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4. STRUCTURAL DESIGN OF LVL STRUCTURES
The structural design of LVL structures is similar in purpose
and principle to any load-bearing wood-based structure: to
verify that the structure fulfils its strength, serviceability and
other structural requirements. The basis of limit state design
is to verify, based on the partial safety coefficient method, that
the design value of an action E
d
is smaller than the design val-
ue of the resistance R
d
of a section, structural member or con-
nection:
E
d
≤ R
d
where
E
d
is the design value of the effect of actions such as internal
force, moment or a vector representing several internal
forces or moments
R
d
is the design value of the corresponding resistance.
4.1 BASIS OF STRUCTURAL DESIGN
The Eurocode design system has been used for this purpose
in Europe since 2010 with country-specific adjustment factors
that are defined in national annexes. For timber construction
the important parts of the Eurocode system are:
EN 1990 Eurocode 0
Basis of structural design
EN 1991 Eurocode 1
Actions on structures
EN 1993 Eurocode 3
Design of steel structures
EN 1995 Eurocode 5
Design of timber structures
Eurocodes are limit state design codes that have two main limit
states: Ultimate limit state (ULS) and serviceability limit state
(SLS). In ultimate limit state (ULS) design the requirement is
to verify that a structure has adequate safety against failure dur-
ing its whole designed service life. What is regarded adequate
is defined in the relevant building regulations. Serviceability
limit state (SLS) design evaluates whether the structure is fit
for purpose. In most cases, the building regulations do not de-
fine exact limit values for this evaluation. They provide recom-
mendations for the maximum level of deformation and, e.g.,
human-induced floor vibration, but it is ultimately up to the
contractor and the client to agree what is acceptable.
The risk of structural failure is depending on the proba-
bility that the expected actions are exceeded and the probabil-
ity that resistance of the structure is lower than calculated in
ULS design. Normally it can be presumed that the actions and
the resistance of a structure are random variables. When their
distribution functions are known, it is possible to calculate the
risk of failure by the methods of probability theories. These
methods, however are usually too complicated for structural
design in practice, but they can be used for code calibration
when the requirements are defined for building regulations and
in comparison between different construction materials. They
also have an important role e.g. in verification of the safety level
of simplified design methods, such as the partial safety factor
method used in Eurocodes
29, 30
.
The primary purpose of the design rules for load-bearing
structures is to prevent failures that can lead to the risk of hu-
man injury and to ensure buildings function properly accord-
ing to their intended use.
Building codes such as the Eurocodes provide approved
verification methods for verifying that the requirements are
fulfilled. They also present the loads and loading combinations
that the structures must resist. The Eurocodes have common
parts that are applied in all countries in connection with na-
tional annexes. The annexes specify country-specific safety fac-
tors and parameters that take into consideration climate and
geology specific conditions. In addition, national choices are
made in some alternative verification methods or adjustment
factors within them.
The general design requirements according to the Euroc-
odes are
29, 30
:
• Choice of structural system, structural design and construc-
tion work must be done by sufficiently qualified and experi-
enced persons.
• Work must be supervised adequately and quality assurance
implemented throughout the construction process from de-
sign offices and factories to workshops and building sites.
• Structures must be made of constructionmaterials and prod-
ucts defined in the Eurocodes or their harmonized standards
or in other harmonized technical specifications.
• Buildings must be adequately and regularly maintained
throughout their design working life.
• Buildings must be used for the purpose for which they have
been designed
The next generation of Eurocodes is currently under develop-
ment and scheduled to be ready for use after some years.
4.1.1 Actions (Loads)
Actions are loads that cause, e.g., bending moment, shear and
axial stresses and deformation of the structures. The determin-
ing actions and their combinations for each structure are evalu-
ated based on the type, magnitude and duration of the action at
the most unfavourable locations of the structure. Different load
combinations are determined as load cases. The load combina-
tions consist of the main loads combined with reduced values
of other loads that may be acting at the same time. Reduction
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