What do you need to look out for when specifying your frame?
If any components in your frame are under or oversized, there is a risk that your frame will not be structurally viable.
A conventional gravity build will stand up, as is reflected in the name, with the help of gravity, whereas a structural frame is a self-supporting structure. As such, the components in it need to be balanced in proportion, size, strength, and positioning – while considering bracing. If you have undersized or oversized components within a self-supporting structure, it will unbalance the frame and potentially leave you at risk of racking or partial or complete collapse of the frame.
Also, the braces within the frame need to be proportional to the frame, and the joints on the braces need to be of a correct fit to the tenons. What you will quite often find is the framers, for ease of direction, will make the tenons on the braces smaller so they can be located after the main beams are in position; this will, in effect, make the peg the joint. The peg should only be there as a locator of the mortise and tenon and that is what is calculated for the structural level.
Alongside this, the peg will have to be positioned in such a way that there is enough material of the tenon around the peg – so that the peg/joint is structurally correct.
What Is Racking?
Racking is the term used for when a building tilts as its structural components are forced out of plumb. This is most commonly caused by wind forces exerting horizontal pressure on the building.
Buildings need to withstand wind forces in order to pass building regulations. This is done through structural calculations via a structural engineer. These structural calculations need to be site-specific, taking into account the orientation of the building as the wind will be stronger in certain directions.
Ways of withstanding racking will vary depending on whether the side is open or enclosed. If the side is open then the usual way is to use knee braces across the corners between eaves, beam, and post. These braces need to be sized by the engineer because they will differ depending on the size of the building. For the enclosed sides of the building, you may only need to use diagonal stud work braces or the elevation may need to be sheathed in OSB3 or plywood. You can also use knee braces and make them a feature on the inside.
What Is Sagging?
Sagging is also called deflection in a beam. When a beam has a load applied to it, as an eaves beam does when the rafters sit on it, then there will be deflection in that beam. This deflection will be downwards because of the weight of the roof, and in the case of a vaulted roof, there will also be sideways deflection. All structural beams need to be sized correctly to withstand this deflection, but since oak is a natural material you will not be able to totally eradicate it. The beams are always going to sag or bow slightly and the structural calculations allow for a certain movement – this can be up to about 8mm.
This is why garage doors on a building will need adjusting over time, as the roof settles and the beams adjust. This is completely normal and will have been allowed for when the calculations were made.
The mortise and tenant joint is of fundamental importance.
Some oak framers under-size the tenant, meaning that the tenant ends up being a lot smaller than the mortise. They do this for ease of construction so they can install the eve beams on the posts and insert the braces afterwards.
In this situation, the peg (which is the locator), actually becomes the joint. Also, if the tenant is under-sized, there likely won’t be enough wood left around the drill hole to meet minimum standards. This will greatly increase the risk of racking.