Siu Kong Fox, MIEAust, CPEng, NER, BPBvic, RPEQ, NSW Structural Engineer
p: 02 9912 8100 e: email@example.com
Nails and screws are the two most common fasteners speciﬁed by engineers and used by builders. When considering the suitable fasteners to use, sometimes we have options and sometimes we don’t. Let’s have a look at some common sizes and types of nails and screws which engineers often specify in the design drawings.
30mm x Ø2.8mm Reinforced Head Nail
This type of nail is suitable for Multi Grips, Joist Hangers, metal and plywood bracing, and the like. It has a reinforced head, is easy to carry and can be found in most carpenter bags. However, from an engineering point of view, its capacity is relatively low compared to bigger sizes of nails.
Fig 1 – Typical application of Multinail Multi Grip with Ø2.8mm nails
Fig 2 – Typical fixing of plywood gussets with Ø2.8mm nails at apex joint of roof truss
Ø3.05mm Gun Nail
The most common lengths of gun nails in timber frames are 75mm and 100mm. They are pneumatically driven nails with equivalent capacities comparable to Ø2.8mm hand-driven nails. It is ideal for fixing two pieces of timber together without any metal ancillaries, however, it is not recommended for fixing metal connectors to timber members.
It is quite hard to achieve the required end or edge distance as per design standards of timber structures using these gun nails. Refer to Table 1 for more information regarding the minimum end and edge distance and spacing for nails.
Note: D=Shank diameter of screws
Gauge 12 (Ø5.59mm) Screw
These screws are available in 35mm, 65mm and 100mm lengths with a much higher capacity than nails. They are also suitable for timber to timber connections with metal brackets or steel angles.
Gauge 14 (Ø6.4mm) Screw
These screws are available in 75mm, 100mm, 125mm and 150mm lengths with capacities higher than Gauge 12 Screws. They are commonly used in timber joints where higher forces need to be transferred. It is also a common screw size for fixing roof battens and purlins.
Fig 3 – Typical application of Multinail Easy Fix Girder Bracket and Green Tip #12 Screws
When designing screw fixing details, minimum end and edge distance and spacing for screws need to be considered as well. Same as nails, Table 2 below listed those critical distances according to AS1720.1
Note: D=Shank diameter of screws
Fig 4 – Typical fixing of timber web member with Gauge 14 Screws
The Stud Screw was introduced to timber framing industry to tie down top and ribbon plates to studs in order to replace traditional stud ties or T-plates. It is installed in the factory of wall frame fabricators, along with a tag attached for inspecting purposes.
Screws can hold two pieces of timber together tighter due to their threaded shafts. They also perform better with moisture changes in timber in long term. Screws can provide great tensile strength however, they don’t have good performance when taking shear. Nails can provide greater shear strength, but they may bend under certain amount of load or pressure.
Nails and screws are both excellent fasteners however, considerations need to be made when selecting which type of fasteners that you are going to use in your design.
Edge and end distance and distance between fasteners. This condition may result in your only option being the use of nails.
Penetration depth. Make sure the fasteners are penetrating deep enough into the second member. For example, screws need to penetrate 7 times of its shank diameter into the connecting member to achieve their full shear capacities.
Timber species. Softwood and hardwood may also have some impact on which type of fasteners you should be using. Fixing fasteners into hardwood requires pre-drilling for screws and sometimes nails.
We hope the information we have shared helps with selecting appropriate fasteners for your timber connections moving forward. If I may be of any assistance, please contact me on 02 9912 8100 or firstname.lastname@example.org.
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