Structural mass timber, in particular glued-laminated timber (GLT) and cross-laminated timber (CLT), are assigned design values based on the recipe of lam grades used to make up the laminated beam or panel. These lam grades are drawn from structural joist and plank grades, with slightly stricter requirements on characteristics that affect the gluing of the lams – for example, wane, skip, checking, etc.
Let’s dig into the critical aspects of structural lumber grading… in the United States, various lumber grading agencies exist under the umbrella of the American Softwood Lumber Standard. These grading agencies are regional – for example, Western Wood Products Association (WWPA) exists for the grading of many western softwood species; and Southern Pine Inspection Bureau (SPIB) exists for the grading of southern pines. In Canada, National Lumber Grades Authority (NLGA) oversees the grading of Canadian softwood species of lumber.
The structural joist and plank grades are categories based on lumber characteristics that affect strength - most importantly, knots. Other characteristics such as pitch, shake, slope of grain, splits, and wane also factor into the grade categories.
Select structural (SS) is the top grade, followed by No.1, No.2, and No.3.
When these structural grades are re-graded for laminating into GLT or
CLT – grading rules for lam grades are outlined in the American National Standards Institute (ANSI) standard ANSI 117 for the United States, and for Canada – in the Canadian Standards Association (CSA) standard CSA-0122.
There are visual lam grades and mechanical lam grades. A visual grade is similar to the SS, No.1, No.2, No.3 above - except that it has the stricter lam grade requirements - these are renamed L1, L2, L3 and tension lam - for Western species. If lumber meets the lam grade, it can then be used in the GLT or CLT recipe – and design values can be assigned to the GLT and CLT.
When lam grades are determined by mechanical means – this means that lumber stiffness is determined by a machine. The machine can be a setup that induces a flex in the board and this directly measures the response needed to determine modulus of elasticity (MOE), often called E. There are other ways of estimating E, such as longitudinal stress wave – where a ping or thump is induced on the end of a board. The faster the speed of sound through the board, the higher the calculated E. Whatever method used to measure or estimate E, the GLT and CLT standards require that these values be calibrated to the truest measure of board stiffness – which is a 100:1 span-to-depth flat-wise test of board deflection.
For lumber strength, we cannot break every board – so strength for the different grades is established by testing samples of boards within each grade. The strength results of these representative samples are the assigned strength of the grade. Safety factors are assigned to these strength results, and we arrive at the design value of lumber in bending (Fb).
Historically, these evaluation lines were stand alone systems and costly to implement, smaller measurement units, such as thumping units, can be integrated into grading lines, allowing manufacturers to establish their own structural grade per board, providing reduced reliability and lower cost of goods when grading lumber.
Grading for visual characteristic has been done manually for more than a century, with operators inspecting each board, assessing, and either marking cut points or sorting full boards into graded piles. This method is tried and true and is still employed across the industry, but depends on the specific operator on any given day.
This method can lead to consistency issues and is dependent on skilled labour, which can be hard to find or hard and harder to replace. However, as technology has progressed, visual scanners are beginning to dominate the industry. Visual scanners, either transverse or through feed, can scan the boards on all four (or even six) faces and using camera systems, lasers, x-ray systems or a combination of all of these. Using these machines, defects can be detected near-perfectly at speeds of more than 1,000 feet per minute. These automated scanners are programmed and trained on specific grading rules, species and end product, with some brands now using AI modules to help train scanners to fine tune detection and grading automatically.
Concurrently, scanners can track and log each board, aiding in quality control practices. These scanners are mainly implemented in automated lines which can either print grade stamps or labels, print exact cut lines on boards, feed data directly to automated cross-cut saws, or feed boards into automated stacking solutions. One scanning system can replace five or more manual graders while improving grading results and improving material yield.
While it does not directly correlate to certain or specific lam-grades, it is also critical to look at lumber’s moisture content. For a rule of thumb, mass timber and engineered wood products look for lumber with a moisture content of less than 16 per cent. The moisture level in the lumber is important for both the stability of the lumber’s shape (so it does not warp and twist as it dries further) as well as for the glue adhesion and reactivity with adhesive components. While most lumber comes kiln dried, not all boards are checked and vetted before shipment and the species, time of year, process and length of kiln drying can cause inconsistent moisture content across a shipment, or even lift of lumber.
Moisture meters are commonly used for moisture measurement of each board, determining if it fits within the allowable range for structural lumber. These meters or scanners can be as simple as a pin-type meter that is inserted temporarily into each board and measures electrical resistance from pin to pin ranging up to a through-feed electromagnetic scanner. Automated moisture scanners, such as the electromagnetic models, can be installed in line, and can detect moisture in real time as a board travels through a system without operator intervention, while also allowing for board density compensation and data tracking of each board for quality control purposes.
Putting all these aspects together, an automated system with a moisture scanner, visual scanner and strength measurement unit can be found in factories across North America, each grading for the highest structural standards while maximizing material yield and minimizing fiber waste. So, the question is, does your process make the grade?
