Design Considerations for Horizontal Penetrations in Glulam Beams

This article references the 2024 National Design Specification (NDS) for Wood Construction. Previous editions also apply. 

Mechanical, electrical, plumbing, and fire protection (MEPF) penetrations through beams are necessary in all structural systems, but there are unique considerations with mass timber. Because structural wood is often the exposed architectural finish, the aesthetics of MEPF penetrations are more important than in other building types. To ensure quality detailing, penetrations through glulam beams (and through all mass timber members) should be well coordinated during the design and preconstruction phases. Holes should be drilled by the mass timber manufacturer using CNC technology in their facility. This provides the cleanest looking holes awhile ensuring accurate placement.  

Structural Impacts of Holes Through Glulam Beams 

As with any structural member, adding holes to a glulam beam can impact its structural capacity. While vertical holes are sometimes necessary, horizontal holes are much more common and this article will focus on horizontal holes only.  

Chapter 5 of the American Wood Council’s National Design Specification® (NDS®) for Wood Construction provides prescriptive guidelines for the design of glulam members. Section 5.4.5 discusses the design of notched glulam members, and Section 5.3.10 notes that the shear reduction factor is applicable to notched glulam members using the provisions of Section 3.4.3.2.  

While the NDS does not discuss glulam members with horizontal holes, Section 3.1.2.1 states: “The net section area is obtained by deducting from the gross section area the projected area of all material removed by boring, grooving, dapping, notching or other means. The net section area shall be used in calculating the load carrying capacity of a member except as specified in 3.6.3 for columns.” When evaluating the structural capacity of a glulam beam with horizontal holes, this reduced cross section, or net section area, is used when determining the applicable stresses at that particular location along the span of the member and at the particular depth within the member. 

APA Technical Note S560J (APA 2020), Field Notching and Drilling of Glued Laminated Timber Beams, provides guidelines for where small holes are permitted without additional analysis or reinforcement. These guidelines are based on a simply supported and uniformly loaded member. They do not allow holes where bending forces are highest (middle three quarters of the top of the beam span, all of the bottom of the beam span, top and bottom quarter of the beam depth), and where shear forces are highest (end quarter of the beam span, middle half of the beam depth).

Source: APA – The Engineered Wood Association

The guidelines for hole location are based on a maximum hole diameter of 1-1/2 in. or one tenth of the beam depth, whichever is smaller. They recommend that all holes be located at least four hole diameters clear from the top and bottom of the beam, and at least eight hole diameters clear from the ends of the beam. Using this design methodology, holes need to be spaced with at least eight hole diameters clear between them, based on the larger of the adjacent holes. Some exceptions are noted when holes have a maximum diameter of 1 in. However, most horizontal holes in glulam members for MEPF penetrations in mass timber buildings will exceed this size.  

As an example, a mass timber project has 6-3/4-in. x 24-in. glulam beams. The above guidelines would allow a maximum hole diameter of 1-1/2 in. (the smaller of 1-1/2 in. and d/10 = 24/10 = 2.4 in.). It is very common for MEPF penetrations in glulam beams to require holes much larger than this to accommodate sprinklers, ductwork, and plumbing or drain lines—as shown below. 

If a glulam beam must accommodate horizontal holes that exceed the prescriptive guidelines noted above, additional analysis and/or reinforcement of the structural member is typically necessary. Additional guidance can be found in two sources: APA Technical Note V700E (APA 2022), Effect of Large Diameter Horizontal Holes on the Bending and Shear Properties of Structural Glued Laminated Timber, and AITC Technical Note 19 (AITC, 2023), Guidelines for Evaluation of Drilled Holes and Notches in Structural Glued Laminated Timber Beams.  Both documents provide a method of calculating the bending stiffness, shear capacity, and bending moment capacity of a member when these larger holes are required. The equations in the document are based on a maximum hole diameter of 2d/3 for glulam members up to 24 in. deep (d = beam depth). For members deeper than 24 in., the document notes a maximum hole diameter of 16 in. It also notes specific hole locations, including required distances from the top and bottom of the beam and bearing supports, and spacing between holes.  

Example: 6-3/4-in. x 24-in. glulam beam (southern pine) with two 6-in.-diameter holes, located as follows:

Analysis of the structural impacts of these two holes, using the analysis method described in APA Technical Note V700E:

Beam checks prior to considering impacts of holes:

Note that the above checks indicate that the glulam beam is easily working for bending (stress ratio = 0.49), shear (stress ratio = 0.44), and deflection (L/1476 and L/703 for live and total load, respectively). 

Beam check considering impacts of holes: 

For the purpose of this example, we’ll analyze Hole 2, which is 13-ft-5-in. from the left support. However, both holes would need structural review. 

Note that Hole 2 has a significant impact on the bending and shear capacity of this member. The bending capacity now has a stress ratio of 0.61. The shear capacity now has a stress ratio of 0.78. 

Per the provisions of AITC Technical Note 19, one additional check should be made to account for the stress concentration at the hole location. For the example problem, assume C_vr, C_M, and C_t are 1.0. 

Some engineers choose to reinforce glulam beams with fasteners adjacent to holes, as seen in the image below. Information on fastener reinforcement design may be available from fastener manufacturers, as in this example. Regardless of the analysis and/or reinforcement method used, the glulam manufacturer should be consulted for input when planning for holes in glulam beams.

Fire Impacts of Holes Through Glulam Beams 

Glulam beams are often left exposed while achieving a 1- or 2-hour fire-resistance rating (FRR), permitted through char calculations within Chapter 16 of the NDS. For more information on the fire-resistive design of exposed mass timber framing elements, see the WoodWorks publication, Fire Design of Mass Timber Members: Code Applications, Construction Types and Fire Ratings.

1030 Music Row / Anecdote Architectural Experiences / StructureCraft / Photo Andrew Keithly

When designing holes through glulam beams that require an FRR, there are two basic design options. The first is to allow charring to occur around the hole perimeter, treating it as an exposed surface. For example, a 6-in.-diameter hole is drilled through a glulam beam that is exposed and requires a 1-hour FRR. Using the char calculations in NDS Chapter 16 provides an effective char depth of 1.8 in., which must be doubled when calculating the post-fire hole diameter because a fire would affect the entire perimeter of the hole. This results in a post-fire hole diameter of 6 in. + 1.8 in. + 1.8 in. = 9.6 in. Accounting for the 1-hour char impacts on the beam cross section as well as the hole, what was a 6-3/4-in. x 24-in. glulam beam in non-fire checks would be reduced to a 3.15-in. x 22.2-in. beam after a 1-hour fire design scenario (beam width = 6.75 in. – 1.8 in. – 1.8 in. = 3.15 in.; beam depth = 24 in. – 1.8 in. = 22.2 in.). This increased hole diameter has a significant impact on the structural capacity of the beam. Note that the allowable bending capacity can be increased under fire design scenarios by a factor of 2.85 using NDS Table 16.2.2. and shear capacity increased by a factor of 2.75 per 2024 NDS Fire Design Spec Table 3.3.2.  

Using the above example, this would result in the following design: 

If calculations indicate that the beam is not structurally adequate, the beam could be increased in size, the effects of fire on the hole could be reduced through protection means, or an alternative engineering method could be used. 

To avoid oversizing holes for char conditions, some designers choose to protect them with a firestopping system—usually a penetrant in the hole surrounded with materials such as intumescent tape, fire caulk, or mineral wool insulation. This solution usually requires an engineered approach since tested configurations are not yet available. These options should be discussed with a firestop manufacturer and fire protection engineer. Some projects create more openings than may be initially used, to enable future flexibility in MEPF routing, which wouldn’t be applicable in this fire protection option. 

The USDA Forest Service’s Forest Products Laboratory (FPL) is working on a testing program to evaluate the fire effects of holes through glulam, both with and without penetrants installed in the openings. FPL and Arup presented the results of a preliminary study at the 2023 World Conference on Timber Engineering and will publish additional information as it becomes available.  

It is usually advantageous to minimize the number and size of holes and openings to avoid significant impacts on the structural and fire design of exposed glulam beams. Perhaps more so than with other building materials, mass timber framing systems should be coordinated early with MEPF to determine how structural grids and MEPF routing/integration can be optimized. For more information on the integration of MEPF systems in mass timber buildings, see the WoodWorks expert tip, Accommodating MEP in Exposed Mass  Timber Buildings.