In various structural situations, engineers may need to evaluate the strength of joints made by welds and mechanical fasteners. Today, mechanical fasteners are usually bolts, but older designs may have rivets.
This can happen during upgrades, renovations, or enhancements to a project. A new design may require bolting and welding to work together in a joint where the material to be joined is first bolted together and then welded to provide full strength to the joint.
However, determining the total load capacity of a joint is not as simple as adding up the sum of the individual components (welds, bolts, and rivets). Such an assumption could lead to disastrous consequences.
Bolted connections are described in the American Institute of Steel Structures (AISC) Structural Joint Specification, which uses ASTM A325 or A490 bolts as tight mount, preload, or sliding key.
Tighten tightly tightened connections with an impact wrench or locksmith using a conventional double-sided wrench to ensure that the layers are in tight contact. In a prestressed connection, the bolts are installed so that they are subjected to significant tensile loads, and the plates are subjected to compressive loads.
1. Turn the nut. The method of turning the nut involves tightening the bolt and then turning the nut an additional amount, which depends on the diameter and length of the bolt.
2. Calibrate the key. The calibrated wrench method measures the torque that is associated with bolt tension.
3. Torsion type tension adjustment bolt. Twist-off tension bolts have small studs on the end of the bolt opposite the head. When the required torque is reached, the stud is unscrewed.
4. Straight pull index. Direct tension indicators are special washers with tabs. The amount of compression on the lug indicates the level of tension applied to the bolt.
In layman’s terms, bolts act like pins in tight and pre-tensioned joints, much like a brass pin holding a stack of perforated paper together. Critical sliding joints work by friction: preload creates downforce, and friction between the contact surfaces works together to resist slippage of the joint. It’s like a binder that holds a stack of papers together, not because holes are punched in the paper, but because the binder presses the papers together and the friction holds the stack together.
ASTM A325 bolts have a minimum tensile strength of 150 to 120 kg per square inch (KSI), depending on the bolt diameter, while A490 bolts must have a tensile strength of 150 to 170-KSI. Rivet joints behave more like tight joints, but in this case, the pins are rivets that are typically about half as strong as an A325 bolt.
One of two things can happen when a mechanically fastened joint is subjected to shear forces (when one element tends to slide over another due to an applied force). Bolts or rivets can be at the sides of the holes, causing the bolts or rivets to shear off at the same time. The second possibility is that the friction caused by the clamping force of the pretensioned fasteners can withstand shear loads. No slippage is expected for this connection, but it is possible.
A tight connection is acceptable for many applications, as slight slippage cannot adversely affect the characteristics of the connection. For example, consider a silo designed to store granular material. There may be slight slippage when loading for the first time. Once slip occurs, it will not happen again, because all subsequent loads are of the same nature.
Load reversal is used in some applications, such as when rotating elements are subjected to alternating tensile and compressive loads. Another example is a bending element subjected to fully reverse loads. When there is a significant change in load direction, a preloaded connection may be required to eliminate cyclic slip. This slip eventually leads to more slip in the elongated holes.
Some joints experience many load cycles which can lead to fatigue. These include presses, crane supports and connections in bridges. Sliding critical connections are required when the connection is subjected to fatigue loads in the reverse direction. For these types of conditions, it is very important that the joint does not slip, so slip-critical joints are needed.
Existing bolted connections can be designed and manufactured to any of these standards. Rivet connections are considered tight.
Welded joints are rigid. Solder joints are tricky. Unlike tight bolted joints, which can slip under load, welds do not have to stretch and distribute the applied load to a large extent. In most cases, welded and bearing type mechanical fasteners do not deform in the same way.
When welds are used with mechanical fasteners, the load is transferred through the harder part, so the weld can carry almost all of the load, with very little shared with the bolt. That is why care must be taken when welding, bolting and riveting. Specifications. AWS D1 solves the problem of mixing mechanical fasteners and welds. Specification 1:2000 for structural welding – steel. Paragraph 2.6.3 states that for rivets or bolts used in bearing-type joints (i.e. where the bolt or rivet acts as a pin), mechanical fasteners should not be considered to share the load with the weld. If welding is used, they must be provided to carry the full load in the joint. However, connections welded to one element and riveted or bolted to another element are allowed.
When using bearing-type mechanical fasteners and adding welds, the load-bearing capacity of the bolt is largely neglected. According to this provision, the weld must be designed to transfer all loads.
This is essentially the same as AISC LRFD-1999, clause J1.9. However, the Canadian standard CAN/CSA-S16.1-M94 also allows stand-alone use when the power of the mechanical fastener or bolt is higher than that of welding.
In this matter, three criteria are consistent: the possibilities of mechanical fastenings of the bearing type and the possibilities of welds do not add up.
Section 2.6.3 of AWS D1.1 also discusses situations where bolts and welds can be combined in a two-part joint, as shown in Figure 1. Welds on the left, bolted on the right. The total power of welds and bolts can be taken into account here. Each part of the entire connection operates independently. Thus, this code is an exception to the principle contained in the first part of 2.6.3.
The rules just discussed apply to new buildings. For existing structures, clause 8.3.7 D1.1 states that when structural calculations show that a rivet or bolt will be overloaded by a new total load, only the existing static load should be assigned to it.
The same rules require that if a rivet or bolt is only overloaded with static loads or subjected to cyclic (fatigue) loads, sufficient base metal and welds must be added to support the total load.
The distribution of load between mechanical fasteners and welds is acceptable if the structure is preloaded, in other words, if slippage has occurred between the connected elements. But only static loads can be placed on mechanical fasteners. Live loads that can lead to greater slippage must be protected by the use of welds capable of withstanding the entire load.
Welds must be used to withstand all applied or dynamic loading. When mechanical fasteners are already overloaded, load sharing is not allowed. Under cyclic loading, load sharing is not allowed, since the load can lead to permanent slippage and overload of the weld.
illustration. Consider a lap joint that was originally bolted tight (see Figure 2). The structure adds extra power, and connections and connectors must be added to provide double the strength. On fig. 3 shows the basic plan for strengthening the elements. How should the connection be made?
Since the new steel had to be joined to the old steel by fillet welds, the engineer decided to add some fillet welds at the joint. Since the bolts were still in place, the original idea was to add only the welds needed to transfer the extra power to the new steel, expecting 50% of the load to go through the bolts and 50% of the load to go through the new welds. It is acceptable?
Let’s first assume that no static loads are currently applied to the connection. In this case, paragraph 2.6.3 of AWS D1.1 applies.
In this bearing type joint, the weld and bolt cannot be considered to share the load, so the specified weld size must be large enough to support all of the static and dynamic load. The bearing capacity of the bolts in this example cannot be taken into account, because without a static load, the connection will be in a slack state. The weld (designed to carry half the load) initially ruptures when the full load is applied. Then the bolt, also designed to transfer half the load, tries to transfer the load and breaks.
Further assume that a static load is applied. In addition, it is assumed that the existing connection is sufficient to carry the existing permanent load. In this case, paragraph 8.3.7 D1.1 applies. New welds only need to withstand increased static and general live loads. Existing dead loads can be assigned to existing mechanical fasteners.
Under constant load, the connection does not sag. Instead, the bolts already bear their load. There has been some slippage in the connection. Therefore, welds can be used and they can transmit dynamic loads.
The answer to the question “Is this acceptable?” depends on load conditions. In the first case, in the absence of a static load, the answer will be negative. Under the specific conditions of the second scenario, the answer is yes.
Just because a static load is applied, it is not always possible to draw a conclusion. The level of static loads, the adequacy of existing mechanical connections, and the nature of the end loads—whether static or cyclic—may change the answer.
Duane K. Miller, MD, PE, 22801 Saint Clair Ave., Cleveland, OH 44117-1199, Welding Technology Center Manager, Lincoln Electric Company, www.lincolnelectric.com. Lincoln Electric manufactures welding equipment and welding consumables worldwide. Welding Technology Center engineers and technicians help customers solve welding problems.
American Welding Society, 550 NW LeJeune Road, Miami, FL 33126-5671, phone 305-443-9353, fax 305-443-7559, website www.aws.org.
ASTM Intl., 100 Barr Harbor Drive, West Conshohocken, PA 19428-2959, phone 610-832-9585, fax 610-832-9555, website www.astm.org.
American Steel Structures Association, One E. Wacker Drive, Suite 3100, Chicago, IL 60601-2001, phone 312-670-2400, fax 312-670-5403, website www.aisc.org.
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