Stress-Rated Lumber

"Engineered lumber" that will influence wood building design, change construction economics, and simplify retail lumber yard ordering and inventorying problems is cominglff the production line of Potlatch Forests, Inc., in Lewiston, Idaho.

It is EMSR lumber, electro'mechanically stress-rated structural lumber with its stifiness and strength so precisely measured that an entire electronic computor system must be used. This allows the accurate strength rating to be marked on each piece as it is tested.

The Continuous Lumber Tester (CLT'I), developed by Industrial Sciences, Portlando Oregon, in cooperation with Potlatch, ac' curately identifies lumber grades by strength rather than by appearance or species.

Here is the step-by-step story, When the first electric eye (1) sees the lumber, the first test section is alerted. Then the lumber is gripped by the powered clamp-roll units (2 and 5) which do two thingsfeed the lumber into the machine and prevent end.whip and vibration from affecting the measurements. The next sensor (4) alerts the second test section. When the wood passes the next sensor (6), it activates the first transducer (3) which deflects the wood 5/l6th of ai inch and transmits the measurements of the foroe necessary to maintain this deflection to the computor (10), This measurement is made every six inches along each piece of lumber. The lumber is advanced to the final powered roll section (8) which grips the wood. The fourth sensor (9) activates the the second transducer 0) which takes readings of the force needed to maintain the 5/16th of an inch deflection in the opposite direction. Finally, when the end of the wood passes the last sensor (9), all the data from the storage section of the computor is transmitted to the brain section which instructs the stamping mechanism to mark the proper E-rating'

In this new system, termed E-rating, each piece of lumber is accurately per' tested to determine its individual struc' tural capabilities. The results are far more precise than can be ascertained through visual stress grading. Under Western Pine Association grading rules, Potlatch's elec' tro-mechanical stress-rated (EMSR) lumber may be sorted into five grades based entirely on strength. Therefore, there are considerably fewer grades for lumber deal' ers to specify and stock than lumber graded by systems using complete sets of spans for various dimension grades for each species.

Visual stress grading, standard through' out the nation's structural lumber industry today, is based upon appearance of each piece of lumber graded. Knots, their sizes and relationship with each other, slope-of' grain, and numerous other visual char" acteristics are considered.

But heavy safety factors are assigned visual stress grades which penalize between 50 and 98% oI all structural lumber. Some pieces tested showed they could do as much as 10 times the work which they are allowed under current grades.

"Well over half o{ all structural lumber produced today is under-rated by the common grading system, and consequently is undersold, and is inefficiently used," reports Dr. H. B. McKean, director of research for Potlatch.

Electro-mechanical stress-rating is a completely automatic process. The testing, (Contirucd on Page 60)