Civil Engineering Reference
In-Depth Information
facade panels, packing pieces, slotted holes and threaded rods.
Fit tolerances tend to be governed by conditions in the field,
so accurate pre‑erection surveys of support structures should
be undertaken prior to erection. See Section 16.10 for further
references.
In most respects, the design processes of aluminium and steel
structures are similar, which in particular explains the sharing
of Eurocode Execution Codes. However, there are noteworthy
differences in their physical and mechanical properties which
must be accounted for in the design process.
Panel strength is time dependent and decreases with load
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duration.
High exposure to solar radiation, warm‑air stratification beneath
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panels and/or other types of sustained loading can cause interlayer
creep.
Structural glass has three types of dimensional tolerance:
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Manufacturing tolerances: dependent on factory quality control.
Environmental tolerances: dependent on exposure and use of
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material.
Erection tolerances: dependent on site construction quality
The heat input in welded aluminium profiles eliminates some of
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favourable consequences from heat treatment or strain hardening.
This decreases the local elastic limit resulting in strength redistri‑
bution along the cross‑section profile.
The coefficient of linear expansion is higher than that of steel.
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control.
European and American codes of practice and material stand‑
ards are well developed with respect to manufacturing, envir‑
onmental and placement tolerances as referenced below.
In most cases structural glass is fabricated in a controlled fac‑
tory environment, allowing accurate tolerances to be achieved.
Fit tolerances tend to be governed by conditions in the field,
so accurate pre‑erection surveys of support structures should
be undertaken prior to erection. See Section 16.10 for further
references.
The brittle behaviour of glass and its potential for accidental
cracking means that potential panel failure and robustness cri‑
teria must be assessed during design such that:
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The low density of aluminium and its high strength to weight ratio
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are the main drivers for its use. These are broadly favourable but
they do present disadvantages; in cyclical loading conditions the
ratio of live/dead load is low as compared to steel making fatigue
design critical; its low density also makes an aluminium structure
prone to vibrations so dynamic behaviour of the structure must be
considered.
The aluminium Young's modulus is about one‑third that of steel;
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for equivalent steel and aluminium sections its deflections are
therefore proportionally higher; combined with its low density,
this also lowers the fatigue strength of aluminium to about half
that of steel.
glass panels must be replaceable;
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Given these constraints, and the fact that aluminium designs
often use slender, thin walled sections, torsional buckling, shear
deformation and shear stability are key design constraints.
Tighter specification of movements and tolerances may be
required in the following cases:
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panel choice and its integration into a structure depend on its po‑
tential failure consequences; progressive collapse must be avoided
by providing residual structural capacity after failure to reduce
post‑cracking hazards.
This usually requires one of the laminates to be heat‑strength‑
ened glass, designed to support loads in a temporary
conditions.
Tighter specification of movements and tolerances may be
required in the following cases:
Facade structures to high‑rise towers with large deflections.■■
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Dynamically loaded facade or canopy structures with fatigue‑sensi‑
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tive connections via quality control of welding and connections.
Highly architectural features such as glass stairs, floors and
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eye‑level facades.
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high‑rise towers and dynamically loaded facade or canopy
structures;
highly architectural features such as glass stairs or floors.■■
16.4.6 Glass
Structural glass exhibits the following material in‑service char‑
acteristics, and structural design and calculation of movements
should account for these issues:
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See Chapter 21:
Structural glass
, for more information on
glass.
16.4.7 Membrane tensile structures
Membranes exhibit the following material in‑service charac‑
teristics, and structural design and calculation of movements
should account for these issues:
Applied load deformation is elastic and time‑dependent; brittle‑
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ness precludes design in the plastic range.
Response to environmental changes and applied loads is
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isotropic.
Response to low temperatures (i.e. non‑fire) loads is linear
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Applied load deformation is elastic and plastic above and below
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elastic.
Laminated panel interlayer strength can degrade in high humidity
yield stress.
Response to environmental changes and applied loads is
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conditions.
anisotropic.
