1 September 1999
shear-bearing type connections are installed "snug tight" since shear strength in this type of
connection is not related to the pretension load in the bolt. The bolts are usually ASTM A307
bolts with an ultimate tensile strength of 450 MPa (60ksi), although "snug tight" high-strength
ASTM A325 and ASTM A490 bolts are also permitted for shear-bearing type connections with
some restrictions. ASTM A325 bolts have an ultimate tensile strength of 830MPa (120 ksi),
and ASTM A490 bolts have an ultimate tensile strength of 1040 MPa (150 ksi).
d. Slip Critical Connections. Slip critical connections transfer shear load by shear-friction
with the normal force provided by pretensioning the bolts. The shear strength at service load
levels is dependent on the pretension load (normal force on joint) and the friction between
joined materials. Slip-critical connections are generally required when loads are repetitive and
fatigue is a concern, where connection slip would effect structure performance and where bolts
and welds must act in unison to resist applied loads. Bolts are pretensioned to at least 70
percent of the bolt-minimum tensile strength. The tension load is applied by tightening the nut,
or otherwise elongating the bolt sufficiently to develop the prescribed pretensioning force.
Turn-of-the-nut, calibrated wrench, and direct tension indicators are all methods used to
assure the bolt has been properly tensioned.
e. Bolts in Direct Tension. When bolts act in tension to resist applied loads, the
possibility of, and the effect of prying action on bolt tension must be considered.
f. Steel Properties. Strength and ductility are properties important to building
steel. Buildings located in high seismic areas must be ductile if they are to survive major
earthquakes. Designers must select steels that meet needed strength and ductility
requirements. They must recognize that ASTM A572 Grade 50 steel will meet ASTM A36
requirements and most likely will be used even though ASTM A36 steel is specified.
Designers specifying ASTM A36 steel must consider the impact a higher yield strength will
have on the seismic design and make sure the substitution does not adversely impact seismic
performance. Connections designed to develop the strength of the connected members may
not perform as intended if the connected members were specified to be ASTM A36 steel and
the contractor elected to substitute members of ASTM A572, Grade 50 steel.
7-6. SPECIAL INSPECTIONS.
Periodic special inspection of the installation and tightening of fully tensioned high-
strength bolts in slip critical connections and in connections subject to direct tension is
Continuous special inspection of all structural welding, except where periodic special
inspection is allowed in FEMA 302, Chapter 3.