Architecture for Athens 2004 - Roof & Facade

Roof&Facade Asia
Feature: Natural Stone Applications
The use of stone in curtain walls
[This is an edited excerpt from a publication produced by Internazionale Marmi e Macchine
Carrara SpA (IMM). Well-known all over the world for the promotion of marble and other
stone materials, IMM has, over the last few years, expanded its range of activities. The
company’s objective is to acquaint people with the qualities of stone and all its possible uses].
Introduction
The use of thin stone in curtain wall systems
is becoming more widespread in Europe.
Aside from their traditional structural
function, stone materials are now being recognized
for characteristics which compare
favourably with those of many other building
materials used for cladding purposes.
These include aesthetic qualities; the ability
of the stone to act as a barrier to prevent
atmospheric and polluting elements from
entering the internal environment; the minimal
requirement for maintenance or renovation;
their capacity to increase the degree
of internal comfort, thanks to the possibility
of creating ventilated curtain wall systems;
and their ability to provide greater
sound reduction in lightweight cladding
systems, due to the mass of the materials.
In order to exploit the above characteristics
and maintain adequate safety levels, especially
in the use of thin stone for large
buildings subjected to adverse environmental
conditions, the important considerations
include the correct choice of stone, the accurate
design of stone fixing systems, and
the accurate installation of stone.
Criteria in the choice of stone
for cladding
As the use of stone for cladding purposes
is a solution for the lifespan of the
building, the choice of the stone should
be on the basis of technical and design
aspects. These apply both generally and
in the specific case of curtain wall systems.
Architectural and aesthetic factors will
determine the initial selection of the stone.
These are dependent upon the vein
patterns and/or grains; the extensive
colour spectra; the proposed cladding
layout which divides units into various
shapes and sizes; and the surface finishes
which best enhance the inherent aesthetic
qualities of the cladding.
The main factors determining the ultimate
choice of stone include the production
technology available for processing stones,
the chemical-physical properties of the
stones and their suitability under the environmental
conditions at the location of the
project, the mechanical properties of the
stones and whether they will enable the material
to withstand the stresses caused by the
load conditions and the fixing systems used,
and the total cost of the cladding.
Stone fixing systems
After completing the theoretical and practical
tests to assess the quality and chemicalphysical-mechanical
properties of the stone
materials selected, and after taking into account
the architectural design of the cladding
and the environmental and loading
conditions, it is necessary to select the most
suitable stone fixing system.
This involves the study of the design and
distribution of the various notches possible,
to accommodate the mechanical fixing elements
in the stone units.
In addition to the mechanical aspects, the
fixing system depends on the cladding construction.
If the cladding system uses trussbacked
panels, a decision must be made as
to whether or not it is better to fix the back
of the stone slabs or their edges to the truss,
depending on whether the truss is prefabricated
on a horizontal or vertical plane.
If the prefabricated panels are to be transported
or handled in a position that is different
from their position on the building,
the fixing system must be able to compensate
for such handling, or extra fixing elements
should be used for this purpose.
The replacement of stone units, where
necessary, is also a consideration. Last, but
not least in the order of importance, is the
cost of the fixing system.
There are two main types of fixing systems
- isostatic and hyperstatic. In the
isostatic system, the stone units are fixed
at very few points, such that it is sufficient
to guarantee the statics. This is the classical
system with four pins inserted into
four holes positioned on two opposing
sides of rectangular panels. Owing to inaccuracies
in the manufacture and installation
of the material, only three out of the
four pins work at any one time, except
when the stone elements and the main
support structure undergo great deformations.
Under the hyperstatic system, the
stone units are fixed to the main structure
using numerous fixing points on the reverse
side of the units, using continuous
profiles fixed at the edges or by gluing the
units to the back support, as in the application
of very thin stone on to panels in a
honeycomb structure.
The main difference between the two
types of fixing, lies in their static behaviour.
In the isostatic system defined by ‘local fixing’,
the design of the stone units and the
metal fixing elements must take into account
the fact that if a fixing point fails, then the
stone unit may detach itself from the supporting
structure and fail. This may require
the use of thicker panels or even the application
of a reinforcement to the reverse side
of the panels, which comes into play if the
panels break. This fixing system makes the
stone units independent of the supporting
structure which is deformed by the external
forces (loads, temperature variations and
other differentials). On the other hand, in the
hyperstatic system defined by ‘spread fixing’,
where the stone panels are fixed at numerous
points, it may be possible to prevent
the panels from falling, if they break. This
system permits the use of thinner panels but
they could then become deformed in the
same way as the supporting structure.
Safety in the application of stone
materials for cladding
The use of stone materials for cladding
purposes, from the point of view of safety,
is partly determined by the static and dynamic
loads applied, the consequent internal
stress conditions, the reaction of
stone materials to environmental conditions,
and the mechanical resistance to
stone, and partly determined by the professional
expertise employed during the
design of the fixing systems, the production
processes used on the stone, and the
installation of the materials.
In order to maintain the necessary
safety levels, while still respecting the
project requirements, it is essential to bear
in mind the factors directly concerning the
stone material itself, the fixing system and
the installation. The uncertainty regarding
the loading conditions has practically been
eliminated by the norms which impose
much greater loads than those which occur
in reality and by tests such as those
conducted in the wind tunnel. The uncertainty
regarding the internal stress of stone
materials is minimised by very accurate
structural analysis carried out with the aid
of sophisticated computer programs.
As the results of surveys carried out on
a number of buildings in the US have
shown, the main causes for the collapse of
stone elements in curtain wall systems are
not directy linked to the inherent mechanical
properties of the stone. They are due,
rather, to the corrosive agents present in the
environment; the unsuitable design of the
fixing systems (as a result of not fully taking
into account the mechanical properties
of the stone around the fixing points); the
inexactness in the cutting of the stone elements,
so great sometimes, that the resistance
around the fixing points is reduced, as
in the case of oversized holes to accommodate
metal fixing pins with axes that do not
respect the project guidelines; the technological
processes which cause micro-cracks
in the material, thereby weakening it; and
inadequate installation by workers who do
not possess the required skills.
All these factors can be checked by
closely examining the behaviour of the materials
selected, in similar projects and under
similar conditions, and by running a
series of tests on the materials, both before
the start of production and during production.
These tests are even more important
for new materials as it is necessary to verify
their suitability and the limits for their application
in curtain wall systems. An even
more effective way of controlling such factors,
is by carrying out quality control procedures,
both at the production and installation
stages, and developing the ‘fail-safe’or
‘safe-life’ design criteria on the basis of the
fixing system chosen.
Fail-safe systems involve a high degree
of hyperstaticity and prevent the total collapse
of the stone elements in the event of
breakage, by keeping them fixed to the support.
Moreover, it is possible to discover and
replace broken stone elements during normal
maintenance operations (cleaning). In
the case of safe-life systems that are mainly
adopted in isostatic systems, the fixing systems
are designed to exclude any breakage
of the systems themselves or of the stone
elements. This makes the cladding as longlasting
as the building itself.
Safety criteria
Safety criteria in the use of curtain wall
systems are of paramount importance if
designers are to be convinced to use stone
in preference to other materials. The absence
of complete information regarding
the chemical-physical-mechanical properties
of stone materials (available for industrial
products) tends to dissuade many
designers from specifying their use.
Unfortunately there are no current
laws to establish minimum safety standards
on the basis of the data available regarding
the behaviour of stone materials.
There are no ‘resistance criteria’ for
stone materials, in order to calculate the
ideal stress, or the breakage point, and to
compare it to an allowable stress taken
from the results of tests on the materials.
What actually happens is that either
the designer or whoever draws up the
project specifications, or the technical adviser
on the end-use, has to determine not
only the ultimate stress but also the acceptable
one. The result is that the same project
conditions can generate a variety of different
safety factors and analysis criteria,
based on the results of the tests.
One answer to this is the ‘stone safety
criteria’. As far as resistance is concerned,
to calculate the ultimate stress, reference
is made to the Huber-Von Mises criterion,
but in some cases involving shear stress,
this proves to be unfavourable.
For example, marble’s ultimate shear
strength, when cut, can even be double its
tensile stress, and so here, HV Mises’s
theory could be replaced by Rankine’s or
Mohr’s, both of which are more suitable
for materials with limited plastic properties.
As regards the definition of the permissible
stress, reference is made to the
normal distribution of probability, and in
accordance with the current rule by which
it is governed, the average value and the
standard deviation are calculated, on the
basis of the results of the tests referring to
the resistance of the stone material. In this
way, the k values are defined, depending
on the stress fractile, the number of results
available and the safety factor laid down
by the project specifications. The safety
factors which differ according to whether
they refer to the overall resistance of the
stone element or just to the resistance
around the fixing points, are given a wide
range of variability. They can however be
given minimum values, close to those
under existing regulations governing
usual building materials, if quality control
procedures are carried out during production
and installation of the materials, and
fixing systems are accurately designed.
Design and execution aspects
Curtain wall systems allow for the manufacture
of prefabricated panels at the
workshop. This has several advantages.
It is possible to manufacture lightweight
panels with a metal truss. This facilitates
installation operations and the sizing
of the perimeter structure of buildings,
especially when they are high-rise buildings
located in seismic areas. Production
is more accurate as all the machines in the
workshop can be made use of, in carrying
out delicate operations such as the waterproof
sealing of joints, and there is better
quality control indoors.
Manufacturing times are reduced and
productivity is increased. The prefabricated
panels can be made immediately after work
on the main structure of the building has
begun, and installed soon after, therefore
anticipating the internal finishing operations.
The assembly of the panels in the
workshop as opposed to the building yard
increases production, with the additional
benefit that weather conditions do not affect
the process.
Continued on page 7 >>>
6 Roof & Facade Asia • October 2004