RENOVATING WITH UHPC PART III: HOW TO USE WALLS AND STRUTS AND WHY

RENOVATION WITH UHPC

This post concludes the mini-series on renovation with Ultra High Performance Concrete (UHPC) and discusses the advantages and challenges of using walls and stiffeners as tools to support the balconies.

For a general introduction to renovation for potential new readers, please refer to the first post in the series on glued connections.

WHY ADD WALLS TO THE OUTSIDE OF A BUILDING?

When renovating, there are two main concerns - this is also true for balconies:

How to make it work structurally
How to make it work aesthetically and functionally for the people living there

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

Walls are, of course, heavy, but the stiffness from an integrated wall that functions as a very tall beam, either cantilevered on brackets at the bottom or anchored via a tension connection at the top and a compression connection at the bottom, allows the balcony slab to be fully supported and thus realized in minimal dimensions.

Finlandsparken during the ongoing installation of new balconies. Note the very small dimensions of both the walls and balcony slabs.

WHY UHPC?

As in the previous blog posts, the biggest advantage of UHPC in this context is the lower weight with larger and more usable balconies for the same loads.

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

This is mainly advantageous when the balconies are attached per floor - if the walls are used as columns supported by a foundation, the weight is much less problematic, and for these projects, regular concrete in larger dimensions is usually sufficient.

However, with UHPC, the loads can be reduced enough that even relatively large balconies are supported by relatively small brackets, as shown below.

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

The walls, functioning as cantilevered beams, can be replaced by stiffeners. While this does not add the same rigidity, it still creates an effective load-bearing system.

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

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 an advantage when supporting the balconies on a per-floor basis.

CHALLENGES

However, there is a problem with using UHPC, and it is - again - tolerances and the small dimensions in general, which create these challenges:

When very thin slabs need to be connected to brackets - base plate or wall - there is very little room for error. For example, drilling to place anchors to establish a tension connection at a bracket as shown above leaves only very limited adjustment 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.

Ensuring a rigid connection between the balcony slab and wall or securing the wall to ensure that horizontal wind loads can be resisted is something that requires thorough consideration with the very narrow dimensions of UHPC elements.

Distance to the edge: 30-40 mm and slab thickness down to 60 mm are not covered by standard tables from manufacturers of mechanical and chemical anchors and require specific testing and documentation.

THANK YOU FOR YOUR INTEREST!

This concludes the mini-series on renovation with UHPC, outlining some different principles for utilizing the properties of UHPC and the potential advantages and challenges when doing so.

We hope it has been of interest!

The presented examples of solutions are just a small selection of what we could have chosen - we have many other examples, and other solutions will also be developed and used over time.