Approximately 5-minute reading time.
MTC Solutions offers fasteners in both coated carbon steel and stainless-steel materials to meet the exposure requirements of the North American timber construction industry. This blog post will define the two main exposure conditions, dry service conditions and wet service conditions, and will explain which type of screws to use in each condition.
1) Dry Service Conditions
Based on the exact needs of the connection, several screw options are available. The ASSY VG CSK and the ASSY Ecofast screws are reliable options with a versatile countersunk head shape that can be used in dry service conditions, as well as being installed countersunk into the steel plate, and installed using MTC’s 90-degree and 45-degree wedge washers which will be explained later in this blog post.
Dry service conditions may generally be considered if the average equilibrium moisture content of the wood material remains below 15% on average, never exceeding 19% over the span of a year at any given time. Connections designed in dry service conditions may utilize both:
Regular ASSY self-tapping screws and SDD are coated with a thin protective layer of blue passivated zinc to resist minor corrosion risk during construction periods and are intended for usage in low corrosion environments such as untreated wood members and dry service conditions.
Connections utilizing MTC Solution SDD’s in combination with knife plates should consider the configuration of the connection and the potential swelling/settling effect of moisture content variation. SDD’s should be installed in dry timber and the connection should be kept at a constant moisture content to prevent the wood from swelling or settling, creating restraining stresses. The fasteners should be installed in dry timber and the connection should be protected from dramatic moisture content changes to prevent the wood from swelling or settling, creating restraining stresses. More information on self-drilling dowels and knife plates, and how to avoid restraining stresses can be found in our Designing Internal Knife Plate blog post.
2) Wet Service Conditions
A connection may be considered in wet service conditions if it exceeds the moisture content requirements of 15% on average. Wet service conditions include direct exposure to the elements but sheltered outdoor applications must also be designed with great attention to detail as corrosion issues may arise due to wind-driven rain, splashing, and poor air circulation. Connections that do not meet the requirements for dry service conditions shall be designed with fasteners with a higher class of corrosion resistance such as the stainless-steel ASSY self-tapping screws available in three models:
Agnostic stainless-steel hardware typically offers lower material strength properties and is more expensive than regular carbon steel alternatives, however, it offers unequalled longevity for the life span of the connection.
2.1) Detailing considerations for wet service conditions
Steel-to-Wood Connections. If a connection in a corrosive environment includes two dissimilar electrochemical metals in physical contact, contact corrosion may occur. As the electro potential difference between two dissimilar metals is the driving force for corrosion, both fastener and steel plate shall be of materials as close together as possible in the galvanic series and the fastener shall be of a material higher in the galvanic series so that it acts as the cathode. Additionally, a physical separation such as a plastic washer fitted between the screws and steel plate may be used.
Wood Material. The usage of glulam and timber is allowed in wet service conditions, however, design resistances of connections is reduced by a service condition factor (CM or KS). The usage of CLT in wet service conditions is generally restricted.
Connection Design. A connection designed in wet service conditions should be detailed in a way that prevents water from accumulating on top or under the wood member.