Essential engineering tips for trouble-free patio doors

The three main problems with patio doors

Patio doors have some basic requirements. They need to:

• Open and close
• Provide security, and
• Keep the weather at bay

But they also need to look and feel good, to operate easily, to be simple to maintain and the threshold needs to be comfortable and safe to walk on or over. The gaps between a door panel and its frames are often small and there is usually some kind of weather-stripping in those gaps too.

Loading and structural movements cause few problems with standard sized door openings but can have very significant consequences in a large opening.

Large openings, particularly those that are 'top hung’ i.e. panels are ‘hanging' from the top track so all the weight is at the top, can be prone to three main problems: deflection, creep and poor connections. These can make doors inoperable over time, or cause cracks or gaps to appear in internal or external walls. And yet such problems can be prevented with simple planning before construction begins.

To ensure that doors remain trouble-free for many years, it is important that the frame in which they operate is stable. Door installers often do their job early on in a build, sometimes with several months until handover. If the building structure moves enough to stop the doors operating perfectly, then it is likely to cost someone money to put it right.

Deflection during construction

During construction, the progressive loading of the structure above the door needs to be considered, particularly where the head also supports upper floor and / or roof and wall loads.

A desirable installation procedure lessens the impacts of gravity load deflection by pre-cambering the head track. The head track is generally set with an upward camber of 3mm from the horizontal. At this stage of the build, the load on the head will typically include the weight of all completed structure over, such as the framework, the roofing and the weight of the doors if they are top-hung.

The upward pre-camber will then work to counter the additional loads applied afterwards, which typically include:

• Wall linings and siding / cladding
• Ceiling linings and insulation
• Upper flooring

Other superimposed dead loads including interior fit-out, furniture and heavy items such as spas can all contribute to long-term deflection.

Wind load effects

The head and / or floor beams may be required to resist dynamic wind loads that might cause deflection of the beam upwards, downwards and laterally. With loading from large areas of walls and doors, these loads can be very high – equivalent in some cases to several tons of load. Lateral loads particularly can be significant if a floor or roof diaphragm cannot be used to resist it; such a load case should be considered early in the design by a structural engineer.

The head size required to limit gravity load and lateral wind pressure deflections will general be adequate to cater for loads such as wind uplift. However, each load case should be checked, particularly in the situation where large areas of wall, floor and / or roof are to be supported. Wind load will also apply a twisting action on a header beam that must be resisted at the connection between header beam and posts, and it is important that the junctions are appropriately designed.

Creep from sustained load on header beams

All building materials move when load is applied and removed, but some header materials move slowly and permanently under sustained loads that are below their yield stress, resulting in a permanent deflection. This process is called ‘creep’. On small openings, the amount of creep generally does not cause problems for the homeowner, but with wide openings, the amount of creep can, and often does, result in doors that no longer open and close properly. Sometimes, a simple adjustment can remedy the situation, but occasionally the fix ends up with costs that are entirely disproportionate to the cost of avoiding the problem through initial good design and planning.

The mechanism and extent of creep depends on the material used for the header beam. Both wood and concrete structural members are designed with long-term deflection as one of their design criteria because creep with both materials is an important factor.

But how much creep is too much*? Any amount that causes serious problems is the only useful answer, but as a rule of thumb keep to the smaller of the following two limits:

1. Total creep ≤ 3.2mm
2. Creep component of deflection ≤ span/2000

These limits are similar to that used for designing support structures for brittle masonry. For spans greater than about 3.6 metres, wooden beams become quite impractical. In many highly loaded situations, a 3.05 metres span is a good practical limit as appropriately designed beams become very deep for wider openings.

Steel beams are the practical answer for wide openings in wood framed construction just as they are in brittle masonry. A well-designed steel beam appropriately connected to steel posts that run from floor structure to either the roof or sub-floor above makes a slim, stable structure that will not sag over time. Bolting wood trimmers under the beam, and either side as well, provides working surfaces that are friendly for conventional wood framing practices.

*Please note: the LABC Technical Manual does not stipulate maximum allowances for creep and it should not be assumed that advice given here will meet our warranty requirements

Author:

Mike Sullivan works at www.centor.com, which manufactures high-quality doors and windows, screening systems and hardware components for the door and fenestration industry.