QUESTION: (FALL2015) I recently came across some fully threaded A325 bolts in the connections of a building structure. Are these bolts permitted? Do they have the same resistances versus bolts with regular thread length? How are they identified after installation? Do they offer any benefit?
ANSWER: A325 bolts threaded full length are permitted under Supplementary Requirement S1 of ASTM A325. They are restricted to bolt lengths within the length of four times the nominal diameter.
Bolt resistances: Since the tensile resistance is based on the threaded area (0.75Ab) it is not affected by the longer thread length. However, the bearing-type shear resistance must be reduced to account for threads intercepting the shear plane. When used in a slip-critical joint with a long grip, the smaller (threaded) bolt area in the entire grip affects the relationship between the clamping force and bolt elongation and may result in a reduction in clamping force when using the turn-of-nut installation method. The significance of this effect is a study in pursuit.
Identification: The bolt head is marked with the symbol “A325T” instead of “A325” as shown in Figure 1.
Benefits: They offer no benefit in terms of bolt resistance. However, the fabricator and erector may find their use viable for certain applications from the standpoint of ordering and inventory control, particularly for applications where thin connected steel parts, instead of the bolts, control the connection shear resistance.
QUESTION: (FALL2015) What is the minimum bolt length requirement for high-strength bolts; must the bolt end past the nut when installed?
ANSWER: A325 and A490 bolts, when installed, must have sufficient thread engagement to develop the tensile strength of the bolt, i.e. they must have the bolt end extending beyond or at least flush with the outer face of the nut.
QUESTION: (SUMMER2015) Are RCSC Specifications mandatory for projects in Canada?
ANSWER: The Research Council on Structural Connections (RCSC) Specification for Structural Joints Using High-Strength Bolts provides state-of-the-art criteria for design and installation of ASTM high-strength bolts and assemblies. These recommendations become mandatory if and when the local code adopts them. In Canada, structural design and inspection of bolted joints and installation of high-strength bolts should comply with CSA Standard S6 or provincial specifications for road bridge structures and S16 for building and other structures to which S16 applies. These standards adopt many recommendations in the RCSC Specification but not all and certainly not all at the same time. In addition, S6 and S16 adopt ASTM specifications for high-strength bolts and bolt assemblies, e.g. ASTM A325 and F1852, by reference. These ASTM Specifications reference other pertinent specifications for testing, etc.
QUESTION: (WINTER2014/2015) The resistances for bolts in tension and shear have increased significantly from those tabulated in the Handbook that I received in 2000. Are the modern high-strength bolts produced to a higher strength or have more recent research and testing substantiated the increase in resistance?
ANSWER: The difference in bolt resistances for the ultimate limit states you noted reflects the increase in the resistance factor for bolts. When the first limit states design standard for design of steel structures, CSA S16.1-74, was introduced in 1974 only two resistance factors were adopted, for simplicity – 0.90 for steel members and 0.67 for welds, bolts, concrete in composite beams and shear connectors. Research studies based on tests and statistic analyses suggest that the resistance factor for high strength bolts can be increased to 0.80, as documented in Guide to Design Criteria for Bolted and Riveted Joints, Second Edition (available via this link: http://boltcouncil.org/files/2ndEditionGuide.pdf.) This increase was first introduced to the Canadian Bridge Design Code when CAN/CSA S6-00 took effect; the change was adopted in S16 when S16-01 was released.
QUESTION: (SUMMER2014) Are bolted moment connections used in a canopy structure required to be slip critical? My question relates to a situation where slip critical connections are not required for deflection control. I have many years of connection design experience but seldom had to provide slip-critical connections for wind-load resisting braced bents or moment frames.
ANSWER: The key question here is whether fatigue is a consideration; will the structure be subjected to repetitive loading and stress reversal? A relatively light canopy type of structure subjected to gusty local wind load may experience stress reversal and a significant number of load cycles to warrant such assessment. The judgement rests with the engineer responsible for the design of the structure. Fatigue design is covered in Clause 26 of S16.
QUESTION: (SPRING2014) Where can I find the standard torque table for pretensioning A325 bolts?
ANSWER: The use of ‘standard torque table’ for pretensioning high strength bolts was discontinued several decades ago. When pretensioning is required, CSA Standard S16 recognizes three bolt installation methods:
a) Turn- of-nut method;
b) Using ASTM F1852 tension control bolt assemblies; and
c) Using an F959 direct tension indicator.Both S16-09 and S16-14 assign a higher slip resistance to bolts installed by the turn-of-nut method versus the other methods for a given Class of contact surface, recognizing the larger pretension typically attained using the turn-of-nut method.
QUESTION: (FALL2013) When ASTM F1852 bolts are used in a simple bearing-type shear connection designed to receive A325 bolts of equal size, does the bolt tension in F1852 bolts due to pre-tensioning reduce the shear strength?
ANSWER: The answer is no. As recognized in CSA S16-09, the bolt in an ASTM F1852 twist-off type bolt assembly has the same ultimate shear strength as an A325 bolt of equal size. The ultimate shear strength of a high strength bolt is not affected by the presence of an initial pretension in the bolt.The Commentary to RCSC Specification for Structural Joints Using High-Strength Bolts offers this explanation:
“When required, pretension is induced in a bolt by imposing a small axial elongation during installation, as described in the Commentary to Section 8. When the joint is subsequently loaded in shear, tension or combined shear and tension, the bolts will undergo significant deformations prior to failure that have the effect of overriding the small axial elongation that was introduced during installation, thereby removing the pretension. Measurements taken in laboratory tests confirm that the pretension that would be sustained if the applied load were removed is essentially zero before the bolt fails in shear (Kulak et al., 1987; pp. 93-94). Thus, the shear and tensile strengths of a bolt are not affected by the presence of an initial pretension in the bolt.”
It should be noted that, for a given Class of faying surface (Class A, B or C), S16-09 assigns a smaller slip resistance to F1852 assemblies versus their A325 counterparts pretensioned by means of the turn-of-nut method, in recognition of the larger pretension typical in the turn-of-nut method of installation.
QUESTION: (SUMMER2013) What are the most common high-strength bolt products used in building construction?
ANSWER: Three-quarter-inch A325 bolts are still very common. Some fabricators/erectors prefer seven-eighth-inch A325 bolts, especially for large projects. A490 bolts are used increasingly in building construction. Typically, they are selected for connections resisting very large forces while A325 bolts may be used elsewhere in the structure. In such applications, care must be taken to prevent A325 bolts from being inadvertently installed in holes designed to receive A490 bolts. It is prudent to segregate them by size, typically, a quarter of an inch difference in diameter.Practical combinations include:
a) 1˝ A490 bolts for heavy connections and ¾˝ A325 bolts elsewhere; and
b) 1⅛˝ A490 bolts for heavy connections and ⅞˝ A325 bolts elsewhere.
Where pre-tensioned installation is required, twist-off type tension-control bolts (assemblies) have emerged to be viable options. ASTM F1852 and ASTM F2280 bolts (twist-off type) share the ultimate-limit-state resistances with A325 bolts and A490 bolts respectively. However, CSA S16-09 specifies smaller values for 5 per cent slip coefficients, c1, for these twist-off type bolt assemblies versus those of high strength bolts pre-tensioned to meet the turn-of-nut method of installation. For further discussion on ASTM F1852 and ASTM F2280, visit Q & A Column in Advantage Steel No. 38. A490 and F2280 products shall not be galvanized.
Use of metric bolts is still rare because they are unavailable unless a special order for a very large quantity is placed with advance notice.
QUESTION: (FALL2012) When designing bolted connections, are seismic loads considered to be static or cyclic?
ANSWER: The Seismic Corner article entitled “Bolted Connections for Seismic Applications” in CISC publication, Advantage Steel No. 31 (Summer 2008), outlined the requirements for bolted connections for seismic applications in accordance with S16-01.The article is available at: https://cisc-icca.ca/ciscwp/product/advantage-steel-no-31/
In NBC 2010 and S16-09, the building height restriction for Conventional Construction where the specified short-period spectral acceleration, IEFaSa(0.2), exceeds 0.35 has been increased. The above mentioned requirements for bolted connections also apply to these taller structures of Conventional Construction.
QUESTION: (SPRING2012) When wide-flange purlins are also subjected to significant axial tension, which is transmitted by connecting the bottom flange to the supports with two transverse lines of high strength bolts, how do I account for shear lag? Specifically, should the effective net area, Ane, be taken as 0.75An, as provided in Clause 220.127.116.11 (c) (ii) of S16-09?
ANSWER: The approach as you described is unconservative. In this situation, the effect of shear lag is more severe than the case for angles connected by one leg with two transverse lines of fasteners. Hence Ane < 0.60An. On the other hand, the lower bound for Ane may be taken as Anf, where Anf is the net area of the connected flange alone. Therefore, Ane should lie somewhere between Anf and 0.60An.
QUESTION: (FALL2010) I have noticed that twist-off bolts are gaining popularity. Are they accepted as high-strength bolts for structural applications? If they are, what are the shear and tensile resistances?
ANSWER: ASTM F1852, twist-off type tension control structural bolt/nut/washer assemblies, are used increasingly in pre-tensioned connections. These bolts feature a splined end which, when properly installed with a special wrench, should shear off when the target pretension is reached (See Figure 2). ASTM F1852 and F2280 bolts have mechanical and chemical properties equivalent to A325 and A490 high-strength bolts, respectively. Specific design requirements can be found in CSA Standard S16-09 Clauses 22.2.5 and 23.8.4 and in Table 3.The tabulated values for ultimate shear resistance in bearing-type connections and tensile resistances of A325 and A490 bolts in Part 3 of the CISC Handbook of Steel Construction may be used for F1852 and F2280 bolts, respectively, whereas smaller values for the 5% slip coefficients, c1, are specified in Table 3 of S16-09 for use of twist-off bolts in slip-critical connections.
Because surface friction is an important factor during installation, these bolt assemblies include hardened washers. Also, the use of tension-control bolts calls for prior testing and particular attention to their handling and storage so as to avoid lubricant deterioration over time.