# Determining the suitability of a timber joint in a given situation

There seems to be a lot of information around calculating the forces exerted on a structure. When joining wood, once the structural analysis is done, how is the performance limit of a certain type of joint determined?

E.g. building a wooden frame in the garden

1. Building a pergola - Relatively weak joins may be used as the loads are negligible compared to strength. This relatively weak joint is fine for supporting the "roof" of the frame

2. Putting solar panels on top of the same pergola - Now there is in increased load the frame has to deal with, as well as even greater forces when there are strong winds. Nobody will be recommending the pergola above for that purpose

I.e. How could an engineer determine if a mortice and tenon joint is sufficiently strong for a given joint or if it would be better to use a butt joint with screws?

• I can't tell you the names of them, but I know that there are handbooks that detail all of this, with the strengths of various joints in various kinds of timber spelled out. Commented Mar 23 at 15:44

## 2 Answers

By destructive testing of the different kinds of joints under different kinds and magnitudes of loads.

This means that joints of various kinds are constructed, then loaded in a particular fashion until they break, taking measurements of the forces needed to deform the joint and/or separate the parts. Then, some sort of analysis of the results is used to produce structural ratings for the different joints in specified conditions; and if a consensus can be reached for some application, a committee is likely to draft a standard to make it easier to specify requirements for a certain design. An example of such a specification is (in the USA) the National Design Specification for Wood Construction.

Here, for example, is an excerpt from a technical handbook referencing empirical methods:

Maximum lateral resistance and safe design load values for small-diameter (nails, spikes, and wood screws) and largediameter dowel-type fasteners bolts, lag screws, and drift pins) were based on an empirical method prior to 1991. Research conducted during the 1980s resulted in lateral resistance values that are currently based on a yield model theory. This theoretical method was adapted for the 1991 edition of the National Design Specification for Wood Construction (NDS). Because literature and design procedures exist that are related to both the empirical and theoretical methods, we refer to the empirical method as pre-1991 and the theoretical method as post-1991 throughout this chapter. Withdrawal resistance methods have not changed, so the pre- and post-1991 refer only to lateral resistance. Wood Handbook, Forest Products Laboratory. 1999. Ch. 7

Not just engineers use this method: carpenters often consider themselves, with varying degrees of justification, as engineers of sorts.

Most woodworking joints have evolved over thousands of years. Woodworkers learned to make them based on their own experience, and the experience of the people who taught them. To make successful joints you need a little experience, a basic knowledge of the workings of wood and glue, and a bit of common sense. But in the 21st century we want a definitive answer. We want numbers and we want proof. We want something in writing we can point to when we’re not sure of ourselves.

Like other magazines, we have tested joints to destruction. Popular Woodworking

• I would trust a good carpenter over an engineer who only dabbles in wood construction. Commented Mar 25 at 20:36

Screws are not advised for connecting frames, They are too brittle. They are usually used only for decking or attaching accessories. Nails are recommended and there are standards on how much shear and pull out force or bending moment they can take.

The mortice and tenon joints can be strong for interior use. They crack and warp when exposed to elements unless their aesthetic value overrules the high maintenance cost.

• Thanks for the response. What I was trying to get at is how does an engineer know to choose one of the other in a more general sense than the exact examples I used. Commented Mar 23 at 18:16