Lumber stress grades and design properties woodweb. Bending, shear and compression paralleltograin shall be adjusted according to code. Similarly, a 26f designation refers to a member with an allowable bending stress of 2600 psi. Lumber sawn from a log, regardless of species and size, is quite variable in mechanical properties. Below is a sample of how to adjust the reference bending value by the factors listed above to arrive at the allowable bending value that would be used to design wood in bending. Since the load caused by the fishing line is cantilevered off the end of the pole and since the cross section of a fishing pole is relatively small, a fishing pole will have high flexural stresses. Fb adjusted allowable bending stress, fb fb cm cf cr cd, where. F b allowable bending stress adjusted by applicable factors. Thus a piece of stress graded lumber with a strength ratio of 75 % in bending would be expected to have 75 % of the bending strength of the clear piece.
Wood is a renewable building material whose structural properties vary by species, natural growth characteristics and manufacturing practices. Continuous beam moment and reaction support forces moment and reaction support forces with distributed or point loads. Allowable bending stress, fb the maximum bending stress permissible for a specified structural member units for stress. Design values for visually graded timbers 5 x 5 and larger see nds 2. The term timber describes sawn wood that is 140mm 512 or more in its smallest dimension. If wood used for the beam has an allowable bending stress of. The design values contained herein are based on the southern pine inspection bureaus standard grading rules for southern pine lumber and modified as required by the american wood councils national design specification nds for wood construction. The following table is structural lumber wood engineering properties. These different stress levels are achieved by varying the percentages and grade of higher quality lumber in the beam layup. Design values for most species and grades of visually graded structural lumber products are determined in accordance with astm standardsincluding. Designs for this species grade are better based on the grade stresses given in bs 52682 for the individual species and grades, see table 5 table 3 strength graded hardwoods assigned to bs en 338 strength classes strength properties nmm. Standard practice for establishing structural grades and. Western wood products association 1500 sw first ave. Select wood species and grade or enter values for modulus of elasticity, e, and allowable stress, f c, after setting species for other, wet service conditions, duration of load factor c d, and effective column height for both axes unless braced at different points, both heights should be the same see fig.
This is certainly a useful yardstick of strength but is by no means the only one. Strength of wood is often thought of in terms of bending strength. The strength ratio of stress graded lumber is the hypothetical ratio of the strength property being considered compared to that for the material with no strengthreducing characteristic. Species grade size classification max fiberstress modulus of elasticity max shear. The design is satisfactory when applied stress is less than or equal to allowable stress. The controlling bending stress and the capacity of the beam in this orientation must be checked to. The allowable strengths and stiffness are for normal load duration.
I found an old listing for wood properties and it does appear that the allowable bending stress is higher than the allowable compression parallel to grain. The allowable unit stresses for fireretardanttreated wood, including fastener values, shall be developed from an approved method of investigation that considers the effects of anticipated temperature and humidity to which the fireretardanttreated wood will be subjected, the type of treatment and the redrying process. Wood strength, including the relationship between grain direction and strength, specific gravity as an indication of wood strength, and additional ways to measure wood strength. The layers are glued together with the grain of adjacent layers at right angles. Elastic ratios for various wood species at approximately 12% moisture content see bottom of webpage. Accordingly, neither sfpa, nor its members, warrant that the design values on which the. Divide the maximum bending movement by the fiber stress of a wooden beam. The beam is supported by a pin at one end and a roller at the distance of 3 m from the pin. Structural wood lumber engineering properties table chart. Stress grades and design properties for lumber, round timber, and ties david e. Floor joists capacities carrying capacities of domestic timber floor joists grade c in metric units. Modulus of elasticity and compression perpendiculartograin shall not be adjusted. Old wood stress values structural engineering general.
For simplicity and economy in use, pieces of lumber of similar mechanical properties are placed in categories called stress grades, which are characterized by a one or more sorting criteria, b a set of properties for engineering. For depths greater than 12, multiply fb by 12depth0. The beam has a rectangular crosssection with width b 250 mm and height h 325 mm. Note that due to sampling inadequacies, these values may not necessarily represent average species characteristics.
Except as otherwise noted, the values are computed in accordance with astm standards based on clearwood tests or on tests of fullsize pieces in specific grades. Select either the 2005 or 2012 national design specification. I was comparing values using nds for southern pine which varies as to the one that is higher. Standard specifications for structural glued laminated timber. A large number of species can be used to produce structural lumber. Veneer for panels covered in this specification is usually rotary peeled rather than. Wood can take heavy loads of short duration very well. Wood is the right choice for a host of construction applications. The fiber stress varies depending on the species of wood. Compares ipe decking with red cedar and pressure treated decking material based on strength, durability, decay, insect resistance, weathering, and looks. Modulus of elasticity of wood, wood engineering design data. It is typically measured in pounds per square inch psi. Wood beams strength of material engineering toolbox. Allowable total uniform load limited by deflection.
Beams and columns deflection and stress, moment of inertia, section modulus and technical information of beams and columns. The miracle in todays wood products is that they make more efficient use of the wood fiber resource. Updated design values for southern pine dimension lumber, effective june 1, 20, are. Z x is similar to the section modulus of a member it is usually a minimum of 10% greater than the section modulus in 3 f b the allowable stress of the beam in bending f y the yield strength of the steel e. Timber chapter 11 bangladesh national building code 2011 6. Methods in astm standard d 245 involve adjusting strength properties of small clear specimens of wood, as given in astm standard d 2555. The choice of species for use in design is a matter of economics typically. Maximum bending stress, f b must not exceed allowable stress, f b. In most species, the moisture content of the sapwood is higher than that of the heartwood table. Depending on the species and type of wood, the moisture content of living wood ranges from approxi mately 30 percent to more than 250 percent twoandahalf times the weight of the solid wood material. E modulus of elasticity adjusted by applicable factors. It is the earths natural, energy efficient and renewable building material. For a better comparison, we need to look at the allowable bending stress of that steel. A number of other strength criteria are described below.
For situations where the snow load is the maximum load we are allowed an additional 15% allowable fiberstress in bending. Compare that to the 2200 psi from wood, and there you have it, as uknerrjor said, steel is about 10x stronger than wood. Dimensional lumber western wood products association. Design properties apa the engineered wood association. Timber design uses standard symbols to denote the types of stresses for strength properties. If wood used for the beam has an allowable bending. A bending stresses a bending stress is not considered to be a simple stress.
Modulus of elasticity also referred to as tensile modulus or youngs modulus. Determine total load based on allowable web and flange bending stress for singlespan beams under uniform load, the total uniform load, w b, based on allowable bending stress in the web and flanges, can be calculated from the following formula. Note that not all of the factors are used to adjust the bending design value. Part of the workshop companion, a collection of information on wood, woodwork, woodworking skills, woodworking materials, and woodworking plans that together form the core knowledge needed by woodworkers, furniture. Bending stress aka flexural stress, aka torque is the stress caused by a moment or a couplea great example of bending stress can be seen in figure 1.
Maximum shear stress, f v shall not exceed allowable shear stress. When loads are applied, structural members bend, producing tension in the fibers along the faces farthest from the applied load and compression in the fibers along the face nearest to the applied load. Select a trial beam size and calculate allowable bending stress, f b see below calculate deflection and check with allowable deflection ratio. It can be used to determine a wood species overall strength. These induced stresses in the fibers are designated as extreme fiber stress in bending fb. Stress classes are designated by primary bending stress and modulus of elasticity. Wood handbookchapter 6lumber stress grades and design. Use of different species may also result in different stress designations. This formula will be discussed later in further detail.
Thus a piece of stressgraded lumber with a strength ratio of 75 % in bending would be expected to have 75 % of the bending strength of the clear piece. Enter values for span ft, spacing, species, grade, live and dead load psf, duration of load and wet service conditions, allowable deflection criteria e. The strength ratio of stressgraded lumber is the hypothetical ratio of the strength property being considered compared to that for the material with no strengthreducing characteristic. Modulus of rupture, frequently abbreviated as mor, sometimes referred to as bending strength, is a measure of a specimens strength before rupture. Pieces may differ in strength by several hundred percent. Each of these stresses will be discussed in detail as follows. Strength properties of commercially important woods the table below provides laboratoryderived values for several mechanical properties of wood that are associated with wood strength. The properties depend on the particular sorting criteria and on additional factors that are independent of the sorting criteria.
The allowable bending stress is tabulated for a standard 12 depth. Stress is the amount of force for a given unit of area. Plywood is manufactured with an odd number of layers, each layer consisting of one or more sheets of veneer thin sheets of wood. Bending and compression strength of wood species used in beams engineering toolbox resources, tools and basic information for engineering and design of technical applications. Strength properties of commercially important woods woodbin. Rectangular beams and stringers width more than 51mm 2 greater than thickness are. Design values provided herein are for western softwood species manufactured and shipped by mills in the 12 contiguous western states and alaska. Lumber design values western wood products association.
1087 1048 1299 1160 1029 1223 694 590 239 1094 526 157 872 952 73 1598 1011 54 218 854 1240 74 750 1317 1436 1114 664 686 65 432 1458