Bolted connections in bridge structures are almost all installed using a torque-controlled tightening method. The use of controlled tightening enables the development of friction force between the plates of the bolted connections; they are therefore slip-critical connections. The resulting friction force allows for an enhanced performance of connections in fatigue conditions, reduces deflections in the structure and prevents bolts from loosening when subjected to impact loads and vibrations.
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The Turn-of-Nut method is widely used to obtain the minimum preload tension force specified for bolts in slip-critical connections. This method consists in first snug-tightening the bolts. This first step is achieved with a few impacts of a pneumatic impact wrench or the full effort of a person using a spud wrench to bring the connected surfaces into firm contact with each another1.
In the most recent edition of the Canadian Highway Bridge Design Code, CAN/CSA-S6-14, the cs and ks coefficients that are used to calculate the slip-resistance of bolted connections have been modified, resulting in a superior slip-resistance calculation with the Turn-of-Nut method than that obtained with the coefficients in the earlier edition. For example, sand-blasted connections (Class B) secured with ASTM A325 bolts will have a 20% greater slip-resistance when calculated with the values indicated the 2014 edition than with those indicated in the 2010 edition.
Reliable connections in structural steel assemblies must accompany superior corrosion protection. Hot-dip galvanized (HDG) coatings produce maintenance-free corrosion protection for many years. The structural connections must provide equivalent corrosion protection as well as structural integrity to ensure maintenance-free performance. A solid structural connection is ensured by providing corrosion protection for the bolt and nut connection and providing clearance for the HDG coating special treatment of the nut. A critical factor in structural connections is the slip factor for the faying surfaces. This article discusses recent changes made by AASHTO to the design parameters for HDG faying surfaces and the increased clearance holes for connections using bolts with a diameter above 1 inch.
Although the hot-dip galvanized coating does not affect design strength considerations for bearing type connections, the design of slip critical connections is affected by the slip coefficient of the hot-dip galvanized faying surfaces. Traditionally, industry standards denote newly hot-dip galvanized steel has a lower mean slip coefficient (μ = 0.30) than blast cleaned bare steel (μ = 0.50) or bare steel painted with Class B coatings (μ = 0.50). As a result, design freedom and cost can be affected when specifying hot-dip galvanizing for corrosion protection. More bolts, holes, and joints are required in the design of high-strength structural connections. Applying Class B zinc-rich paints (ZRP) over HDG faying surfaces can significantly increase the slip coefficient and provide a greater variety of coating options to the specifier/designer without affecting long-term corrosion protection. Recent slip factor and tension creep testing performed by the American Galvanizers Association (AGA) indicates higher slip coefficients (μ = 0.45 or μ = 0.50) can be achieved by applying Class B zinc-rich paints to hot-dip galvanized faying surfaces which have been prepared with a chemical pre-treatment/conversion coating. Based on these findings, the 8th edition of the AASHTO Load Resistance Factor (LRFD) Bridge Design Specification includes updated class definitions to include any blast-cleaned surfaces coated with zinc-rich paints (Class D, μ = 0.45).
Eventually, specifications related to structural connections used in other industries may be similarly revised. Slip critical testing on different HDG surfaces indicates that wire brushing of the HDG faying surface is no longer required as the brushing can smooth the coating and reduce the slip factor.
For the FHWA/bridge customer, the revisions allow a greater variety of coating systems that can be used to design high strength slip critical connections. Specifically, it will become easier and more economical for the specifier to select hot-dip galvanizing and metalizing for corrosion protection. Although the slip coefficient for hot-dip galvanized surfaces is reduced from 0.33 to 0.30 in this revision, it is anticipated that the new value will have minimal impact on design. However, there is a potential for a small increase in the number of bolts used in connections with HDG fasteners. Regardless, customers will benefit from the removal of additional labor previously required to roughen HDG faying surfaces. For the new Class D (μ = 0.45) surface condition, a slightly lower slip coefficient value is provided than for Class B (μ = 0.50). However, the value will not significantly impact the overall number of bolts required for most high-strength bolted connections. Therefore, the addition of Class D simply provides a greater variety of coating options to the specifier/designer, including the use of HDG surfaces with zinc-rich paints.
The combination of HDG coating and paints containing zinc silicate can increase the design slip factor and provide a decrease in the number of bolts required for a slip critical connection. Two specific paint materials that have been tested for slip factor and creep properties when applied to HDG coatings are:
Because hot-dip galvanizing is a coating of corrosion-inhibiting, highly abrasion-resistant zinc on bare steel, the original steel becomes slightly thicker. When talking about tapped holes and fasteners, the increased thickness is an important design consideration. Previously, the thickness of hot-dip galvanizing left the specifier choosing between reaming out through-holes after hot-dip galvanizing or further reducing slip resistance by specifying oversized holes that would allow sufficient clearance for the bolt. As a result, design freedom and cost can be affected when specifying hot-dip galvanizing for corrosion protection as more bolts, holes, and joints are required.
The resistance of bolted slip-critical connection is verified on a simple example of a bolted splice connection. The resistance in CBFEM is slightly decreased by small tensile forces, and therefore, the resistance by CBFEM is lower by 3%.
Abstract:Composite structures have become increasingly popular in civil engineering due to many advantages, such as light weight, excellent corrosion resistance and high productivity. However, they still lack the strength, stiffness, and convenience of constructions of fastener connections in steel structures. The most popular fastener connections in steel structures are slip-critical connections, and the major factors that influence their strength are the slip factors between faying surfaces and the clamping force due to the prevailing torque. This paper therefore examined the effect that changing the following parameters had on the slip factor: (1) replacing glass fiber reinforced plastic (GFRP) cover plates with stainless-steel cover plates; (2) adopting different surface treatments for GFRP-connecting plates and stainless-steel cover plates, respectively; and (3) applying different prevailing torques to the high-strength bolts. The impact on the long-term effects of the creep property in composite elements under the pressure of high-strength bolts was also studied with pre-tension force relaxation tests. It is shown that a high-efficiency fastener connection can be obtained by using stainless-steel cover plates with a grit-blasting surface treatment, with the maximum slip factor reaching 0.45, while the effects of the creep property are negligible.Keywords: GFRP composite structures; slip-critical connection; stainless-steel cover plates; surface treatment; prevailing torque
Fastener Finish: Hot-dip galvanizedConnection Type: Bearing & slip criticalLoading Type: Tension/shearLoad Combinations: Gravity loads, wind loads, and seismic loads (SDC A-F)Load Interaction:
Fastener Finish: Hot-dip galvanized, Mechanical galvanized, Electroplated, Thermal Oxide (blackened), Magni (Zinc Rich)Connection Type: Bearing & slip criticalLoading Type: Tension/shearLoad Combinations: Gravity loads, wind loads, and seismic loads (SDC A-C)Load Interaction: AISC Specification Section J3.7 or J3.9ICC-ES Report: No current third-party test report availableInstallation Video: View siteProduct Website: View site
An important assumption when determining the shear in bolts WHERE THE APPLIED CONNECTION FORCE IS CONCENTRIC WITH THE BOLT GROUP is that all shear planes in a connection see the same applied shear STRESS, fv. This is an appropriate assumption because all bolts are constrained by the connected members to deform the same. If they all have the same shear deformations, then they all have the same shear strains and hence they all have the same shear stress. If all the bolts have the same diameter then, all shear planes see the same shear force. This is true for both slip critical and bearing connections.
The bolt cross sectional area is critical to the prior calculation. The area of bolt available for use by a shear plane will depend on whether or not the shear plane is in a region of the bolt that is threaded. If the shear plane is in a region that is threaded then there is less area available to resist the applied shear force. The AISC specification requires you to determine whether or not threads are included in the shear plane or not when computing the capacity of a bearing type connection. This is not an issue in slip critical connections since the force is transferred by friction between the connected members and not by actual shear in the bolt. We will look at this some more when we start computing actual bolt capacities.
When a connection slips, there is slip everywhere except for at the IC. It then follows that at the moment of slip the magnitude of the reaction force associated with each bolt equals it capacity to resist slip. Theoretically, the force that causes slip at a bolt equals the normal force (bolt pretension) times the coefficient of static friction. In the Section 4.7, we will investigate the SCM equations for determining slip capacity. 2ff7e9595c
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