{"id":2218,"date":"2019-10-25T10:53:18","date_gmt":"2019-10-25T10:53:18","guid":{"rendered":"https:\/\/blogs.napier.ac.uk\/cwst\/?p=2218"},"modified":"2019-10-25T10:53:19","modified_gmt":"2019-10-25T10:53:19","slug":"c-and-t-classes","status":"publish","type":"post","link":"https:\/\/blogs.napier.ac.uk\/cwst\/c-and-t-classes\/","title":{"rendered":"Strength classes C and T"},"content":{"rendered":"<p>The 2016 version of EN338 <a href=\"https:\/\/blogs.napier.ac.uk\/cwst\/en338-whats-happening-with-grades\/\">brought a few changes<\/a>. One of these changes was the addition of a table of tension strength classes (T classes).<\/p>\n<p>Tension strength classes are intended for use in making glulam \u2013 and they have been in use for quite a long time already. The inclusion in EN338 is intended to simplify matters across all timber related standards.<\/p>\n<p><a href=\"https:\/\/blogs.napier.ac.uk\/cwst\/quick-summary-timber-strength-grading\/\">Grading is based on three primary properties: strength, stiffness and density<\/a>. For <a href=\"https:\/\/blogs.napier.ac.uk\/cwst\/c-d-stand\/\">C and D strength classes<\/a> the key strength is the major axis bending strength. Since you can only destructively test timber in one way, the other properties have to be conservatively estimated from the three properties we measure when we establish the grading rules (see EN384:2016 Table 2).<\/p>\n<blockquote><p>So, for C and D classes, tension strength is a secondary property, conservatively estimated from bending strength. (The same is true of shear strength and compression strength parallel to grain).<\/p><\/blockquote>\n<p>The difference between tension grades and bending grades is that tension grades are established based on the results of tension tests, rather than bending tests. That means that bending strength for the strength class is estimated from tension strength rather than the other way around. This explains why, if you compare the values for C and T grades you will see that tension strength is better for T grades.<\/p>\n<blockquote><p>So, for T classes bending strength is a secondary property, conservatively estimated from tension strength. (The same is true of shear strength and compression strength parallel to grain).<\/p><\/blockquote>\n<p>The consequence of this is that <strong>there is no simple one-to-one correspondence between C and T classes<\/strong>.\u00a0 You can say that the requirements of a T class can be met by a certain C class &#8211; but this equivalence does not work in the other direction.\u00a0 You can find the first kind of correspondence (T met by C) in Table 1 of EN14080:2013 but be aware that this is not correct to the 2016 version of EN338 for classes T12 and below because this revision also changed the tension strengths of the bending classes (based on <a href=\"https:\/\/www.vtt.fi\/inf\/pdf\/workingpapers\/2011\/W179.pdf\" target=\"_blank\" rel=\"noopener noreferrer\">Gradewood test data<\/a>).\u00a0 Table 1 of EN14080:2013 has a footnote with dated reference to EN338:2009.<\/p>\n<p>You can work out the correspondence of C and T classes by comparing all property values in the most recent version of EN338.\u00a0 At time of writing, this is EN338:2016 and the correspondence is as follows:<\/p>\n<p>T classes can be met by C classes<br \/>\nT8 by C16<br \/>\nT9 by C18<br \/>\nT10 by C18<br \/>\nT11 by C20<br \/>\nT12 by C22<br \/>\nT13 by C22<br \/>\nT14 by C24<br \/>\nT14.5 by C24<br \/>\nT15 by C27<br \/>\nT16 by C30<br \/>\nT18 by C30<br \/>\nT21 by C35<br \/>\nT22 by C40<br \/>\nT24 by C40<br \/>\nT26 by C45<br \/>\nT27 by C45<br \/>\nT28 by C50<br \/>\nT30 by C50<\/p>\n<p>C classes can be met by T classes<br \/>\nC14 by T9<br \/>\nC16 by T10<br \/>\nC18 by T12<br \/>\nC20 by T14<br \/>\nC22 by T15<br \/>\nC24 by T18<br \/>\nC27 by T21<br \/>\nC30 by T22<br \/>\nC35 by T26<br \/>\nC40 by T30<br \/>\nC45 (not possible)<br \/>\nC50 (not possible)<\/p>\n<p>The difference in direction is quite big at the high end.\u00a0 T24 can be met with C40, but T24 is only sufficient to meet C30. C40 requires T30.<\/p>\n<p><a href=\"https:\/\/blogs.napier.ac.uk\/cwst\/wp-content\/uploads\/sites\/23\/2019\/10\/T-and-C.png\"><img loading=\"lazy\" decoding=\"async\" class=\"aligncenter wp-image-2220 size-medium\" src=\"https:\/\/blogs.napier.ac.uk\/cwst\/wp-content\/uploads\/sites\/23\/2019\/10\/T-and-C-300x119.png\" alt=\"\" width=\"300\" height=\"119\" srcset=\"https:\/\/blogs.napier.ac.uk\/cwst\/wp-content\/uploads\/sites\/23\/2019\/10\/T-and-C-300x119.png 300w, https:\/\/blogs.napier.ac.uk\/cwst\/wp-content\/uploads\/sites\/23\/2019\/10\/T-and-C-768x306.png 768w, https:\/\/blogs.napier.ac.uk\/cwst\/wp-content\/uploads\/sites\/23\/2019\/10\/T-and-C-1024x407.png 1024w, https:\/\/blogs.napier.ac.uk\/cwst\/wp-content\/uploads\/sites\/23\/2019\/10\/T-and-C.png 1214w\" sizes=\"auto, (max-width: 300px) 100vw, 300px\" \/><\/a><\/p>\n","protected":false},"excerpt":{"rendered":"<div class=\"mh-excerpt\"><p>The 2016 version of EN338 brought a few changes. One of these changes was the addition of a table of tension strength classes (T classes). <a class=\"mh-excerpt-more\" href=\"https:\/\/blogs.napier.ac.uk\/cwst\/c-and-t-classes\/\" title=\"Strength classes C and T\">[&#8230;]<\/a><\/p>\n<\/div>","protected":false},"author":57,"featured_media":510,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_crdt_document":"","jetpack_post_was_ever_published":false,"footnotes":""},"categories":[1],"tags":[152,68,20,151],"class_list":["post-2218","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-uncategorized","tag-bending","tag-en338","tag-strength-grading","tag-tension"],"yoast_head":"<!-- This site is optimized with the Yoast SEO plugin v27.3 - https:\/\/yoast.com\/product\/yoast-seo-wordpress\/ -->\n<title>Strength classes C and T - Centre for Wood Science &amp; Technology<\/title>\n<meta name=\"robots\" content=\"index, follow, max-snippet:-1, max-image-preview:large, max-video-preview:-1\" \/>\n<link rel=\"canonical\" href=\"https:\/\/blogs.napier.ac.uk\/cwst\/c-and-t-classes\/\" \/>\n<meta property=\"og:locale\" content=\"en_US\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"Strength classes C and T - Centre for Wood Science &amp; Technology\" \/>\n<meta property=\"og:description\" content=\"The 2016 version of EN338 brought a few changes. 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