fff
fff
fff
fff
The Bolted Bracket
CONFIDENTIAL INFORMATION: DO NOT DISTRIBUTE OR COPY
ABSTRACT
Traditional welded joints for steel moment frame buildings have been used to fulfill the
expansion of commercial and retail real estate on the U.S. West Coast for the last 40
years. These types of buildings currently account for trillions of dollars of structures in
high seismic zones stretching from San Diego to Vancouver, Canada. Failures of welded
steel moment frame connections in the Northridge earthquake in 1995 and subsequent
discovery of similar failures in the San Francisco Bay Area due to the 1989 Loma Prieta
earthquake have proven the vulnerability of these structures to catastrophic financial
losses in moderate earthquakes. Efforts since 1995 to find ways to improve the reliability
of welded joints has been controversial and met with limited success. This has created
the opportunity for development of a new paradigm for construction of steel moment
frame structures based on technology that dates back to the very beginnings of steel
frame structures in the U.S.: rigid bolted joints.

Historically bolted connections have been relegated to only gravity or semi-rigid moment
connections in steel frame buildings. However, tests of bolted connections by ICF Kaiser
Engineers at Lehigh University and elsewhere have shown that bolted connections are
capable of providing rigid moment connections with cyclic plastic rotational capacities in
excess of equivalent welded joints but with the same rigidity as welded connections. The
cyclic testing conducted exceeded ATC-24 minimum standards on beams that ranged
from 16 inches to 36 inches in depth. Field installation of bolted connections have
proven fast and cost effective. Bolted Brackets for moment frame connections are now
underway in hundreds of connections for both retrofit and new construction.

INTRODUCTION
In 1995, the City of Los Angeles enacted an ordinance requiring inspection and repair of
steel moment frame connections in geographic areas strongly affected by the 1993
Northridge earthquake. At least 600 buildings were directly effected by this ordinance.
Likewise, adjacent cities have conducted their own inspections and uncovered
earthquake damage. These cities are now in the process of enacting their own
inspection and repair ordinances. Inspections for damage are not limited to buildings
affected by ordinance requirements. The inspections of steel moment frame
connections is also becoming a standard part of the “due diligence inspection”, along
with the typical inspections for asbestos and lead, required in most real estate
transactions.

While few inspections have occurred outside the Los Angeles area, damage has also
been found in steel moment frame buildings in the San Francisco-San Jose regions of
Northern California due to the 1989 Loma Prieta Earthquake. Typical post-earthquake
inspection procedures were not sufficient to uncover weld failures prior to the Northridge
earthquake. However, Loma Prieta earthquake ground motions, particularly in the San
Jose region, better known as “Silicon Valley”, are similar to Northridge earthquake
motions felt away from the epicenter that caused damage to steel moment frame
connections.

Damage to moment frame connections from the Northridge earthquake generally
occurred well below the yield capacity of the framing members and by all indications
were brittle fractures rather than ductile yielding in the failure zone. Typically, the majority
of these failures occurred in either the weld or the column flanges and, to a lessor extent,
the beam flanges and web. Bolted Brackets can be used directly to repair damage
without welding and reduce the repair cost of divots and column flange replacement
when used in conjunction with the welded repairs of the column flanges.

Manufacture of the Bolted Brackets
The Brackets are produced to engineered specifications by select foundries. The
chemistry of the high strength steel is proportioned to ensure excellent welding
properties under ASTM standard A148. Prior to beginning manufacture of the brackets,
the foundry models the casting process by computer to optimize the gates and risers
used to pour steel into the molds. Prototype castings are cast to ensure that the molding
process produces a sound casting. The prototypes are 100% X-Ray and Magnetic
Particle inspected to Commercial Grade Level-3 ASTM standards. Samples made with
the prototype are tested in our full-scale joint assemblies in accordance with ATC-24
“Guidelines for Cyclic Seismic Testing of Components of Steel Structures”.

The steel is cast in green sand from custom molds produced by the foundry. After
cooling, the castings are removed from the molds; sand blasted cleaned, hand finished
and heat-treated. The heat treatment bakes the steel to over 1700°F, refining the steel
crystalline structure and removing stress concentrations induced by temperature
shrinkage in the casting process. The final product is 100% visually inspected and
magnetic particle inspected by random sampling. Brackets are cast to order for each job
and require 6-8 weeks from receipt of order to delivery. Brackets can also be specially
machined to match slopes in roofline steel for a small additional charge.

The Brackets are supplied with the high strength bolts required for field installation. The
bolts to the column are typically A490, 1 3/8” to 1 ½” in diameter. These large diameter
bolts A490 bolts are made to order for each job. The large diameter A490 bolts are
manufactured by hot forging the head of the bolts onto high strength steel stock, which is
cut to size and heat-treated. The heat treatment of the high strength bolts is required to
meets ASTM specification that sets lower and upper limits to the yield strength. Setting
upper limits to the bolts helps ensure ductile behavior in the bolts. Each bolt is Magnetic
Particle inspected prior to shipment. The bolts are shipped with heavy hex nut and extra
thick (5/16”) washers.

Testing of Bolted Connections
The bolted connections currently being installed steel moment frame buildings in
California and Oregon were designed and tested by ICF Kaiser Engineers at Lehigh
University, in Pennsylvania, and Wyle Laboratories in California. The bolted connection is
now available through Steel Cast Connections LLC. A total of fifteen full-scale connection
tests were performed in three phases, with the results listed in the table below. The key
performance parameter is the “Maximum Plastic Rotation”, which measures the ability of
the connection to rotate, under earthquake forces, by large, plastic deformations of the
steel. The minimum accepted performance level is .0025-.003 radians of rotation

Field Installation of Retrofit Bolted Brackets
Although the Bolted Bracket is a proprietary device details and cut sheets are provided at
no charge to the design professional. The SCC Bolted Bracket arrives as a finished
product to the building site. No welding is required to install the bracket. The brackets
weigh between 200 and 300 pounds. They are lifted into place with lightweight lifting
platforms more commonly used to install ductwork in ceiling spaces. These lifting
platforms have a narrow base and can be wheeled through doorways and down narrow
aisles. The brackets come to the field with all the holes pre-drilled in the bracket. The
holes in the column flange are drilled in the field using a lightweight drill, which clamps
itself to the column with its own magnetic base. The column flange holes are up to 1 3/4”
in diameter. However, because the drill cores a round slug out of the metal rather than
chipping out the metal like a traditional drill, each hole can be drilled in under 5 minutes
through steel flanges up to 3 inches thick. The column flange bolts act in tension so the
column flange holes are standard holes, oversized from the nominal bolt diameter by
1/8”.

The bracket is then lifted into place and several column bolts are set to support the
weight of the bracket. Using the bracket as a template the beam flange holes are drilled
into the beam in the overhead position, again using lightweight magnetic based drills.
The beam flange bolts are bearing bolts so the holes for the beam bolts are less than
1/16” oversized and are tightened in with load indicator washer or tension controlled
bolts with twist-off tightening studs. The column bolts are large diameter and require a
hydraulic torque wrench to achieve full tension load. Load is verified in the large column
bolts using either load indicator washers, “turn of the nut” method, or by calibration of the
torque wrench.

In occupied areas bolting requires only modest protection to prevent injury to the
finishes. Since no welding takes place, only minor precautions are necessary to prevent
grease and dirt from spoiling finishes. The drill process produces very little odor or heat.
Typically, a bracket can be installed overnight in under eight hours, with minimum impact
on the use of the space in occupied areas.

The Bolted Bracket can be used to replace damaged welds by simply installing the
bracket in lieu of repairing the connection. When column flanges are damaged the
bolted brackets can still be used to advantage. The column flange will still require
welded repair, which consists of filling in the crack or divot with new weld material.
Instead of rewelding the beam flange, a bracket is installed to provide the rigid flange
connection. The bracket is simpler to install than a new, reinforced, full penetration
flange weld. In addition, unlike a new flange weld, the bracket has does not put tension
stresses on the face of the column flange built from weldments.

Installation of Bolted Bracket for New Construction
The foundry ships the brackets directly to the fabricator. The brackets are mounted to the
beam flange in the shop by the fabricator and fillet welded to the beams in the shop. The
fabricator is provided with the Weld Procedure Specification, WPS, for the fillet welding of
the brackets to the beam flanges, which is pre-qualified by weld tests,.... The beams are
typically square cut at the end and the Brackets are mounted to overhang the end of the
beam by 1/8” to ¼”. The Brackets have vertical short slots for the column flange bolts to
provide vertical fit-up,.... The holes in the column are oversized by 1/8” and the holes in
the Bracket, which are vertical short slots, also have an internal outward taper which
provides room for additional fit-up at the column face. The beams are hung on the shear
tabs, and the shear tab bolts are installed but not tightened. The fit between the column
and bracket is adjusted by shims. After the Bracket column bolts are installed the beam
shear tab bolts are fully torqued and the beam connection is complete...

The high strength bolts use a hydraulic torque wrench for field installation. The torque
wrench comes with a portable hydraulic pump weighing from 65-80 pounds. The torque
wrench attaches to the pump with flexible hoses up to 25 feet long. The wrenches work
quickly using 110 or 220V power and can full tighten a bolt in several minutes. The bolt
torque can be inspected by one of three accepted methods:

•  Use of a Calibrated Hydraulic Torque Wrench: The torque wrench is calibrated on a
direct tension device (“Skidmore”) by the inspector. The calibration determines the
pressure required at the pump to tension the bolts to 70% of yield. Thereafter, the
calibrated wrench is used to torque the bolts in cooperation with the field inspector.
•  Turn of the Nut Method: Turn of the nut can be used on high strength bolts. It is
recommended that this method be verified by use of a direct tension device (“Skidmore”).
The bolts are snugged tight and then turn from snug a specified angle of rotation. The
ironworkers mark the bolts before tightening so the inspector can verify the nut has been
rotated the required amount.
•  Direct Tension Indicators: Load indicator washers are now available for large diameter
A490 bolts. The washers come with ridges stamped on their face. Torquing the bolts
squeezes the ridges flat and the space between the washer and bolt head is checked by
feeler gauge. With load indicator washer the inspection can verify the bolts with the feeler
gauge supplier by the manufacturer of the washers quickly and on his own schedule.

Conclusion
Often times modern day solutions to complex problems are best solved by returning to
basics. The Bolted Bracket is a classic example of this maxim: applying new materials
and technology to an old solution to solve a very modern problem.
REFERENCES
Anderson, J.C., and Linderman, R.R., (1991) “Post Earthquake Repair of Welded
Moment Connections”, Report No. CE 91-04, Dept. of Civil Engin., Univ. of Southern
California, Los Angeles, CA May 1991
Engelhardt, M.D., and Sabol, T.A., (1994) Testing of Welded Steel Moment
Connections in Response to the Northridge Earthquake”, Northridge Steel Update,
AISC, Chicago, IL Oct. 1994
Kasai, K., and Bleiman, D., (1995) “Preliminary Test Results of Kaiser/Lehigh Bolted
Repair Method”, Report to AISC Special Task Committee on the Northridge
Earthquake, American Institute of Steel Construction, Chicago, IL, June 19, 1995
Kasai, K., and Bleiman, D., (1996) “Bolted Brackets for Repair of Damaged Steel
Moment Frame Connections”, Proceedings, 7th U.S.-Japan Workshop: Leasons
Learned From Northridge and Kobe, Applied Technology Council, CA Jan. 1996
CONFIDENTIAL INFORMATION: DO NOT DISTRIBUTE OR COPY
BOLTED BRACKETS FOR STEEL MOMENT FRAMES:
A NEW TWIST ON AN OLD IDEA
by David Bleiman
fff