Strength and Stiffness of Spliced Nail-Laminated Posts Part 2: Testing and Design Procedures

Transcription

1 Dave Bohnhoff, Ph.D., P.E. University of Wisconsin-Madison Dave Bohnhoff is an Assistant Professor in the Agricultural Engineering Department at the University of Wisconsin-Madison. He teaches courses in Agricultral Structures and Environment, Metal Building Systems, and Residential Construction. The majority of his research has involved computer modeling and laboratory testing of mechanically-laminated posts and mechanical fasteners in wood. He is very active in local, state and national ASAE activities, and currently chairs the committee which is reviewing the ASAE snow and wind load standard. Russell C. Moody U.S. Forest Products Laboratory Russell C. Moody is a research engineer in charge of the Engineered Wood Products and Structures Research Work Unit, U.S. Forest Service, Forest Products Laoratory, Madison Wisconsin. He received his B.S. degree in Civil Engineering from the University of Wisconsin-Platteville, and his M.S. degree in Engineering Mechanics from the University of Wisconsin and Colorado State University. Russ has been with the Forest Products Laboratory since 1964 and his personal research has been primarily on the structural properties of light-frame wood systems and glued laminated wood products. His 80 publications include reports on structural evaluations of light-frame wood systems, conventional glued-laminated timber, products laminated from veneer, and packaging materials. 4 Volume 3, Number 2 Strength and Stiffness of Spliced Nail-Laminated Posts Part 2: Testing and Design Procedures Introduction The increased usc of nail-laminated posts and greater emphasis on post-frame building engineering have spurred considerable research on the bending strength and stiffness properties of the assemblies. This paper presents methods for characterizing these properties for design. Part 1, which was published in the last issue of this journal, provided an overview of computer modeling and laboratory test data for nail-laminated posts. The procedure used to determine allowable bending capacity and bending stiffness is different for spliced and unspliced nail-laminated posts. For unspliced posts, the bending strength is almost always calculated using the repetitive-member-use bending stress values listed in the National Design Specifications (NDS) (NFPA 1986). These values are 15% greater than the NDS single-member bending design values (ASTM 1989). The strength and stiffness of spliced nail-laminated posts, unlike that of unspliced posts, are highly dependent on nail type, size and location as well as the relative location of the butt joint or joints in each layer, and the type, amount, and location of butt joint reinforcement (when such reinforcement is used). Because of the complex interaction of these variables, the strength of spliced posts is currently determined by a laboratory test of representative sample of actual assemblies. A two-point load, applied in accordance with ASTM D 198 (ASTM 1989), is commonly used to establish the ultimate bending strength of each post. To arrive at an allowable bending moment for design, the fifth percentile of the distribution of the ultimate bending moment for all sample posts tested is divided by 2.1, which is product of a load duration factor of 1.6 and a traditional safety factor of 1.3 (ASTM 1989: Hoyle and Woeste 1989). The load duration factor is used to adjust the strength determined in a 5-min. test to that expected under a load with a duration of 10 years. Because different procedures are used to arrive at the allowable bending moment values for spliced and unspliced posts, it is possible to find a design value for a spliced post with butt joint reinforcement that is greater than that calculated for an unspliced post fabricated using the same size, grade, and species of lumber. For example, the allowable design bending moment of the spliced post design tested by Woeste and others (1988) was calculated to be 4,430 ft-lb (6.01 kn-m) (see Part 1, Table 1 of the January 1991 issue of Frame Building Profession). This is 9% higher than the allowable design bending moment of 4,065 ft-lb (5.50 kn-m) assigned to a three-layer unspliced post fabricated from the same FBP / March 1991

2 KD 15 No. 1 Dense southern Pine lumber (NDS repetitive-member-use bending stress = 2,150 lb/in 2 (14.8 MPa). Numbers such as these can confuse a designer, who is led to conclude that the spliced region of the post is stronger than the unspliced region. This is seldom, if ever, true because relatively light reinforcing is currently used in spliced nail-laminated posts. The bottom line is that if the same procedure used to determine the allowable bending moment for spliced posts were used for unspliced posts, the allowable bending stress for unspliced posts would, in most instances, be higher than that based on published allowable stress values. Another problem confronting design engineers is post stiffness. Because the flexural rigidity of spliced posts varies, the posts must be divided into two or more elements (discretized) for analysis if they are to be accurately represented in a plane-frame structural analog. Exactly where to section a spliced post and what properties to assign each section are complex functions of several design variables. Unfortunately for the designer, the necessary data and a procedure for accomplishing these two tasks have not been well established. Strength Of Posts As previously mentioned, the only way to determine allowable design values for spliced posts at present is to actually build and laboratory test a number of specimens. Since this is a costly and time-consuming process, we propose a procedure whereby allowable values for spliced posts are calculated by multiplying the design values for unspliced posts by strength modification factors. Before such a procedure can be implemented, (1) the conservativeness of the design values for unspliced posts must be investigated and (2) considerably more testing or analysis, if determined appropriate, is needed to establish strength modification factors. Design Values for Unspliced Posts As reported in Part 1, of the January 1991 issue of Frame Building Processional the design bending stress of three-layer posts has been found to be as much as 46% higher than that of a single member. However, the repetitivemember-use criteria of the National Design Specification (NDS) allow a designer to use a design bending stress value for a three-layer assembly that is only 15% greater than the single-member value. To justly reward those designers who currently use unspliced naillaminated posts, design criteria should allow the engineer to take advantage of higher design values than are currently allowed. These criteria should address the number of layers, the spacing between layers, the type, number, and location of fasteners, and the method of loading. Specifically, the design criteria should include provisions that limit the gap between two Continued on page FBP / March 1991

3 Continued from page 6... adjacent layers and do not allow nail fasteners to be located too close to member edges. In all cases, specifications should be written to ensure that the applied loads will either be distributed uniformly to all layers or applied through a load-distributing element that forces all layers to have similar displaced geometries. Once a design methodology using higher values is implemented, post manufacturers will need to establish a quality assurance system to assure that these design values are maintained. Strength Modification Factors for Spliced Posts We propose to determine a strength modification factor by obtaining the ratio of the design strength of a spliced post to that of an unspliced post. To establish such strength modification factors, spliced and unspliced posts must be laboratory tested under the same conditions. This includes using randomly selected lumber from the same lot to fabricate both post types. Without such a procedure, variabtity in strength caused by lumber characteristics (such as species and grade) cannot be differentiated from differences in strength caused by splicing. Only recently has an experiment comparing spliced and unspliced posts been conducted (Bohnhoff and others 1990). Both the spliced and unspliced assemblies were fabricated from the same lot of lumber (design information and test results for the spliced assemblies were reported in Part 1, Table 1, Reference 5 of the January 1991 issue of Frame Building Professional). The bending strength modification factors calculated on the basis of these data demonstrated a significant reduction in strength associated with splicing. The design bending strength of three-layer spliced posts without butt joint reinforcement was found to be 40% to 45% of the design strength of the three-layer unspliced assemblies. With reinforcement at the butt joints, the spliced posts were found to be 50% to 55% as strong as unspliced posts (Table 1). Because two of the three layers are continuous at each joint in an unreinforced spliced post, a common belief is that the post should have a bending strength that is approximately two-thirds that of an 20 FBP / March 1991

4 unspliced post. The problem with this assumption is that the following three factors are not taken into account: (1) the bending moment values in the two continuous layers adjacent to a joint can be quite different because of the redistribution of forces in the vicinity of the joint, (2) nail forces are much higher in spliced posts and precipitate failures in the posts that are not common to unspliced posts, and (3) design strength values are dependent on the relative variability of post strength, which may be higher for spliced posts than it is for nail-laminated posts without splices. Another reason that bending strength modification factors were lower than typically perceived is that test results for spliced posts are often incorrectly compared to NDS allowable values for unspliced posts. The NDS allowable bending moment value for the type of unspliced posts evaluated by Bohnhoff and others (1990) is only 59% of the value determined from the test. This is due, in part, to the relatively high quality of the material used in the test. When engineers compare the design bending moment values found by Bohnhoff and others (1990) to the NDS allowable value for the unspliced posts, they can be mistakenly led to conclude that splicing is much less critical than is actually the case. Before bending strength modification factors can be used to assign design values to spliced posts, additional testing and modeling will have to be performed. This is because bending strength modification factors are dependent on such design variables as overall splice length, number of layers, grade and species of wood, lumber size, nail size and density, nail joint stiffness, and type and location of butt joint reinforcement. Although several studies involving spliced nail-laminated posts have been conducted, their usefulness is limited because they did not evaluate individual pieces of lumber. Lumber is quite variable, even within the same grade and species. If the strength and stiffness of individual pieces of lumber are not known, it is impossible to determine where the batch or lot of lumber ranks with respect to other batches and lots of the same grade. Without design values for unspliced posts, strength modification factors cannot be calculated and dissimilar spliced post designs cannot be compared. To avoid confusion with respect to the strength of spliced posts, the allowable design bending strength values for the posts have been expressed as moment values and not as stress values: spliced posts are complex structural systems and not homogeneous, rectangular members. Engineers who assign allowable bending stress values to spliced vertically laminated assemblies must assume some effective moment of inertia or section modulus. These engineers commonly use the gross outside dimensions of the post to calculate the section properties. When the individual layers are the same size, the resulting bending stress value is the average of the bending stress values of the individual layers. Such an average value does not indicate the level of stress in the individual layers. Stiffness Of Posts To determine the distribution of forces in an indeterminate plane-frame structure, the bending stiffness of the components that make up the structure must be known. The bending stiffness of a homogeneous member is related to EI where E is the modulus of elasticity and I is the moment of inertia. For this paper, the product of E and I will be referred to as the bending stiffness of the member. The bending stiffness of a spliced naillaminated post, unlike that of homogeneous members, varies along the length of the post. Consequently, to be accurately represented in a planeframe structural analog, each post must be divided into elements. Exactly where to section a spliced post and what bending stiffness to assign each section are functions of several design variables. To standardize the procedure for accomplishing these two tasks, we propose a method by which spliced posts are sectioned into elements with or without joints. Sections without joints will be treated like unspliced naillaminated assemblies and will be assigned bending stiffness values accordingly. The stiffness of each section containing a splice or splices will be obtained by multiplying the bending stiffness of the unspliced section or sections by a stiffness modification factor. 22 FBP / March 1991

5 Bending Stiffness of Unspliced Posts Unspliced posts are assumed to have no slip between the individual layers, a valid assumption when the layers are forced by a load-distributing element to have the same displaced geometry. When there is no slip between individual layers and each layer has the same moment of inertia, the bending stiffness of the post is equal to the average modulus of elasticity of the layers multiplied by the total moment of interia of the post. Stiffness Modification Factors The stiffness modification factor is the ratio of the effective bending stiffness of the spliced section in question to the bending stiffness of the adjacent unspliced section or sections. The effective bending stiffness for the section with joints can be determined with the aid of special analytical methods (Bohnhoff and others 1989) or it can be calculated using load-deflection data from laboratory tests. The effective bending stiffness assigned to a section with joints depends on how the length of the section is defined. Figure 1 shows a laminated post, sectioned at two different locations. In Figure 1a, the sections without joints (outside sections) have a bending stiffness twice that of the section with joints (middle section). As the length of the middle section is expanded to include more of the stiffer outer regions (Fig. lb), its effective bending stiffness increases. Figure 2 and the equations in Table 2 can be used to obtain stiffness modification factors from laboratory test data. The equations only apply to posts tested under a symmetric two-point loading; they were derived using the conjugate-beam method. (Singer, 1962) Use of these equations requires a good estimate of the effective stiffness of the unspliced section, EI. For the stiffness modification factor to be meaningful, EI must be determined by a laboratory test of lumber representative of that used to fabricate the spliced assemblies (either individual pieces or unspliced assemblies can be tested). The equations in Table 2 were used to obtain the factors in Table 3. A value for b was selected that made the length of the spliced region equal to the overall splice length plus 4 ft (1.22 m). In other words, the spliced section included those portions of the post within 2 ft (0.61 m) of the joints. Note that the values in Table 3 were based on a limited number of assembly tests and are included to demonstrate the approach only. The magnitude of these factors is highly dependent on nail type and location, along with reinforcement type, size, and location. Modeling offers the possibility of determining the effect of many variables on the stiffness modification factors. Research is underway to obtain lateral load-slip properties on a variety of nail joints required for computer modeling. Summary Design procedures that would ultimately reduce the amount of laboratory testing of spliced nail-laminated post designs and thus lower design costs are introduced. Before such procedures can be implemented, strength and stiffness modification factors for spliced posts must first be established. This requires that unspliced and spliced posts be tested under the same conditions. This includes using randomly selected lumber from the same lot to fabricate both post types. FBP / March 1991 Printed on recycled paper 23