Nailing connector technique
Best practice in nails and connectors goes way beyond whacking it with a hammer. Dean Ashton from Simpson Strong-Tie explains why the correct installation is critical.
There is a wide range of products in the marketplace for joining timber members together – from simple joist hangers for joining a timber joist to a beam, to more advanced connections that are used in mid-rise timber construction. A lot of times these connectors may be hidden behind cladding materials and not given much consideration; however, they are a structural component that can aﬀect the overall performance of the structure.
Traditional construction with timber often required bolted connections which typically governed the size of the members required. Larger timber members were needed to allow for the required edge or end distances, as well as the spacing of the bolts.
Metal connector brackets, that are nailed and/or screwed, have been available for decades and they allow for better optimising of timber sizes as well as faster and more eﬃcient construction methods. A number of the manufacturers that supply these connectors invest considerable amounts of time and eﬀort into research and development, as well as product testing to ensure that the connectors perform to engineering speciﬁcation.
For these connectors to perform correctly, they also need to be installed correctly. When ﬁrst introduced into the market, these connectors were typically hand nailed with the good old fashioned hammer. The general rule of thumb was that if there was a nail hole in the connector – drive a nail through it. And not just any old nail. There are speciﬁc connector nails for this application. The connector manufacturer will specify the length and diameter of the nails required as well as the required number of nails.
It is also important to use the correct connector and install it as per the manufacturer’s instructions. Modifying connectors by cutting or bending them may have serious consequences unless the connector manufacturer speciﬁes what alterations can be made. There has been a lot of publicity recently about non-compliant and nonconforming building products. The key is that the product must be ﬁt for purpose. This also applies to connectors.
The durability of the connector and the associated nails should also be considered. If the connector is to be in a corrosive environment, such as building a sheltered deck in close proximity to a coastal location, heavier galvanising or stainless steel connectors may be required. And when using stainless steel connectors, stainless steel nails should always be used to prevent galvanic or dissimilar metal corrosion.
A common mistake when installing connectors is not installing the correct number of nails. If the correct number of nails are not used, the connector may not fully resist the loads intended which may result in damage to the structure.
Most building sites now use some form of pneumatic nailing and the hammer is left hidden in the tool box. The use of pneumatic nailing introduces a wide range of issues when installing the connectors. The ﬁrst issue that needs to be considered is safety when nailing through metal products, and it is recommended that you consult the nail gun manufacturer for any speciﬁc guidelines.
As mentioned above the general rule of thumb for installing connectors is to drive a nail through the nail hole. Most pneumatic nail guns do not have a nail locater and the end result is that the nail is driven directly through the metal. This may reduce the overall capacity of the connector by reducing the amount of steel in the connector. If the nail is located too close to a nail hole it may increase the hole size and further reduce the capacity of the connector. The pressure used in the nail gun may also have an eﬀect. Too much pressure and the nail may be overdriven (often seen if the nail dimples the connector or in some cases punches completely through) which again will reduce the capacity.
The actual locations of the nails will also aﬀect the capacity of the connector. For the nails to be eﬀective they require suﬃcient edge distances to the steel and the timber. Too close to the edges may result in the connector and/or timber to split. If the nails are too closely spaced, they lose their eﬀectiveness to hold into the timber and again reduce the capacity of the connector.
Pneumatic nails guns can drive nails that are a smaller diameter than the speciﬁed hand driven connector nails. The size of the nail used is critical for the performance of the connector as smaller diameter nails will have a lower shear capacity and shorter length nails will have lower nail withdrawal capacity. Some manufacturers may give information about reduction factors, others may recommend additional nails be installed, while others will only recommend the hand nailed option. It is important to understand that the published load data will have speciﬁc nail lengths and diameter. Any variation to these criteria at point of installation may impact on the engineered load carrying capacity.
Building inspectors are becoming more aware of the installation errors of the connectors and in some cases projects have been delayed, along with the additional costs, due to rectiﬁcation works needed to be carried out to poorly installed connectors.
So what can be done to ensure that your connectors are correctly installed and that the structural integrity of the building is not aﬀected? It is recommended that you refer to the connector manufacturer’s installation instructions. Follow these and your connections will perform as intended.
Your timber connectors may end up getting hidden behind the cladding, but it is important for the performance of the structure that they are installed correctly.