Joining FRP and timber
presents several interesting advantages and potentials. The
principal need is to provide additional strength and stiffness
to structural member made of timber or glulam, with respect
to the mechanical properties that timber alone is able to provide.
In the past, before starting to think about using FRP for structural
strengthening, several retrofit techniques have been tried in
order to confer strength and stiffness increase to structural
timber members. Due to difficulty of installation, excessive
costs and the low versatility of the approached technological
solutions, such solutions have not been adequately divulgated
and commercialized and almost none of these have become of use
in engineering practice.
With respect to the aforementioned problems, advanced composite
materials present several advantages such as ease of installation
and extreme versatility, both in retrofitting existing structures
as well as in designing new ones. Joining FRP and timber is
particularly successfull because of the excellent compatibility
between these two materials. One of the most important characteristics
of timber in fact is the lightness, that absolutely is not subjected
from FRP strengthening. At the same time, the most remarkable
defects of timber, such as the high mechanical disomogeneity
due to a high number of defects, are abolished thanks to the
sinergy with another structurally efficient material.
Thanks to the recent publication of the CNR (National Research
Italian Council), “Guide for Static Strengthening of Timber
Structures through Fiber Reinforced Composites” (CNR-DT
201/2005), that guarantees tecnological/analitical concepts
for the design and installation of FRP, the use of such materials
has registered a remarkable increase.
The main applications of FRP in strengthening/restoring timber
strucutres regard:
1. Flexural
strengthening: such as single beams, floor joists or single members of more complex structural systems (roof trusses or frames). The strengthening consists of bonding FRP laminates or sheets of different fiber type to the section in order to improve strength and stiffness.
2.
Strengthening for in-plane loads: a typical example is the
application to timber floors, that offer limited stiffness and
low efficiency in terms of transmission of the in-plane horizontal
loads, despite presenting obtimal characteristics in terms of
lightness, strength, acustic isolation and compartmentation.
The aforementioned characteristics, that are necessary for seismic
design, can be easily realized creating a grid that solidarize
the timber floor with the thin reinforced concrete slab on top,
by creating a grid of FRP sheets installed on top of it.
3. Strengthening
of timber member joints: the use of FRP guarantees the reduction
of the rupture risk due to tensile stresses acting perpendicular
to wood fibers, to reduce the distance between the connections
and their offset from edges, increase their ultimate capacity
and their ability to dissipate energy when subjected to cyclic
loads. In practice, the most common retrofits regards joints
subjected to axial, shear and flexural stresses. We can distinguish
two types of FRP retrofit:
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