Questions and Answers

Question: The LSF’s (lightweight steel framing’s) track is aligned with the edge of the concrete floor slab on your diagrams for the movement and tolerance series. However, it is rare when the edge of the concrete slab could be aligned in such way. Could you please expand on the position of the LSF and the edge of the slab?

Answer: In the movement and tolerances course series, the LSF track was indeed aligned with the edge of the slab. This is rarely the case on-site. This is due to the imperfections from the pouring of large volumes of liquid concrete. As a result, the edge of the slab is almost always irregular. In the UK, the installation tolerances of the concrete are restricted to 20mm. This means that the designed dimension could be pushed inwards or outwards by 10 mm. The lightweight steel framing systems usually allow for the slab-edge tolerance by oversailing the tracks. This oversail is limited to a third of the width of LSF’s track. This results in 50mm for 150mm wide LSF, and just over 30mm for 100mm wide system. It is a common practice to oversail the LSF by 15mm and to establish this as a design principle. Furthermore, there are cases with recessed LSF or protruding floor slab. The slab protrusion is related to the bracketry supporting a façade. For example, in this detail, the slab protrudes to shorten the distance between the primary framework and the back of the brickwork. This shortening was needed to allow for a more common and therefore more cost-effective masonry support bracket. However, the cases with recessed LSF or protruding slab require special attention to the membrane to avoid ponding and water ingress.

Configuration of the Floor Slab Edge and Fire Separation

The concrete down-stand, and an up-stand had become increasingly common in the UK.  

The main purpose of the upstands and the downstands, is structural. They are assisting the concrete structure, in minimising the deflection, between the long-spans of the columns.

More recently, the downstands and upstands, are assisting façade compartmentation, for fire safety. They help creating continuous fire compartmentation, across the façade.  

The current normative base, requires cavity closers to be in line with all internal compartments, on every floor.  

The areas in front of the floor slab, usually pose issues. These areas, are usually busy with the facade framing components, such as masonry support brackets, or balcony brackets. Meaning that, there may not be enough space for both, framing elements, and the cavity closers, in the slab area.  

In order to provide continues fire compartmentation, the slab can be extended upwards, or downwards, creating more space, to accommodate the framing components, and fire-safety elements.  

If we do not require an up-stand for structural reasons, it can be stopped once there is enough space for both, framing, and cavity closer, within the standard slab thickness. An isolated bolted-on balcony, is a good example of this.  

The upstand could be extended slightly past the balcony, to create enough clearance for the cavity closer overlaps.

For condensation risk analysis the following parameters are required. External and internal temperature and relative humidity. These are so called boundary conditions. 

The choice of these parameters depends on the building use and the location. It is prescribed by the local building regulations or warranty providers specific requirements. 

If we are to analyse the same detail to standard minus four degrees outside and plus twenty-one inside with fifty percent relative humidity they will pass. Dew temperature is expected at around nine degrees. 

However, if exactly the same details to be analysed to minus two externally and plus twenty internally with sixty percent relative humidity. Dew temperature is expected at around twelve degrees. (These are more stringent warranty providers requirements for houses in the UK). Dew point line will go through the internal finishes at interfaces. This will mean the condensation risk. 

Question: on your diagram in precast concrete joint drainage there is wheep hole the vertical joint. I have never come across this detail. Why is it required here?

As you can see from the case study diagram, there are two types of drainage for the joint between the precast concrete panels.

The chamber between the two silicone seals can be drained though the vertical joint and the horizontal.

On our diagram we show both types used simultaneously. Though this detail is a rare one. It is a common practise for the precast concrete contractors to use only the horizontal. However, that depends on the installation contractors judgement on the panels geometry and precast concrete panels arrangement on the façade.

The centres of these holes in the horizontal joint created by small tubes penetrating the outer seal depend on the panel length. Usually there are two per panel.

The holes are quite small in size and nearly invisible in the recessed joint.

Using both types of drainage in our case study would be a belt and braces detail.

The location of the second seal could be closer to the face of the cladding if it is applied from the outside from the practicality point of view. This again depends on the specialist contractor’s judgement.

Semi-recessed balustrade

Question from forum. - why in the course on semi- recessed terrace. The the balustrade is required to be out of metal bars. Can it be glass?

In this case study we recommend the balustrade to be metal and not laminated glass. Even though it is a terrace. It is semi recessed and partially made out of prefabricated bolted-on elements, which could be interpreted as an attachment to the facade. 

That would mean that it could be viewed as part of the facade. Hence, laminated glass could not be used due to combustible interlayer. 

If this would be a fully recessed terrace of the penthouse, the balustrade resting on the slab could be viewed as part of the roof. A laminated glass could be used then. 

In the near future we will release the course case study with similar detail to investigate this condition further. 

Stay tuned. 

Models Lighting And Navigation

We are trying our best to model and light all of our details so that they are easy to view from all angles and are quick to load. 

However there are areas in some models that could be difficult to view close up due to direction of the light and application of material. 

Here are several tips how to solve this. 

If you view it in the desktop or a laptop browser. 

You can switch to the full-screen mode by hitting the F-key on your keyboard.

Then you can try holding Alt- key on the windows and Option-key on the Mac and clicking and draging your mouse or a track pad to rotate the lighting scene left and right. 

The shading can be switched off by pressing 2 on you keyboard and switched on by pressing 1. 

If you are viewing the model on your mobile phone you can simply rotate the lighting scene with three finger drag. 

45 mm drip detail

One of Our members noticed that some details from courses on cavity closers and aluminium cladding with recessed windows do not have sills and copings with drip detail 45mm away from the face of the cladding Like we recommend in most of the other courses. 

It is a very valid observation. 

We believe that the flashing with the drip would be the best practice detail.

However to our knowledge it is not prescribed by any other normative other than some of the warranty providers requirements for insulated render. External render stains and the 45mm drip detail is very Much a requirement for it. 

With other claddings and other facade assembly methods it is a recommendation. A 45mm drip detail is a best practice detail. We do recommend using it wherever possible. But this is a recommendation. We are not aware of a normative document which would stop you from doing it for any facade treatments other than render. 

Thank you for raising this question.