.2 where f_0.2 is the 0.2% proof stress and f_u is the ultimate stress
K₁ is 1.0 for rivets, 0.95 for close tolerance bolts and 0.85 for normal clearance bolts
A_es is the effective area of shear, which may be the area of the shank or threaded part of the bolt depending on the location of the shear plane.
g_m is the material factor, taken as 1.2.

Rearranging the above equation, the required area of the bolt is given by:

A_es = V x g_f / a_s K₁ ( p_f / g_m)

where V is the applied shear force and g_f is the partial safety factor for loads.

Assuming bolts of stainless steel with f_0.2 = 210N/mm² and f_u = 500N/mm² hence p_f is 252N/mm². Substituting the known values in the equation:

A_es = 7680 x 1.2 / 0.7 x 0.85 x (252 / 1.2) = 73.75 mm²

As the bolt is in double shear the required cross-sectional area of the bolt is half this value and an 8mm bolt with an area of 50mm² would be satisfactory. It is however necessary to check that the bearing surfaces of both the bolt and mullion have adequate strength.

The bearing strength (B_RF) of a steel bolt is given by:

B_RF = d_f x t x 2 x p_f / g_m

where:

d_f is the nominal bolt diameter, mm
t is the thickness of the connected ply, mm

The minimum thickness of bolt bearing in this case is bearing against the mullion wall with a thickness of 2mm. Substituting the known values in the above equation gives:

B_RF = 8 x (2+2) x 2 x 252 / 1.2 = 13440N

These values should be compared with the applied load of 7680N multiplied by the partial factor of safety (g_f) of 1.2 (i.e. 9216N). Clearly the capacity of the bolt is greater than that required and the bolt is satisfactory.

The load capacity of the connected aluminium ply is given by the lesser of the following

the bearing capacity of the ply which is given by:

B_RP = c x d_f x t x p_a / g_m

the tear out resistance of a fixing close to the edge of the material given by

B_RP = e x t x p_a / g_m

where:

c is dependent upon d_f / t {where d_f / t < 10, c = 2},
d_f is the nominal diameter of the fastener, mm,
t is the thickness of the connected ply, mm,
p_a for the material of the connected ply is the lesser of 0.5(f_0.2+f_u) and 1.2(f_0.2).

For 6063 aluminium alloy in temper T6,

f_0.2 = 160 N/mm²

and f_u = 185 N/mm²

hence p_a = 172 N/mm²,

e is the distance from centre of the hole to the adjacent edge in the direction the fastener bears,
g_m is the factor of safety for material strength.

Substituting the known values in the above equations gives:

B_RP = 2 x 8 x (2+2) x 172 / 1.2 = 9173N

and

B_RP = 40 x (2+2) x 172 / 1.2 = 22933N

As above, the required capacity is 9216N and the tear out resistance is clearly satisfactory. The shortfall in the bearing capacity is less than 0.5% and hence within the accuracy of the design formulae. It may, therefore, also be considered acceptable. If a greater shortfall in bearing capacity were found it would be necessary to use a larger bolt. Alternatively, the bearing area in the aluminium could be increased by drilling a larger hole and using a steel bush between the aluminium and the bolt.

Provided that the wall thickness, edge distance to the hole and material strength of the bracket are at least as great as for the mullion, the bearing of the bracket will also be adequate. If these conditions are not met additional calculations will be required.

The calculations above show that the single 8mm diameter bolt is adequate to carry the loads on the intermediate bracket. The same size bolt would be suitable for the bracket at the head of the mullion, however the image above shows two bolts for the final fixing. Two 6mm diameter bolts would give slightly higher load capacity than the single 8mm diameter bolt and would therefore be sufficient.

Bracket
The bracket will be subject to a combination of direct and bending stresses. For the bracket arrangement shown here the critical points that need to be checked are the connecting bolts between the shoe and the angle bracket and the bending stress in the angle bracket. It may be necessary for the bracket to incorporate a fillet to sustain the bending moment.

The design of the bolts will follow the same procedures as given above except that the bolts will also be subject to tension resulting from both negative wind loads and bending of the bracket due to the dead load.

The tensile capacity of the bolt is given by:

P_RT = a x p_f x A_tb / g_m

Where:

a is 1.0 for steel and 0.6 for aluminium
p_f is the limiting stress which is the yield stress in the case of mild steel. For stainless steel it is the lesser of 0.5 (f_0.2+f_u) and 1.2 f_0.2 where f_0.2 is the 0.2% proof stress and f_u is the ultimate stress
A_tb is the stress area of the threaded part of the bolt.

Where bolts are subject to shear and tension simultaneously the following condition should be satisfied:

( P / P_RT)² + ( V / V_RS)²£ 1

Code requirements - Fixings
The fixing connecting the bracket to the structure will be subject to a combination of shear and tension. Fixings should comply with Section 2 of Approved Document A of the Building Regulations, which requires a factor of safety of 3 to be applied to the strength of fixings determined from tests. The basic strength to which the factor of safety is applied is the mean test result less three times the standard deviation. This is a global factor of safety and in this case it would not be necessary to apply a partial factor of safety to the loads. The connection between the bracket and the structure will usually make use of proprietary fixings and the design will be based on the manufacturer’s recommended safe loads.

Rainscreens
The load carrying capacity of any existing or new wall to which it is intended to fix a rainscreen cladding should be properly assessed before any rainscreen design is started. Fixings should be designed to carry all loads on the brackets. For rainscrens these may include moments to be transferred from the cladding to the primary structure, and also moments arising from eccentric vertical or horizontal loads. In particular the dead load of the cladding system will be offset from the line of support of the brackets.

Fasteners
All fasteners or bonded joints, connecting panels to supporting rails or a supporting grid, should be capable of transferring the required loads with the desired factor of safety. Bonded joints and panels have to remain adequately bonded throughout the design life of the facade.

The factor of safety for fasteners should be 2.0 for fasteners into ductile materials and 4.0 for fasteners into brittle materials. For factory bonded joints the factor of safety should not be less than 10.0 when tested to ultimate failure; this is to take account of the variability within the manufacturing process. A factor of safety of 25.0 should be used for site bonded joints to take account of site conditions and the variation in the levels of quality control and standard of workmanship.

Bonded joints should only be used following consideration of the consequence of failure the ability to inspect joints and any backup mechanical retention. Reputable structural adhesives, prescribed procedures and fully trained installers should be specified when considering the use of bonded joints.

CWCT FACETS

04.09 Connections and fixings