Journal of Civil Engineering and Architecture 11 (2017) 16-28 doi: 10.17265/1934-7359/2017.01.003 Design and Experimental Analysis of NLT Trussed Rafter in Yellow Pine Emanuelle Graça Recco 1 , Jorge Daniel de Melo Moura 1 and Everaldo Pletz 2 1. Center of Technology and Urbanism, Department of Architecture and Urbanism, Londrina State University, Londrina CEP: 86057-970, Paraná, Brazil 2. Center of Technology and Urbanism, Department of Structures, Londrina State University, Londrina CEP: 86057-970, Paraná, Brazil Abstract: The roof system of social housing in Brazil generally consists of components made out of native forest lumber of high market value. Taking into account the increasing number of planted forests and the need to develop new products and to add value to this timber, this work deals with the development and structural analysis of a roof system using yellow pine plantation wood (Pinus spp), a sustainable material which however presents many defects. The NLT (laminated nailed timber) technology was chosen as it allows the use of shorter length and smaller cross section pieces, eliminating major defects. Seven samples of structural trussed rafters in NLT were tested; six made out of graded timber and one ungraded in order to verify the impact of the wood grading in the structural performance of the model. The results showed that the trussed rafter system in NLT meets the necessary structural performance requiring poor conditions of infrastructure for manufacture process, and that the graded wood samples showed better performance than the ungraded one. Key words: Roof systems, yellow pine wood, laminated nailed timber, trussed rafter, design methodology. 1. Introduction When considering the social circumstances of the Brazilian housing deficit and the precariousness of living conditions today, it appears that there is a vast field of research of manufacturing and use of prefabricated building systems with plantation lumber. The construction sector has shown the increasing need of product development and processes that promote the reduction of costs while improving the quality of construction systems and housing. Plantation wood occupies increasing space in the market, due to its rapid growth. The good quality wood is the one that has fewer defects, whether related to the wood itself or those from the production chain. Although the yellow pine (Pinus spp) wood presents good features that make it easy to transport Corresponding author: Emanuelle Graça Recco, M.Sc., research fields: wood construction and wood structures. E-mail: [email protected]. and use for various purposes, it also presents aspects that deserve further attention during the tree’s growth to obtain a final product with better quality. The traditional construction systems in Brazil have not contributed to overcoming the existing housing deficit and, consequently, prefabrication is a viable path to be followed. It is known that in prefabricated structures, the connections are extremely important. According to Ref. [1], connections enable the transportation of the parts, being responsible for the continuity and the overall behavior of the buildings as well as being the cause of most problems and structural flaws. The trussed rafter system is known and widely used as a prefabricated subsystem for roofing in European countries, the United States and Canada. The NLT (nail laminated timber) is an effective alternative to enable the use of smaller structural timber pieces, considering its design and standard requirements. This study aims to develop an alternative for D DAVID PUBLISHING
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Journal of Civil Engineering and Architecture 11 (2017) 16-28 doi: 10.17265/1934-7359/2017.01.003
Design and Experimental Analysis of NLT Trussed
Rafter in Yellow Pine
Emanuelle Graça Recco1, Jorge Daniel de Melo Moura1 and Everaldo Pletz2
1. Center of Technology and Urbanism, Department of Architecture and Urbanism, Londrina State University, Londrina CEP:
86057-970, Paraná, Brazil
2. Center of Technology and Urbanism, Department of Structures, Londrina State University, Londrina CEP: 86057-970, Paraná,
Brazil
Abstract: The roof system of social housing in Brazil generally consists of components made out of native forest lumber of high market value. Taking into account the increasing number of planted forests and the need to develop new products and to add value to this timber, this work deals with the development and structural analysis of a roof system using yellow pine plantation wood (Pinus spp), a sustainable material which however presents many defects. The NLT (laminated nailed timber) technology was chosen as it allows the use of shorter length and smaller cross section pieces, eliminating major defects. Seven samples of structural trussed rafters in NLT were tested; six made out of graded timber and one ungraded in order to verify the impact of the wood grading in the structural performance of the model. The results showed that the trussed rafter system in NLT meets the necessary structural performance requiring poor conditions of infrastructure for manufacture process, and that the graded wood samples showed better performance than the ungraded one.
Fig. 11 Deflection curves to the seven samples—service loading (6 kN).
Fig. 12 Load (kN) when reaching the service state limit (L/200 and L/300).
second cycle, Sample 1 showed the lowest K value
and the ungraded timber sample resembled Sample 6
staying among the lowest values and thus showing
lower stiffness.
To represent the global behavior of the truss, vertical
displacement was measured through gages placed
underneath the three posts.
The NBR 7190/97 determines that the service
deflection must be less than L/200 and the revision
text of the same standard, proposes that it should be
less than L/300, L being the total span. Since the total
span considered is 6,000 mm, the limit deflection to
the application of the service loading should be less
than or equal to 30 mm and 20 mm (text under
revision). In Fig. 11, one can see that the deflections
to all samples did not exceed 14 mm.
Design and Experimental Analysis of NLT Trussed Rafter in Yellow Pine
27
Sample 1 showed the lowest deflection, less than
5 mm and the ungraded timber sample showed the
highest deflections.
In all cases, the service state limit was the
deflection limit. Fig. 12 shows the load under which
the deflections reached 30 mm (L/200) and
20 mm (L/300) (former and new standard
limitations, respectively). To the 30 mm
deflection, the load ranged from 27 to 35 kN; and the
to 20 mm deflection, the load ranged from 22 to
27 kN.
The weakest point of the structure in terms of
failure is the connections and not the wood, since all
samples displayed highly elevated failure load, and
low variability. However, concerning stiffness, the
graded timber samples performed better than the
others.
5. Conclusions
This paper presented the analysis of seven samples
of trussed rafters made out of yellow pine nail
laminated timber-NLT, and the following conclusions
could be drawn:
The trussed rafters are widely used as roof structures
in the US, Canada and some European countries, that is,
its efficiency is proven. The fabrication of structural
elements in NLT proved to be a viable option
considering the use of pine plantation timber, which
features lots of natural defects.
Before the test of the structure, the experimental
assessment of splices in NLT tensioned bars was
performed in order to check their capacity, and to
determine the nail type to be used.
The tensile test showed that concerning splices in
NLT beams, it is necessary to ensure a minimum
spacing between two consecutive splices of two blades,
determined from the applied load. The experimental
results showed that the three types of nailed
connections are different and therefore require specific
mathematical equations unlike the only model proposed
by the standard NBR 7190/97.
Connections with smooth nails showed the worst
performance on both resistance and rigidity.
Connections with nails with bent tips showed the best
results of rigidity, while connections with ardox nail
achieved the best results with respect to resistance.
These latest two connection types showed results
close to each other and above the smooth nailed
connections.
The connections presented the necessary ductility
and for testing of the full-scale trussed rafter samples
the ardox type nail was chosen due to its higher
resistance and lower coefficients of variation.
Seven samples of trussed rafters were tested, six
fabricated with graded timber (as required by the NBR
7190/97) and one with ungraded timber. All samples
showed similar behavior during the test and the failure
occurred in the tensioned bar of the bottom chord
close to the side post. The tensile strength results were
quite high, with average of 60 kN, and low coefficient
of variation.
With the application of service load (6 kN),
deflections were below 1.4 cm lower than that required
by the Brazilian standard (30 mm current requirements
and 20 mm to the new under-revision text respectively).
The service state limit occurred under the load ranging
from 27 to 35 kN for 30 mm deflection and from 22 to
27 kN to 20 mm deflection.
The deflections were higher in the ungraded timber
sample, showing that grading benefits the performance
of the structure. However, in this case, it did not
invalidate the structure since the load values, when
reaching the service state limit, were above that which
is expected and necessary for the proper functioning of
the system.
The results of failure showed the system is highly
dependent on the quality of the connections. Regarding
the rigidity, the quality of wood had greater
influence, since the ungraded sample showed higher
deflections.
The study of tensioned connection with dimension
timber was very helpful and led to improvements in the
Design and Experimental Analysis of NLT Trussed Rafter in Yellow Pine
28
connecting system of the splices of NLT beam, and
consequently allowed enhancement in the
stiffness performance and strength of the NLT trussed
rafter.
The results could also confirm that the provision of
an adequate number of nails is able to convey
effectively the stresses and significantly increase the
performance of the structure.
The trussed rafter focused on here, achieved a
performance highly above the requirement (10 times
the service load), allowing resizing parts with
consequent saving of timber and nails.
The trussed rafter developed in this study was
supposed to be mostly applied to social housing
however, due to the excellent performance observed, its
application to several situations in civil construction is
possible.
Acknowledgments
We are very grateful to the Fundação Araucária for
the scientific and technological development of Paraná
state for funding this research.
References
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