Field Notching and Drilling of Glued Laminated Timber Beams

Transcription

1 T E C H N I C A L N O T E Fiel Notching an Drilling of Glue Laminate Timber Beams Number EWS S560C June 1999 Introuction Glue laminate timber (glulam) beams are highly engineere components manufacture from specially selecte an positione lumber laminations of varying strength an stiffness properties. These fabricate beams are often specifie where the span, strength or stiffness requirements for a specific application call for a highly engineere woo prouct. As such, glulam beams are generally esigne for an use in applications where they will be highly stresse uner esign loas. For this reason, fiel moifications such as notching, tapering or rilling not shown on the esign or shop rawings shoul be avoie, an never one without a thorough unerstaning of their effects on the structural integrity of the member. As mentione above, glulam beams are fabricate using selecte lumber laminations of various strength, stiffness an species types. The beams are assemble with the lower grae material in the mi-epth of the beam an higher grae material positione on the top an bottom. The highest grae of material is use as the outermost laminations on the tension sie of the beam. Because of this, any rilling, apping or notching that takes place in these outermost tension laminations has a two-fol effect on the strength an serviceability of the beam. First, such moifications reuce the section of the beam. Secon, they remove woo fiber from the laminations having the highest strength. Given this, along with the fact that stress concentrations occur in such areas create by notching or rilling, it is easy to see why a cautious approach to fiel moifications is vital. This Technical Note provies recommenations for fiel notching, tapering an rilling glulam beams. Beams illustrate herein are assume to be simple span an are shown with the compression sie up. All equations an notching guielines are presente using the same assumptions. For continuous or cantilevere beams, this information can be use, but only with extreme caution an only after careful analysis base on soun engineering jugment. It is important to unerstan that improperly mae fiel notches or holes may reuce the capacity of a properly esigne member to the point where a structural failure may occur. The effects of any fiel notching or rilling shoul be checke by a esigner competent in engineere timber esign. Notching Notching of bening members shoul be avoie whenever possible, especially on the tension sie of the member (see Figure 1). Tension sie notching of glulam beams is not permitte except at member ens an then only uner specific conitions. The notching of a bening member on the tension sie results in a ecrease in strength cause by stress concentrations which evelop aroun the notch as well as a reuction of the area resisting the bening an shear forces. Such notches inuce perpenicular-to-grain tension stresses which, in conjunction with horizontal shear forces, can cause splitting along the grain, typically starting at the insie corner of the notch. Stress concentrations ue to notches can be reuce by using a graually tapere notch configuration in lieu of a square-cornere notch. Rouning the square corner of a notch with a raius of approximately 1/2 inch is also recommene to reuce stress concentrations in these areas. For square-cornere notches occurring at the ens of beams on the tension sie, the esigner may consier the use of reinforcement such as full-threae lag screws to resist the tenency to split at the notch. (See Figure 2.) A number of ENGINEERED WOOD SYSTEMS

2 1999 ENGINEERED WOOD SYSTEMS ALL RIGHTS RESERVED. ANY COPYING, MODIFICATION, DISTRIBUTION OR OTHER USE OF THIS PUBLICATION OTHER THAN AS EXPRESSLY AUTHORIZED BY EWS IS PROHIBITED BY THE U.S. COPYRIGHT LAWS. FIGURE 1 TENSION ZONE LOCATION IN SIMPLY SUPPORTED BEAM CONFIGURATION /2 /2 Note: Sketch is not to scale. FIGURE 2 Uniformly istribute loa Tension zone Outer tension lams (5% of beam epth) RECOMMENDED REINFORCING OF TENSION-SIDE NOTCH 0.9 minimum Depth of 0.1 notch = maximum Lag screw extens past the neutral axis into the upper portion of beam Washer Potential crack zone Use one or more fully threae lag screws Neutral axis 2

3 esign methoologies exist for sizing such screws. The esign methoology selecte an subsequent fabrication etails are the responsibility of the esigner/engineer of recor. If lag screws are use, lea holes shall be prerille in accorance with accepte practice. Where glulam members are notche at the ens for bearing over a support, the notch epth shall not excee 1/10 of the beam epth. Figure 3(f) is provie to assist in evaluating the associate reuctions to beam strength ue to notching on the tension sie. For notches on the compression sie, a less severe conition exists an equations for the analysis of the effects of these notches are also given in Figure 3. The equations given are empirical in nature an were evelope for the conitions shown. As this guieline is limite to single span, simply supporte beams, the notches shown in Figure 3 occur in areas of high shear an effectively zero moment. For this reason, the esign equations given are shear equations. In situations where compression sie notches exten into areas of significant moment, the bening capacity of the beam shoul also be checke using the remaining section of the beam an the appropriate allowable stresses for those laminations remaining at the notch location. Contact Engineere Woo Systems for more information regaring this type of notch. When it becomes necessary to cut a small notch in the top of a glue laminate timber (in the compression zone) to provie passage for small iameter pipe or conuit, this cut shoul be mae in areas of the beam stresse to less than 50 percent of the esign bening stress. The net section in these areas shoul be checke for shear an bening stresses to insure aequate performance. All fiel notches shoul be accurately cut. Avoi overcutting at the root of the notch. Drilling a pilot hole in the member at the interior corner location of a notch as a stop point for the saw blae provies both a roune corner an minimizes overcutting at the corner. Holes Horizontal Holes Like notches, holes in a glulam beam remove woo fiber, thus reucing the net area of the beam at the hole location, an they introuce stress concentrations. These effects cause a reuction in the capacity of the beam in the area of the penetration. For this reason, horizontal holes in glue laminate timbers are limite in size an location to maintain the structural integrity of the beam. Figure 4 shows the zones of a uniformly loae, simply supporte beam where the fiel rilling of holes may be consiere. These non-critical zones are locate in portions of the beam stresse to less than 50 percent of esign bening stress an less than 50 percent of esign shear stress. For beams of more complex loaing or other than simple spans, similar iagrams may be evelope. Fiel-rille horizontal holes shoul be use for access only an shoul not be use as attachment points for brackets or other loa bearing harware unless specifically esigne as such by the engineer or esigner. Examples of access holes inclue those use for the passage of wires, electrical conuit, small iameter sprinkler pipes, fiberoptic cables, an other small, lightweight materials. Fiel-rille horizontal holes shoul be space at least 2 feet on center in the non-critical zones, as efine in Figure 4. Uner no circumstances shoul holes be rille in the outer four laminations of a flexural member on the tension sie in the zone or zones stresse to 50 percent or greater of the esign stress. Holes with a iameter larger than the thickness of one lamination (typically 1-1/2 inches) or 1/10 of the beam epth, whichever is smaller, are not recommene. However, for glulam members that have been oversize for architectural reasons, this limit may be increase base on an engineering analysis. Regarless of the hole location, the net section of the beam remaining shoul be checke for flexure an horizontal shear. In aition, holes rille horizontally through the member shoul be positione an size with the unerstaning that the beam will eflect over a perio of time uner in-service loaing conitions. This eflection coul cause istress to supporte equipment or piping unless properly consiere. Vertical Holes As a rule of thumb, vertical holes rille through the epth of a glulam beam cause a reuction in the capacity at that location irectly proportional to the ratio of 1-1/2 times the iameter of the hole to the with of the beam. For example, a 1-inch hole rille in a 6-inch wie beam woul reuce the capacity of the beam at that section by approximately (1 x 11 2) 6 = 1/4. For this reason, when it is necessary to rill vertical holes through a glulam member, the holes shoul be positione in areas of the member that are stresse to less than 50 percent of esign 3

4 FIGURE 3 SHEAR DESIGN EQUATIONS FOR NOTCHED AND TAPERED BEAMS Compression sie Compression sie f v = 3V 2b f v = 3V 2b (a) Square En Bearing (b) Slope En Bearing 3 maximum 3 e maximum e 0.4 Max. /3 Min. e e f v = 3V 2b e f v = 3V 2b e e e (c) Slope En Cut for Roof Drainage () Compression-sie Notch Where e e 3 e maximum e 0.4 Max. e e f v = 3V f 2b v = 3V e 2b e e Uner No Circumstances shoul e be<0.9 (e) Compression-sie Notch Where e> e (f) Tension-sie Notch Not Recommene f v = shear stress (psi) = epth of beam (in.) V = shear force at notch location (lb) e = effective epth as shown (in.) b = with of beam (in.) e = length of notch as shown (in.) 4

5 in bening. In a simply supporte, uniformly loae beam, this area woul be locate from the en of the beam inwar approximately 1/8 of the beam span. In all cases, use a rill guie to minimize wanering of the bit as it passes through knots or material of varying ensity, an to insure a true alignment of the hole through the epth of the beam. Holes For Support of Suspene Equipment Heavy equipment or piping suspene from glulam beams shoul be attache such that loa is applie to the top of the members to avoi introucing tension perpenicular-to-grain stresses. Any horizontal holes require for support of significant weight, such as suspene heating an cooling units or main water lines, must be locate above the neutral axis of the member an in a zone stresse to less than 50 percent of the esign flexural stress. (See Figure 4.) Fasteners supporting light loas such as light fixtures must be place at least four laminations away from the tension face of the member. The esign capacity of the beam shoul be checke for all such loas to insure proper performance. Protection of Fiel-Cut Notches an Holes Frequently, glulam beams are provie from the manufacturer with the ens seale by a protective coating. This sealer is applie to the en grain of the glulam beams to retar the migration of moisture in an out of the beam ens uring transit an jobsite storage. fiel cutting a notch in the en of a beam can change the moisture absorbtion characteristics of the beam at the notch location. This can result in seasoning checks or even localize splitting eveloping at the root of the notch. To minimize this possibility, all notches shoul be seale immeiately after cutting using a water-repellent sealer. Sealing other fiel-cut locations as well as fiel-rille holes is also recommene. These sealers can be applie by brush, swab, roller or with a spray gun. FIGURE 4 ZONES WHERE SMALL HORIZONTAL HOLES ARE PERMITTED IN A UNIFORMLY LOADED, SIMPLY SUPPORTED BEAM /8 /8 /2 /8 /8 /4 Moment critical zone /2 Shear critical zone Shear critical zone /4 Moment critical zone Bearing critical zone Bearing critical zone Zones where horizontal holes are permitte for passage of wires, conuit, etc. Zones where horizontal holes are permitte for loa-carrying fasteners or for passage of wires, conuit, etc. 5

6 We have fiel representatives in most major U.S. cities an in Canaa who can help answer questions involving APA an APA EWS traemarke proucts. For aitional assistance in specifying engineere woo proucts or systems, get in touch with your nearest APA regional office. Call or write: WESTERN REGION 7011 So. 19th St. P.O. Box Tacoma, Washington (253) Fax: (253) EASTERN REGION 2130 Barrett Park Drive, Suite 102 Kennesaw, Georgia (770) Fax: (770) U.S. HEADQUARTERS AND INTERNATIONAL MARKETING DIVISION 7011 So. 19th St. P.O. Box Tacoma, Washington (253) Fax: (253) Internet Aress: PRODUCT SUPPORT HELP DESK (253) Aress: (Offices: Antwerp, Belgium; Lonon, Unite Kingom; Hamburg, Germany; Mexico City, Mexico; Tokyo, Japan.) For Caribbean/Latin America, contact heaquarters in Tacoma. The prouct use recommenations in this publication are base on the continuing programs of laboratory testing, prouct research, an comprehensive fiel experience of Engineere Woo Systems. However, because EWS has no control over quality of workmanship or the conitions uner which engineere woo proucts are use, it cannot accept responsibility for prouct performance or esigns as actually constructe. Because engineere woo prouct performance requirements vary geographically, consult your local architect, engineer or esign professional to assure compliance with coe, construction, an performance requirements. Form No. EWS S560C Revise June 1999/0100 ENGINEERED WOOD SYSTEMS 6