Holes and notches in EWP need careful placement, but the guides can be confusing. By Craig Kay.
In the recent past, when most floor systems were constructed from sawn solid timber elements, it was standard practice for builders to run bulkheads below floors for services that needed to run perpendicular. Bulkheads reduce ceiling heights, so it was common to see bulkheads close to walls in low trafficked areas, and on top of built-in cupboards and benches. It was also common to see service pipes from the upper storey wet areas and toilets extend down the outside of external walls, creating a negative aesthetic effect, exposing PVC pipe to UV from sunlight and making maintenance more difficult.
The introduction of EWP elements such as wood I-Joists and parallel chord floor trusses revolutionised the two-storey floor construction market by allowing the services to be routed through these elements, reducing or removing the need for ceiling bulkheads. Happy days! Problem solved. Well, not quite.
Modern two-storey house architecture typically involves both the upper storey having less floor area than the ground floor and irregular shapes, requiring the upper storey to be supported on a grid of support beams, both perpendicular and parallel to the floor joists. With plumbing services for the upper storey now contained within the building envelope, and usually directed to one of more service ducts, it is common for the services to be ‘ring-fenced’ inside heavy solid elements such as steel beams, LVL or glulam.
An example scenario is shown in Image 1 (below), where a 100mm service pipe needs to penetrate the solid section primary support beam to connect to the sewer outlet via the services chute.
When the designer seeks to structurally analyse the effect of the penetration in the beam, there is little or no information available to them within AS 1720.1: Timber Structures – Design methods.
AS 1684 Section 4 does provide some deemed-to-satisfy solutions in Figure 4.1 for bearers and joists. For an element 200mm or deeper, the deemed-to-satisfy maximum hole size is 50mm Ø. In this case the 50mm hole is not sufficient for the above scenario, so that advice doesn’t provide a solution. Where to now?
While on the topic of Figure 4.1 from AS 1684, I do have some personal concerns about the consequences of undisciplined use of item (i) in cyclonic areas, (see Image 2, below). When this figure was first introduced into 1684, most timber subfloors would have been typically on an 1800mm grid of bearers and joists, with a joist section size of 90 x 45mm and the bearer typically 90 x 75mm or 2/90 x 45mm members laminated together.
In cyclonic areas in particular, the provision in (i) to allow a vertical hole of a diameter of D/4 at 6B intervals in a joist/bearer would most commonly be used for 12mm Ø cyclone tie down rods, or braced wall anchoring requiring 13kN resistance. When bearers were typically 75mm wide and only spanning 1800mm, this rule of thumb provision is reasonable, but it may become problematic with the longer spans and spacing available with EWP products such as LVL or GLT.
Currently, where EWP Suppliers’ Design Centres fiercely compete to provide the most economical solution, there is technically nothing stopping a designer looking up a span table, or via software, and placing a single 3000mm-long, 300 x 45mm LVL floor joist under an external parallel load-bearing wall.
Once on site, the builder could theoretically use the provisions of AS 1684 and drill 11mm Ø vertical holes at a minimum spacing of 270mm along the beam, as a deemed-to-satisfy solution.
Additionally, the provisions of (e) in Figure 4.1 (Image 3, below) could be adopted, and a 25mm notch up to 100mm wide could be created under these holes, to allow the nut and washer to not protrude below the bottom of the beam.
Is that what the creators of AS 1684 envisaged? I think not. Unfortunately, we see these sorts of issues reported from our technical support team.
After that long diversion, back to the original topic, where do engineers get help in determining if that 100mm pipe can penetrate this beam?
There are three references that I would recommend:
Effect of Large Diameter Horizontal Holes on the Bending and Shear Properties of Laminated Veneer Lumber. APA Technical Note number V900C April 2020.
Design of Reinforcement of round Holes in Laminated Veneer Lumber (LVL) Beams by Manoochehr Ardalany, Massimo Fragiacomo, Peter Moss and Andrew Buchanan. New Zealand Timber Design Journal Vol 2, Issue 4.
NZ Wood Design Guides – Reinforcement of Timber Members, Chapter 12.6, September 2020
All references approach the analysis of the effect of the holes slightly differently, with the second two references providing guidance on the engineering design for the reinforcement of the hole.
The second element of today’s modern design is that these primary beams supporting the upper story often extend out into the roofline of the external lower storey walls, and therefore need to be tapered/chamfered at their end to avoid interference with the roof cladding.
Here, AS 1684 provides a deemed-to-satisfy solution for the allowable chamfer for a strutting beam (Section 7.2.9, see Image 4, below) to be reduced to a minimum end dimension of 100mm or D/3 whichever is greater, however, Section 4 of AS 1684 which deals with Floor Framing, remains silent on this issue.
The industry by and large seems to have taken a deemed-to-satisfy provision for a roof strutting beam with its unique loading pattern and applied that chamfer principle to all primary timber beam elements, which from an engineering perspective, may be a dangerous assumption.
Engineers can design these primary timber beams for the effect of tapers, chamfers and notches etc by using appendix C of AS 1720.1.
Extra references which provide shear equations for tapered ends include:
- APA – The Engineered Wood Association, Technical Note: Field Notching and Drilling of Glued Laminated Timber Beams, No S560J, February 2020.
- APA – The Engineered Wood Association, Technical Note: Field Notching and Drilling of Laminated Veneer Lumber, No G535B, February 2020.
I have listed both the APA documents, one for GLT and one for LVL, because it is interesting that the APA offers shear equations for a minimum height of GLT of D/3, whereas it does not offer shear equations for LVL for a residual depth below 0.6 D, almost double the depth for GLT.
Tilling’s new SmartFrame 2021 software, now undergoing Beta testing before selected release, will allow appropriately registered users the ability to place notches, tapers etc. in the element, thus removing the need to perform hand calculations to carry out the analysis or, because that is laborious, assume.
Reputable EWP suppliers have engineers on staff who provide advice to users of their product, and to help train those many consulting engineers in the construction industry that are experts on steel and concrete, but unfortunately are not as skilled in the finer points of timber engineering.
With the support available, there really is no excuse for holes inappropriately cut into primary support elements, but awareness of the dangers in these areas is still a work in progress.
For more information on this topic, contact Craig Kay and the Tilling engineers via email at email@example.com