Banging Bolt SyndromeMay 02 2018 · 0 comments · NISHKIAN DEAN, Steel-framed, Technical notes ·0
By Deke Black, P.E.
Banging bolt syndrome happens when structural bolts, usually in a beam shear tab, slip from a friction condition into a bearing condition — the transition occurs instantaneously, releasing strain energy which creates a loud banging noise that sounds similar to a gunshot. The violent transition is followed by a small vibration in the building in the area of the connection. The noise and vibration associated with this phenomenon while very disturbing to building occupants is not something to be concerned about.
To understand this phenomenon, it is important to understand how the A325 high-strength bolts—the most common structural connection used in steel-to-steel connections in buildings—are designed. There are two common design approaches:
- Slip-Critical Connections. This method of design involves tightening an A325 bolt so that the two steel plates being joined are held very tightly together. All shear transferred through the steel plates is done so through friction of the two plates. There is a large amount of tension in the bolts, but no shear goes through the bolt itself. A slip-critical connection must have properly prepared faying (plate interface) surfaces so that an accurate slip coefficient between the two plates is known. Additionally, the bolts must be tightened in a specific sequence and method to ensure that all bolts are tensioned to a specific value and so that an accurate tension for all bolts in the connection is known. For reference, a ¾” diameter bolt in a slip critical connection has an allowable shear of 7.36 kips in a single shear connection. Slip-critical connections accommodate the use of oversized holes to ease erection and can offer some erection economies.
- Bearing Connections. This method relies on transferring shear through the bolt. The shear comes in though one plate, bears on the bolt, and transfers the shear within the bolt to where it bears on the second plate. No shear is transferred through friction between the plates and the bolts are not tensioned. There are two types of bearing connections, one with threads included in the shear plane and one where threads are excluded from the shear plane, but both are the same concept where the shear is transferred through the bolt. For reference, a ¾” diameter bolt in a bearing connection has an allowable shear of 10.6 kips (for threads included in the shear plane) or 13.3 kips (for threads excluded from the shear plane). You should note that whether threads are included in or excluded from the shear plane, the capacity of the bolt in bearing exceeds the capacity of the bolt in shear when comparing it to the same size bolt in a slip critical connection by 44% to 81%, respectively. In other words, it is typically more economical to use a bearing connection because you can theoretically use fewer bolts to achieve the same allowable capacity and there is less effort involved in tightening them.
Now that you understand the two types of connections that can be designed with high-strength bolts, it is important to understand the steel building erection process. When a steel-frame building is being constructed, it is beneficial to have a little forgiveness in the connections (whether that be short-slotted holes or oversized holes). This allows members a little accommodation in the fabrication so that all beams and girders can be installed to plumb columns. However, once the frame is in place and all members are installed and columns are plumb, it is necessary that the connections be tightened and fully secured.
It is a common occurrence for the steel erector to pre-tension the bolts in the newly erected steel frame to prevent movement of the frame while construction is finished. This is true even if the connections themselves were designed as bearing connections. It is important to note that if these connections are designed as a bearing connections, it is most likely that the faying surfaces were not prepared (and may in fact have mill oil or have burrs) and the order of tightening bolts has not been carefully adhered to, meaning that tightening some bolts may loosen others, resulting in various tensions on the different bolts. All the erector is looking for is to hold the frame in place until construction can be completed and floor and roof diaphragms can be put in place. Even if a building has connections designed for bearing, it is quite possible that the bolts are fully tensioned or tightened, and shear is being transferred through friction like in a slip-critical connection. This is somewhat inherent in connections that use twist-off (ASTM F1852) bolts which are typically fully tightened when installed. Even though these connections use full tightened or pretensioned bolts they won’t necessarily develop the friction to hold the plates together and keep them from slipping.
Banging bolt syndrome may occur when the shear loading on the connection exceeds the shear friction between the plates and the bolts slip into bearing. From a structural integrity perspective, this is not a concern because the shear capacity of the bolt in bearing is greater than the shear capacity of the bolt in a slip-critical connection. There are three typical scenarios regarding bolts and their potential to slip:
- The bolts can slip during construction. If the tightening of the bolts was not very extensive, it is very likely the bolts will slip into a bearing connection during construction. Concrete composite decks, which are common in steel construction, account for a significant amount of dead load. The moving of materials over the floor can also load up specific beams.
- The bolts can slip during occupation of the building. If the tightening of the bolts was a little more robust, it is still possible for the plates to slip when furniture and people begin loading into the office. Or it may take a larger loading, such as a seismic event, to force the slip.
- The bolts may never slip. If the bolts were all fully tensioned and there was good, clean surface, it is possible that the actual load may never exceed the capacity of the friction connection, even if the bolts were designed as a bearing connection.
In most steel buildings that are designed with bearing connections, either scenarios #1 or #3 are the most commons cases. The #2 scenario is not common, although it still happens occasionally. And while the noise can be quite disconcerting to occupants of the building during a “banging bolt” occurrence, it is important to note that if a connection does slip from friction to bearing, it only happens once at that connection. If it does happen at one connection in a building, one can expect that other connections may be likely to slip as well, assuming that the same construction techniques were used throughout the structure. While the loud bang and slight vibration of the floor can be unnerving; no instances have been reported in the press of banging bolts have ever shown structural damage. For connections that slip during occupancy loading, it is common for all the connections that will slip to do so within the first year or two of occupancy.
There are several ways in which the possibility of banging bolts during occupancy can be minimized:
- The connection can be designed and designated on the drawings as a slip-critical connection. From a design perspective, this may lead to requiring more bolts. More importantly, from a construction standpoint, it would require preparation of the faying surfaces to meet a specific roughness requirement and a well-regulated method of tensioning the bolts in a proper sequence to an appropriate tension. This adds to the construction cost, but ensures that the proper capacity of a slip-critical connection is obtained, and that occupancy loading would not exceed the limit.
- The connection can be designed as a bearing connection, where bolts are not tensioned beyond snug tight.
- Change bolted connections to welded connections. These are significantly more expensive but would eliminate the issue of using bolts altogether.
- It is possible that during construction, spans can be loaded with the full occupancy load, forcing the connections that have the potential to slip to do so before occupancy. This would require a fairly substantial effort to properly load all of the slab areas and is not particularly feasible.
On rare occasions in structural steel buildings a phenomenon occurs where high-strength bolted connections slip from a friction or slip-critical connection to a bearing one. The almost instantaneous transition occurs with a loud, disturbing “bang” and shudder. While disconcerting this does not diminish in anyway the structural integrity of the building. Feel free to contact our office if you have questions or concerns about banging bolt syndrome. We are always happy to help.
Deke Black, P.E. is Senior Engineer with Nishkian Dean a structural engineering consulting firm in Portland, Oregon.
- The Banging Bolt Syndrome, by Robert L. Schwein, Modern Steel Construction, November 1999
- Banging Bolts – Another Perspective, by R.H.R. Tide P.E., W.E., D.Sc., Modern Steel Construction, November 1999