Renovating with UHPC part III - How to use walls and struts and why...

The housing area Finlandsparken in Vejle renovated with CRC i2® balconies. Integrated walls provide privacy. 

This post concludes the mini-series on renovating with UHPC and discusses benefits and challenges using walls and struts as the tools for supporting the balconies.

For a general introduction to renovating to potential new readers, I refer to first post in the series on glued connections.

 

Why add walls to the outside of a building?

Often with renovations there are two concerns – equally important when it comes to balconies:

• How do I make it work, structurally?
• How do I make it work aesthetically and functionally for the people living there?

In my two previous posts most of the focus was on the structural side, but with walls - or struts with light privacy screens - it is possible to create private outdoor spaces with some shielding from wind and rain, with the potential side effect of having an effective connection system.

Walls are of course heavy, but the stiffness from an integrated wall acting as a very high beam, either cantilevered placed on brackets at the bottom, or restrained via a tensile connection at the top and a compression connection at the bottom, allows the balcony plate to be fully supported and therefore be realized in minimal dimensions.

 

Why UHPC?

As in the previous posts, the main benefit of UHPC in this relation is reduced weight with larger and more useable balconies for the same loads being the aim.

With UHPC the weight of especially the wall can be kept to a minimum, while maintaining the stiffness, durability and fire resistance needed for the support of the plate.

This is mainly beneficial when the balconies are attached per floor – if the walls are used as columns supported by foundations, the weight is much less of an issue, and for those projects, conventional concrete in larger dimensions is normally sufficient.

But with UHPC, the loads can be reduced enough for even relatively large balconies to be be supported by comparatively small brackets, as shown below.

 

Finlandsparken during installation of the new balconies – notice the very small dimensions of both walls and balcony plate.

 

Assembly is simple with placement of the balcony on four or five supporting brackets, depending on size.

 

The walls acting as cantilevered beams can be replaced by struts. That does not add the same amount of stiffness, but still creates an efficient load carrying system.

To provide privacy, light screens are typically added at the location of the struts, as in the example below.

 

Renovation project in Aalborg with UHPC plates and steel frames supported by struts where light non-structural privacy screens have been added between apartments.

 

To summarize, integrated walls are a good solution when privacy screens are needed anyway because they also make the connection to the building effective when used structurally, and in this case the low weight and high strength of UHPC is a benefit when supporting the balconies on the building pr. floor.

Challenges…

 

There is one issue using UHPC though, and it is – again – tolerances and the small dimensions in general that creates them:

When very thin plates are to be connected to brackets - bottom plate or wall - there is very little room for error - e.g. drilling to place anchors to a create a tensile connection at a bracket as shown above leaves only very limited setting depth and often requires actual testing to determine the design capacity of the anchors because they fall outside the anchor load table values designed for regular concrete.

 

Edge distances down to 30 - 40 mm and plate thickness down to 60 mm are not covered by standard tables from the manufacturers of mechanical and chemical anchors and require specific testing and documentation.

 

Also, ensuring a stiff connection between balcony plate and wall, or fixing the wall to make sure horizontal wind loads can be resisted is something that requires careful thought with the very narrow dimensions of UHPC elements.

 

Thank you for your interest!

This concludes the mini-series on renovating with UHPC, indicating some different principles for utilizing the properties of UHPC and the potential benefits and challenges when doing so. We hope it has been of interest.

The presented examples of solutions are of course just that – examples - and many other examples could have been chosen, and other solutions will in time be developed and used.