T E C H N I C A L N O T E

Field Notching and Drill ing of Glued Laminated Timber Beams

ENGINEERED WOOD SYSTEMS

Number EWS S560E

November 2003

Introduction

Glued laminated timber (glulam) beamsare highly engineered components manu-factured from specially selected andpositioned lumber laminations of varyingstrength and stiffness properties. Thesefabricated beams are often specifiedwhere a certain span, strength and/orstiffness is required. As such, glulambeams are generally designed for andused in applications where they will behighly stressed under design loads. Forthis reason, field modifications such asnotching, tapering or drilling not shownon the design or shop drawings shouldbe avoided, and never done without athorough understanding of their effectson the structural integrity of the member.

As mentioned above, glulam beams arefabricated using selected lumber lamina-tions of various strength, stiffness andspecies. The beams are “assembled” withlower grade material in the mid-depth of the beam and higher grade material

positioned on the top and bottom. Thehighest grade of material is used as theoutermost laminations on the tensionside of the beam. Because of this, anydrilling, dapping or notching that takesplace in these outermost tension lamina-tions has a two-fold effect on the strengthand serviceability of the beam. First, suchmodifications reduce the section of thebeam. Second, they remove wood fiberfrom the laminations having the higheststrength. Given this, along with the factthat stress concentrations occur in suchareas created by notching or drilling, it iseasy to see why a cautious approach tofield modifications is vital.

This Technical Note provides recommen-dations for field notching, tapering anddrilling glulam beams. Beams illustratedherein are assumed to be simple spansubjected to uniform loads, and areshown with the compression side up. Allequations and notching guidelines arepresented using the same assumptions. Ifthis information is applied to continuousor cantilevered beams, it should be usedwith extreme caution and only after care-ful analysis based on sound engineeringjudgment.

It is important to understand thatimproperly made field notches or holesmay reduce the capacity of a properlydesigned member to the point where astructural failure may occur. The effectsof any field notching or drilling should bechecked by a designer competent inengineered timber design.

Notching

Notching of bending members should beavoided whenever possible, especially onthe tension side of the member. Tensionside notching of glulam beams is notpermitted except at end bearings andthen only under specific conditions. Thenotching of a bending member on thetension side results in a decrease instrength caused by stress concentrationswhich develop around the notch as wellas a reduction of the area resisting thebending and shear forces. Such notchesinduce perpendicular-to-grain tensionstresses which, in conjunction with hori-zontal shear forces, can cause splittingalong the grain, typically starting at theinside corner of the notch. Stress concen-trations due to notches can be reduced

Note: This version is superseded by a more current edition. Check the current edition for updated design and application recommendations.

by using a gradually tapered notch con-figuration in lieu of a square-cornerednotch. Rounding the square corner of anotch with a radius of approximately 1/2 inch is also recommended to reducestress concentrations in these areas.

For square-cornered notches occurring atthe ends of beams on the tension side,the designer may consider the use ofreinforcement such as full-threaded lagscrews to resist the tendency to split atthe notch. (See Figure 1.) A number ofdesign methodologies exist for sizingsuch screws. The design methodologyselected and subsequent fabricationdetails are the responsibility of thedesigner/engineer of record. If lag screwsare used, lead holes shall be predrilled inaccordance with accepted practice.

This procedure can also be used as a fieldremedy to minimize further propagationof an existing crack.

Where glulam members are notched atthe ends for bearing over a support, thenotch depth shall not exceed 1/10 of thebeam depth. Figure 2(f ) is provided toassist in evaluating the associated reduc-tions to beam strength due to notchingon the tension side. For notches on thecompression side, a less severe conditionexists and equations for the analysis ofthe effects of these notches are also givenin Figure 2. The equations given areempirical in nature and were developedfor the conditions shown.

As this guideline is limited to single span,simply supported beams, the notchesshown in Figure 2 occur in areas of high

shear and effectively zero moment. Forthis reason, the design equations givenare shear equations. In situations wherecompression side notches extend intoareas of significant moment, the bendingcapacity of the beam should also bechecked using the remaining section ofthe beam and the appropriate allowablestresses for those laminations remainingat the notch location. ContactEngineered Wood Systems for moreinformation regarding this type of notch.

When it becomes necessary to cut asmall notch in the top of a glued lami-nated timber (in the compression zone)to provide passage for small diameterpipe or conduit, this cut should be madein areas of the beam stressed to less than50 percent of the design bending stress.The net section in these areas should bechecked for shear and bending stresses toinsure adequate performance.

All field notches should be accuratelycut. Avoid overcutting at the root of thenotch. Drilling a pilot hole in the mem-ber at the interior corner location of anotch as a stop point for the saw bladeprovides both a rounded corner andminimizes overcutting at the corner.

Holes

Horizontal HolesLike notches, holes in a glulam beamremove wood fiber, thus reducing the netarea of the beam at the hole location, andthey introduce stress concentrations.These effects cause a reduction in thecapacity of the beam in the area of the

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Bearing length Use one or more fully

threaded lag screws

Washer Potential crack zone

dNeutral axis

Lag screw extends past the neutral axis into the upper portion of beam

Depth of notch ≤

0.1d or 3 inches, whichever

is less

FIGURE 1

A REINFORCEMENT TECHNIQUE TO MINIMIZE CRACK PROPAGATION AT END BEARING NOTCHES

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Accessed from the ICC website pursuant to License Agreement. No further reproductions authorized.

penetration. For this reason, horizontalholes in glued laminated timbers arelimited in size and location to maintainthe structural integrity of the beam.Figure 3 shows the zones of a uniformlyloaded, simply supported beam wherethe field drilling of holes may be consid-ered. These non-critical zones are locatedin portions of the beam stressed to lessthan 50 percent of design bending stressand less than 50 percent of design shearstress. For beams of more complex load-ing or other than simple spans, similardiagrams may be developed.

Field-drilled horizontal holes should beused for access only and should not beused as attachment points for brackets or other load bearing hardware unlessspecifically designed as such by the engi-neer or designer. Examples of accessholes include those used for the passageof wires, electrical conduit, small diame-ter sprinkler pipes, fiberoptic cables, andother small, lightweight materials. Thesefield drilled horizontal holes should meetthe following guidelines:

1. Hole size: The hole diameter shouldnot exceed 1-1/2 inches or 1/10 thebeam depth, whichever is smaller.

2. Hole location: The hole should have aminimum clear distance, as measuredfrom the edge of the hole to the nearestedge of the beam, of 4 hole diameters tothe top or bottom face of the beam and 8hole diameters from the end of the beam.Note that the horizontal hole should notbe drilled in the moment critical zone, as defined in Figure 3, unless approvedby an engineer or architect qualified in

engineered timber design.

3. Hole spacing: The minimum clearspacing between adjacent holes, as meas-ured between the nearest edge of theholes, should be 8 hole diameters basedon the largest diameter of any adjacenthole in the beam.

4. Number of holes: The maximum number of holes should not exceed 1 holeper every 5 feet of beam length. In otherwords, the maximum number of holesshould not exceed 4 for a 20-foot longbeam. The hole spacing limitation, asgiven above, should be satisfied separately.

For glulam members that have been over-sized, the guidelines given above may berelaxed based on an engineering analysis.

Regardless of the hole location, holesdrilled horizontally through the membershould be positioned and sized with theunderstanding that the beam will deflectover a period of time under in-serviceloading conditions. This deflection couldcause distress to supported equipment orpiping unless properly considered.

Vertical HolesWhenever possible, avoid drilling verticalholes through glulam beams. As a rule ofthumb, vertical holes drilled throughthe depth of a glulam beam cause areduction in the capacity at that locationdirectly proportional to the ratio of 1-1/2times the diameter of the hole to thewidth of the beam. For example, a 1-inchhole drilled in a 6-inch-wide beam wouldreduce the capacity of the beam at that section by approximately (1 x 11⁄2)=1/4.________

6For this reason, when it is necessary todrill vertical holes through a glulam

member, the holes should be positionedin areas of the member that are stressedto less than 50 percent of design inbending. In a simply supported, uni-formly loaded beam, this area would belocated from the end of the beam inwardapproximately 1/8 of the beam span. Inall cases, the minimum clear edge dis-tance, as measured from either side ofthe member to the nearest edge of thevertical hole, should be 2-1/2 times thehole diameter. Use a drill guide to mini-mize “wandering” of the bit as it passesthrough knots or material of varyingdensity, and to insure a true alignment ofthe hole through the depth of the beam.

Holes for Support of Suspended EquipmentHeavy equipment or piping suspendedfrom glulam beams should be attachedsuch that load is applied to the top of themembers to avoid introducing tensionperpendicular-to-grain stresses. Any hori-zontal holes required for support of signif-icant weight, such as suspended heatingand cooling units or main water lines,must be located above the neutral axis ofthe member and in a zone stressed to lessthan 50 percent of the design flexuralstress. (See Figure 3.) Fasteners support-ing light loads such as light fixtures mustbe placed at least four laminations or 25%of beam depth, whichever is greater, awayfrom the tension face of the member. Thedesign capacity of the beam should bechecked for all such loads to insureproper performance.

Protection of Field-Cut

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FIGURE 2

SHEAR DESIGN EQUATIONS FOR NOTCHED AND TAPERED BEAMS

(a) Square End Bearing

(c) Sloped End Cut for Roof Drainage

(b) Slope End Bearing

(d) Compression-side notch – Where e ≤≤ de

(e) Compression-side Notch – Where e > de (f) Tension-side Notch

Compression side

d

fv = 3V2bd

d d

fv = 3V2bde

fv = 3V2bde

Compression side

d

fv = 3V2bd

3dmaximum

deded/3

Min.

0.4dMax.

d

3de or 1/3 of the span,

whichever is less

0.4dMax.

3Vfv = 2b d –

(d – de)ede

3V2bde de

d( )fv =

e

de

dde

dn

fv = shear stress (psi) V = shear force at notch location (lb) b = width of beam (in.)d = depth of beam (in.) de = effective depth as shown (in.) e = length of notch as shown (in.)

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Maximum depth of end notch, dn, is 0.1d or 3 inches, whichever is less

3de or 1/3 of the span,

whichever is lesse

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Zones where horizontal holes are permitted for passage of wires, conduit, etc.

Bearing critical zoneBearing critical zone

d/4

d/2

d/4

Shear critical zone Shear critical zone

Moment critical zone

ll/2l/8l/8 l/8l/8

FIGURE 3

ZONES WHERE SMALL HORIZONTAL HOLES ARE PERMITTED IN A UNIFORMLY LOADED, SIMPLY SUPPORTED BEAM

Protection of Field-Cut Notches and Holes

Frequently, glulam beams are provided

from the manufacturer with the ends

sealed by a protective coating. This

sealer is applied to the end grain of the

glulam beams to retard the migration of

moisture in and out of the beam ends

during transit and job site storage. Field

cutting a notch in the end of a beam

can change the moisture absorption

characteristics of the beam at the notch

location. This can result in season-

ing checks or even localized splitting

developing at the root of the notch. To

minimize this possibility, all notches

should be sealed immediately after

cutting using a water-repellent sealer.

Sealing other field-cut locations as well

as field-drilled holes is also recom-

mended. These sealers can be applied

with a brush, swab, roller or spray gun.

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ENGINEERED WOOD SYSTEMSENGINEERED WOOD SYSTEMS

We have field representatives in most major U.S. cities and in Canada who can help answer questions involving APA and APA EWS trademarked products. For

additional assistance in specifying engineeredwood products or systems, contact us:

APA – THE ENGINEEREDWOOD ASSOCIATION

HEADQUARTERS7011 So. 19th St.

Tacoma, Washington 98466(253) 565-6600 ■ Fax: (253) 565-7265

PRODUCT SUPPORT HELP DESK(253) 620-7400

E-mail Address: help@apawood.org

The product use recommendations in thispublication are based on the continuing pro-grams of laboratory testing, productresearch, and comprehensive field experi-ence of Engineered Wood Systems. However,because EWS has no control over quality ofworkmanship or the conditions under whichengineered wood products are used, it can-not accept responsibility for product perform-ance or designs as actually constructed.Because engineered wood product perform-ance requirements vary geographically, con-sult your local architect, engineer or designprofessional to assure compliance with code,construction, and performance requirements.

Form No. EWS S560ERevised November 2003/0100

www.apawood.org@Web Address:

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