Introduction
Recently, a new system of national building codes based on the «State Building Codes» (DBN), which aims to replace the previous system based on the «Construction Standards and Regulations» (SNiP), has been implemented at an accelerated pace. At the same time, unfortunately, the new codes do not always show high quality of their development, thereby can be repeated several times with the introduction of appropriate clarifications, adjustments or even a fundamental change of certain parts of the document.
In some cases, the codes developed in one of the building structure design areas are not entirely consistent, but sometimes even directly contradict the rules of another industry. And by themselves, the new DBNs are not rarely intrinsically controversial and do not fully reflect the state of affairs and the available factual data in a particular area of design. All of this is due to the lack of a single focal point for the development of a national regulatory framework in Ukraine, on the one hand, and the lack of practical testing of building codes before they are put into operation, on the other hand.
Purpose
It is precisely this situation that is observed in the design of wooden building structures. From 01.02.2018 the new state standards DBN B.2.6-161:2017 have been introduced into the practice of designing such structures [4]. They replace the recently drafted codes DBN B.2.6-161:2010 [5], which in turn were elaborated to replace SNiP II-25-80 [9].
Since, unfortunately, nowadays the tradition of accompanying codes with special comments or explanations in which their «bottlenecks» are further disclosed (such as «Manual» [10]), it only remains to the users to interpret them at their discretion. Therefore, the author of this publication would like to draw the attention of developers and potential users of the codes DBN В.2.6-161:2017 to the points that are not entirely clear, which at first reading caused him certain difficulties in terms of their practical use. This is the main purpose of this publication.
Methodology
The author of this publication for more than 15 years has been teaching the discipline «Wooden and Plastic Structures» for the 5th year students (previously – for specialists, now – for masters) of the specialization (specialty) «Industrial and Civil Engineering». Therefore, the accumulated experience in the use of normative literature in educational practice, as well as the available experience during laboratory studies with students, made it possible to estimate the recently introduced codes DBN B.2.6-161:2017 from a practical point of view.
Findings
The development and implementation of the codes DBN B.2.6-161:2017 was carried out by «V.N. Shimanovsky Ukrainian Institute of Steel Construction» LLC with participation of a number of experts of leading universities of Ukraine, in particular KhNUCEA, Lviv Polytechnic National University, KNUCEA. In the previous codes DBN В.2.6-161:2010 the main developer is the State Enterprise «State Research Institute of Building Structures», as well as specialists of the PSACEA. The codes SNiP II-25-80 were developed by specialists from four research institutions in the late 70's of the 20th century.
Immediately I would like to note that the introduced codes DBN В.2.6-161:2017 by their quality of preparation are much more elaborated and have a much smaller number of complicated issues and clear mistakes than the previously abolished codes DBN В.2.6-161:2010.
In general, the new codes DBN В.2.6-161:2017 support the course to harmonize the domestic and European regulatory framework in the field of construction. They use the terminology and the system of designations of characteristics oriented on Eurocode, such as the codes for designing concrete and reinforced concrete structures DBN B.2.6-98:2009 [3], as well as stone and reinforced masonry structures DBN B.2.6-162:2010 [6], and unlike the standards for the design of steel structures DBN В.2.6-161:2017 [8] and aluminum structures DBN В.2.6-165:2011 [7], which use the traditional domestic system of symbols. However, traditional design approaches typical of domestic practice are also preserved. This definitely extends the scope of application of the domestic codes, contributing both to changing the mentality of design engineers and to improving the design process itself. In particular, the main method for calculating the wooden structures in the codes is the method of boundary states. However, the probabilistic method is not even mentioned, even though it is recommended for use in accordance with the requirements of DBN B.1.2-14-2009 [2].
Also, among the positive points I would like to note the introduction of settlement expressions in the work of elements on torsion, as well as a section for evaluating the dynamic characteristics of bending elements, which were not present in the previous codes SNiP II-25-80. Particular attention is paid to modern wood products and its derivatives (LVL, OSB, MDF). Also, the wooden structure work theory with the consideration of nonlinear properties was substantially developed, which the author emphasized in his previous works (see, for example, [11]).
Additionally, the new codes DBN B.2.6-161:2017 regulate the issues related to ensuring the protection of wooden structures from fire, biological pests and corrosion. These issues in the previous codes SNiP II-25-80 were covered in a rather narrow way, and there was no explicit separation of constructive protection measures for various external factors.
However, the new codes DBN B.2.6-161: 2017 also have a number of negative points, which I would like to dwell upon in more detail. They are listed in the order according to the recommendations in the code text.
1. The exposure duration load class values given in clause 5.3.2 are not consistent with the classification of loads given in DBN B.1.2-2:2006 [1] both quantitatively and qualitatively. In particular, the order of distribution of load duration is not clear, since, for example, a seismic load can take several minutes, which, as a result, should be attributed to short-term loads. Also, there is the question of limiting the duration of permanent loads for 10 years.
2. The operating classes listed in clause 5.3.3 include a rather narrow temperature-humidity range, as compared to SNiP II-25-80. In turn, a number of calculation characteristics depend on these classes, which cannot be determined for other conditions, for example, conditions of unheated premises or in dry humidity mode, etc.
3. In the previous codes SNiP II-25-80, the specific conditions of the wooden structure work are factored in a number of special coefficients, denoted as m (par. 3.2). However, the new codes DBN B.2.6-161: 2017 present the system of only two coefficients – kh and kdef. The first of these is related to the size of the elements, served only as the recommended, actually replaces the four coefficients according to the previous codes for different types of sections. The second coefficient takes into account the rheological properties of wood and actually «replaces» two coefficients according to the previous codes for different types of loads. However, the open question is to take into account certain features of the real conditions of operation of structures. These include, for example, taking into account the impregnation of wood with flame retardants or antiseptics, or an elevated (lowered) operating temperature.
4. Section 9 and, in general, the text of the codes does not set the limits as to the boundary flexibility, at least for compressed elements This is, in the opinion of the author of this publication, is fundamentally wrong, since limitation of flexibility is one of the basic moments when designing structures from any materials [3, 6-8]. In the previous codes SNiP II-25-80, the boundary flexibility is also limited (table 14 [9]).
5. In clause 9.2, when calculating the stretched elements, the area on which all the weakening of the calculated section must be combined is reduced from 20 cm (according to the recommendations of the previous SNiP II-25-80 codes) to 15 cm. There is no justification for such a recommendation, which creates certain difficulties in the calculation of connections with numerical openings, for example, for nail joints.
6. In clause 9.4, when calculating bending elements, a recommendation is made regarding the need to combine all weakening of the cross-section, located at a length of 15 cm in one design section. This is a new requirement, absent in the previous codes SNiP II-25-80. However, such an approach raises a difficult question when calculating, for example, the horizontal element of the skew notch (Annex K). Traditionally, according to domestic professional literature [12, 13], this element is tested by two sections – weakened for central compression and complete for non-center compression. According to the new requirements it follows that the test should be performed only for the cross-section for non-center stretching, which requires additional justification
7. Paragraph 12.5 on the cylindrical nailed connections takes 22 pages out of 78 pages of the text of the codes (28%). For comparison, in the previous codes SNiP II-25-80 this paragraph occupied 4 pages out of 27 pages of the text of the codes (15%). In our opinion, this paragraph in the new codes DBN B.2.6-161:2017 would be expedient to structure, singling out a sequence of determining the bearing capacity of the connections of the type under consideration, the constructive requirements to them, as well as the scope of their application. Instead, all of these recommendations are actually in a mixed style, which makes it very difficult to implement them.
8. Among the standard characteristics of tensile strength listed in Annex B, there is no bearing stress. Instead, a link to the following characteristics is provided in annex K, which is dedicated to the contact connection. Also, according to the author, the codes paid little attention to the contact connections, although wooden structures with such connections (especially with skew notches) are found to be in operation not so rarely, for example, in previously constructed structures with wood truss covering.
9. There is a lack of complete information on such constructional elements as the dowel beams, and in particular the connection on the tongue pieces. Certainly, this kind of connections is not widespread and quite effective, however, as in the case of contact connections, similar structures are found in the design practice, even in the modern foreign one [15, 17].
10. Such a widely used in the domestic design practice concept as «wood grade» [12, 13] is mentioned only in one place of the codes (reference appendix G). The linkage of this concept with the classes of timber strength is given without any justification. Also, in our view, it would be advisable to supplement the codes with the recommendations or method for determining the type of wood for practical purposes, since this question is treated fairly freely in the existing professional and reference literature.
11. The transition to the use of strength classes should be accompanied by detailed explanations regarding the methodology for determining these classes for different wood materials, which is completely absent in the codes. In fact, in design practice there is a difficult situation, which is to correctly refer the existing wood or a product thereof to a specific strength class. As far as can be understood from the text of the codes for this purpose, it is necessary to conduct special laboratory tests, during which certain characteristics of strength will be determined. However, the use of the very class of strength with such an approach is simply superfluous.
In this regard, let us dwell separately on the comparison of the characteristics of the calculated strength of wood, which should be determined according to the new codes DBN B.2.6-161:2017 in relation to the previous codes SNIP II-25-80.
Under the new codes, the calculated resistance of wood should be determined by the expression:
, (1)
where fk – characteristic value of wood resistance by the Appendix B of DBN B.2.6-161:2017; γM – material reliability coefficient by the table 6.1 of DBN B.2.6-161:2017; kmod – conversion factor by the table A.1 of the Appendix А of DBN B.2.6-161:2017.
According to the previous codes, the calculated resistance of wood was determined without any additional calculations directly by the Table 3.
We will perform the comparison of the calculated resistance values for the most widespread case of solid softwood, for which М = 1.3 according to the new codes. Let us consider the cases of the most common, in design practice, types of loads by exposure duration – permanent, long-term and short-term. The results obtained are shown in Table 1, where the values for the 1st and 2nd operational classes according to the new codes (operation conditions of A1 and A2 according to the previous codes) were given in the numerator, and in the denominator – for the 3rd operating class according to the new codes (operation conditions of A3 according to the previous codes).
As can be seen from this table,
in general, the calculated resistances according to the previous
codes are higher than those by the new codes. To quantify this
difference the last column «Difference» of Table 1 shows the
calculated ratio of the largest value in the corresponding line of
wood resistance according to the previous codes to the smallest
value according to the new codes within the grades. It is seen that
the difference reaches
3 times.
The values of the calculated resistance in Table 1 are given for a constant load, when the coefficient kmod = 0.6 (0.5 for the 3rd operating class) according to the new codes DBN В.2.6-161:2017. For a long-term load this coefficient is equal to kmod = 0.7 (0.55 for the 3rd operating class), and for short-term load kmod = 0.9 (0.70 for the 3rd operating class). This means that for these types of loads, the difference in the values of the calculated resistance will decrease and will reach 2.5 and 2 times, respectively.