Facade - structure tolerances: the buffer zone

A visit to the Arup central library in London is always a great experience. The other day I found this book, out of catalog and unavailable in Amazon: 'Interfaces - Curtain wall connections to steel frames'.
One of the interesting sections is devoted to discussing the tolerances that a designer should always consider between the plane of the building facade and the alignment of the structural steelwork. For the purpose of this post, the main structure can be steel or concrete, and the lightweight facade can be a curtain wall or any other system.

The idea of writing about this issue comes from an architect with whom we are working these days. The project name and location aren't relevant. Our architect wanted to stick the curtain wall plane to the front plane of a steel column, with no space in between. This would surely make a nice detail as shown on a Scheme Design drawing: all flush and neat. We have had to go some length to explain something quite clear to us facade engineers, but obviously not so to architects in general. The point in discussion was the need to consider a separation between the main structure and the inner plane of a curtain wall. Why is it necessary, and how big should that space be?

Let's go with the why first 
The structure of a building is not built by Swiss watchmakers. For example, the edge of the steel decking is is often set out from the centre line of the steel beam. In a concrete structure, the formwork is positioned in relation to the axis of the adjacent columns. Both are not proper methods - the setting out line should be the main axis lines at each floor. The result is that dimensional variations in the positioning of beams or columns are transferred to the alignment of the edge of slab. Add movements of the formwowk (in case of concrete) and you have a wobbly line instead of a perfect vertical plan.

Bosch CST Berger laser

The same, at a much smaller scale, happens with the curtain wall or lightweight facade installation. Facade contractors use accurate positioning tools as laser beams for the setting out of their lines and brackets. The image to the left is a high quality laser level, capable of self-leveling in horizontal, vertical and plan. The reader unit can find the laser beam as far as 800m away. But its accuracy is not perfect: +/-3mm vertical misalignment every 30m. Add small mistakes when marking lines with chalk on the concrete slab, bracket positioning and profiles drilling, and there you go with a certain degree of variation.





Now the dimensions
How big should the tolerance space be? What I liked about the book above is the way the authors divide the requested space - the total tolerance - in three areas:

1. Tolerances which define the zone within which the main structure should be built. Their value depend on the national building codes, the material, the quality of workmanship and the building size and shape.
2. Tolerances to absorb the misalignments of the facade elements in relation to their theoretical plane. These are smaller than the first ones, a rule of thumb is three times smaller.
3. A 'buffer zone' between the two tolerances above. This is an additional contingency against excessive dimensional errors. In practice, all structural frames exceed their specified tolerances at least in some points of the structure. 

The buffer is the way to avoid breaking arms and legs in order to fix a facade in every building. But remember, they are not intended to be a relaxation of the structural specification (or of the facade installation), rather they are a recognition that sometimes work does not go as planned - even in Switzerland.
The drawings below show, nº 1 for steel structure + curtain wall and nº 2 for concrete structure and precast, my personal rule of thumb for a general structure - facade tolerance. The three areas are shown in each case.

Steel structure plus curtain wall tolerances

Concrete structure plus precast concrete cladding tolerances

The buffer must not be excessively large, since fixing details which have to trasfer the cladding loads across the zone will themselves become significant and costly structural elements. As the cladding contractor effectively carries the cost of the buffer, the designers should consult him to assess a realistic dimension, at least in special cases.

You've got the point now. Let's assume this is too detailed to remember; what should be the real rule of thumb for architects? Easy: the theoretical distance between outer structure and inner wall should be 60mm for curtain walls, 70mm for precast. This is easier to keep in mind.

Matteoli: Water and air tightness of external windows

This post inaugurates the recovery of old documents from which we must still learn a lot: oldies but goldies. The first one is almost 40 years old, it is written in Italian and was published in 1971. Its title is 'Tenuta all'aria e all'acqua dei serramenti esterni'. The author is Lorenzo Matteoli.

Why was it important?
Italy in the 70's - as all Southern Europe - had very low quality windows. The typical window frame was made in aluminium, although there were still many hot formed steel sections. Their water and air tightness design was based in the steel sections of European 1930's suppliers: the best ones had two gaskets, one on the outside and one on the inside of the frame.
Lorenzo Matteoli was leading the testing laboratory of the Istituto di Elementi Costruttivi, a branch of the Architectural School at the Politecnico di Torino. By 1971 his team had conducted six years of window testing, and he had extracted some design principles to improve the window performance. I assume he was aware of the work by O. Birkeland and G.K. Garden about rain-screen and pressure equalization, appeared in 1963-64. But in my opinion the bulk inovations of his paper came from analysing the window tests conducted at the laboratory.

After its publication in 1971 this paper became inmediately influencial for facade contractors and window suppliers, but it was especially followed among aluminium extruders who had window profiles to sell. The aluminium extrusion companies were big conglomerates with very basic products, which they had to improve if they wanted to win in the market for new housing, then booming. This paper explained the new idea of pressure equalization and how it could be applied to windows, and it did so starting from scratch. The text is so basic that there are no drawings or references to central gaskets at all. But the concept of a gasket located in the center of the frame and not on the outside was already being applied in Germany, and with Matteoli's help it became obvious what a central gasket could do, and why it was better to have an external open joint between the frame and the sash.

But what did it say?

1. The water running along the outer surface of a window...

Matteoli's paper has an additional advantage: it can be understood almost without reading it, because it comes with a number of hand sketched drawings and captions that are the real summary of the paper. I assume not everyone reads Italian, but please have a look at the first four sketches with translated captions here below, and judge by yourselves.

2. ... through vertical and horizontal joints...

3. ... gets in contact with the outer gasket. Since Pe is higher than Po, water tends to come in, helped by capillarity processes...

4. ... by pressure differencial and by air movement inside the intermediate chamber, until it reaches the inner gasket where, helped again by capillarity and pressure differencial (Po is higher than Pi), it finds a way to the inside.

Now a personal comment 

I first found this document while I was studying architecture in Madrid, at the school Library, together with other oldies as the curtain wall series published by Folcrá in the seventies. It didn´t make an impression to me then, but some years later I was going to be working for one of those big aluminium extruders: Hydro Aluminium, owner of the window brand Domal. At Hydro's technical department near Milan, led by Massimo Dampierre, this document appeared again. They told me the story of the evolution from the two gaskets to the central gasket, the move from punched hinges to a channel where all accessories could be fitted and removed (the 'European channel'), the birth of thermal break profiles. The Italians had coined a hip name for the new family of windows that became popular in Southern Europe: these new windows were 'systems', just because their elements were interchangeable. You can use the same glazing bead profiles, gaskets or acessories for different window types and it works, saving time and reducing stock at the window manufacturer. Now it's pretty obvious and has become standard, but it all started at a hidden testing place in Turin...

A real pleasure to read. The whole document is just seven pages long: go to the link and check your Italian!

From Wigwarm to Ikea (houses)

'I find it incredible that there will not be a sweeping revolution in the methods of building during the next century. The erection of a house wall, come to think of it, is an astonishingly tedious and complex business: the final result is exceedingly unsatisfactory'

Wells, H.G. 'Anticipations of the reaction of mechanical and scientific progress upon human life and thought', Chapman, London 1901. 
Taken from Alan Brookes, 'The turning point of building'


Who said H.G. Wells was not a man of vision? Some might argue, though, that Mr Wells failed by not predicting the arrival of mass housing prefabrication systems in the 20th century. Well, if that was the question, I'm afraid the answer is even tougher: he didn't have to predic anything, because house prefabrication was well known by the turn of the century. And again, if he new it, he didn't fail in his predictions at all: prefab houses today are as much 'the next thing to come to building' as they were in 1900. Only that we are still waiting for this to happen...

One of the first succesful examples of prefab housing in the USA is the E.F. Hodgson Company, active in Dover, Massachusetts between 1894 and 1944. Their houses came under the brand Wigwarm. The word is a mixture of wigwam, an Indian tent similar to a teepee, and warm. Its meaning: quick to assemble but comfy.

A Hodgson cottage in Dover, MA, as appeared in the 1935 E.F. Hodgson Catalog

One of the remaining Hodgson houses, built in 1940, in pristine condition inside and outside

The Wigwarm construction was a framed house, lighter than a standard timber frame construction, based on several timber sections fastened together with key bolts of special design. With just a blow of a hammer the wedge key tightened up the bolt, saving time during erection or dismantling. Frames were covered with a very heavy waterproof fibre or lining and then with a rabetted siding. The basic modulus for the houses was 6 x 12ft (1.80 x 3.60m). Mr Hodgson didn't just prefabricate houses, he also sold brooders, tool houses, dog houses, car garages - there is one in the top image to the right - or, during war periods, barracks for the military. In the '20s and '30s you could see Hogson houses in places as Europe (Belgium and Italy), Israel, Africa and South America.

Was it a commercial success? It really was, and it was simply based on selling by mail. A response to an ad in a newspaper would get the potential client a full catalog with photographs, floor plans and comments by satisfied customers. The company did not survive its founder: Mr Hodgson, without a son to continue the business, sold it in 1944, four years before he died.

The story is surprisingly similar to today's prefab, portable houses through catalog: the brand new Ikea/BoKlok house. Even the look is familiar. BoKlok, the webpage tells, builds homes for ordinary people who want to have money left over. But the idea for this house was born in very different circumstances, as a dialogue between IKEA and Skanska chairmen in 1996. A retailer and a construction company sharing forces to create affordable apartments. Up to now they have sold more than 4,000 houses in five countries, and the idea is booming, from Scandinavia to Germany to the UK. 

Who knows, maybe now it's the time for the sweeping revolution in the methods of building that H.G. Wells was asking for: his period of 100 years is finally over...