Precautions for High Strength Bolt
By Qu lijuan , Li yimin, Qian yachen
Abstract: During HSB connection
construction, some problems are always happened such as slippage between connection
plate and parent body and bolt broken and so on. In this article, the job practices
and solutions from production practices are proposed for reference.
Key words: HSB (high strength
bolt), connection, construction
The high strength bolt connections
are popularly used on the building steel structures, bridges and heavy machinery
steel structures. In a way, the high strength bolt partially supersedes the
rivet. Although in China there have been relevant technical standards and documents
specify the HSB construction, there are too many kinds of standards and the
corresponding contents of relevant technical design manuals are various. Various
problems are often happened on the HSB construction at site, so the progress
and quality of construction are influenced. Therefore, in this article the problems
of HSB connection construction are discussed combining with production practices
and national standards, and the frequently asked questions of HSB are presented
for reference to the designers and constructors.
Torque for HSB Connection Set
In HSB connection construction, many designers indicate the final torque of
HSB on the construction drawings, but there are two cases always to be happened,
slippage between connection surfaces is happened during loading after installation
and bolt broken is happened during construction.
In technical documents and drawings,
the specified torque from different companies are various and some data have
big difference, but the HSBs to be used comply with the same national standard.
While purchasing, HSB with different batch number from different companies has
different torque coefficient, therefore, it is better not indicate the specified
torque on the drawings, so that the construction quality will not be affected
by the big difference between the specified torque and the actual demanded torque.
At construction site, some installation
companies have to replace the bolts or weld the connection plates at joint line
in case of HSB is broken or there is slippage between joint lines, so the construction
torque of HSB is very important. The construction torque must be reasonably
determined before constructing in order to ensure the final construction quality.
The earliest usage of HSB is
thirty year ago. At that moment, generally companies designed and manufactured
HSB by themselves, and the national standard for HSB is not normative and integral.
The HSB is also not normatively manufactured. During manufacturing, the blanks
for bolts and nuts are usually mechanical machined and are not die forged as
modern technology, and the thread is machined by normal lathe and not by thread
rolling machine; after machining, the bolt will be treated with bluing and some
companies will galvanize the bolts. The heat treatment technology is not perfect.
In the past, products made of the same metal material often can not meet the
requirement of mechanical property. The torque coefficient of bolt and nut machined
by the normal lathe is much bigger, and the torque coefficient in the earlier
design manual and calculation method is 0.18-0.19. However, at present the torque
coefficient of local HSB is 0.11-0.14 due to the change of machining process
and the change of treatment method. Some high strength bolts’ surface is treated
with Parkerizing process and saponification, and the actual torque coefficient
needs to be supplied by the manufacturer or to be achieved by the test. So,
generally the construction torque of HSB is smaller than the construction torque
of old HSB with the same size.
The imported HSBs are usually used on the equipment cooperatively manufactured
with foreign company or on the imported equipment and the torque to be used
has been indicated on the drawings, therefore the indicated value can be adopted
during construction. But if the local HSB will be used, the constructing torque
of local HSB shall be discussed and changed through discussion with the importer
so as to avoid the construction fault. One point needs to be emphasize is the
constructing torque of European HSB is bigger than the constructing torque of
local HSB due to the machining process, treatment method and material are different,
generally the torque coefficient of imported European HSB is 0.13-0.16. There
is also difference between foreign standard and national standard. It is very
important that the national standard can not be simply applied for calculating
the constructing torque when the imported bolts are used and the foreign constructing
torque also can not be directly applied when the local HSB is used.
When the HSB connection set complied
with national standard is used, the constructing torque recommended is as following:
The constructing torque for big
hexagon HSB is calculated by the formula:
value of torque coefficient for HSB connection set ；
pretension of HSB (kN)，see table 1 ；
of HSB screw rod (mm)；
Table 1 Big Hexagon HSB Construction
Bolt nominal diameter
1.05 is damping factor, it is assumed when the pretension is applied, there
will be a certain excessive tension.
The torque coefficient is supplied
by the HSB manufacturer or obtained via test.
In addition, some foreign equipment manufacturers will request to apply the
MoS2 on the thread as lubricant before applying the torque, in such
case the torque coefficient will be very small so the constructing torque will
also be very small, therefore the constructor must pay much attention on it,
otherwise the HSB will be broken during construction.
When using torque wrench during
construction, the deviation of torque value shall be paid special attention
on. The correct value will deviate during purchase, transport, storage and usage,
so the torque wrench for HSB construction must be calibrated before usage and
the torque deviation shall be no more than±5% .
wrench can be used only if it is qualified. The torque deviation of torque wrench
for calibration shall be no more than ±3%.
not allowed to use the normal wrench or normal electric wrench for HSB construction.
Sometimes torque wrench will be damaged and it shall be found and replaced in
time. The torque wrench with big deviation value can not be used.
Requirement for Bolt
The bolt, nut and shim can be used only if they are qualified and come from
the legal companies. The guarantee period for the connection auxiliary torque
coefficient is six month since ex-works date. When the bolt, nut and shim
are at following status, they are unacceptable and can not be used.
a. the origin (manufacturer)
is not definite;
b. mechanical property is not definite;
c. torque coefficient k is not definite;
d. there is crack, flaw, burs, bending, rust or damaged thread; it is immersed
in the water or the rain or it is defect;
e. there is no performance test report enclosed with them;
f. they are mixing with bolts of other batch;
g. the bolt with short length, i.e. after tightening, the head of bolt can not
expose from the end face of nut. Normally, the exposing length from the end
face of nut is 2～3 threads.
h. the guarantee period of connection auxiliary torque coefficient is exceeded
(the guarantee period of torque coefficient for normal HSB is 6 months);
j. the big hexagon HSB and nut
can not be replaced by the normal 8.8 class bolt or 8 class nut ;
k. the used and removed HSB and nut can not be used anymore.
The big hexagon HSB and nut are
replaced by GB5728 or GB6170 bolt and nut.
HSB from different companies
and with different batch no. can not be used together if the torque coefficient
is not clear.
At site, it is easily happened
that the HSB and nut will not be carefully stored. If the surface of HSB is
rust or determined, the HSB shall be not used anymore and the new HSB and nut
shall be used.
for Connection Surface
Generally, the connection plate
and connection parent body shall be treated with sandblasting or blasting. During
transportation and construction, the roughness and cleanness of connection surface
shall be ensured and it is not allowed that the mud, sand and rubbish contaminate
the connection surface. When the roughness of the surface can not meet the design
requirement, the sandblasting or blasting shall be carried out at site before
construction. The installation company shall protect the cleanness of connection
surface between connection plate and parent body and the characteristics of
Requirement for HSB Installation
When assembling the HSB connection
set, the nut truncated cone side shall be against the chamfer side of the shim.
When assembling the big hexagon HSB connection set, the chamfer side of the
shim under the bolt head shall be against the bolt head.
When installing the HSB, it is
prohibited that the bolt is inserted by force (e.g. using hammer). When the
bolt can not be inserted freely, this hole shall be trimmed by the reamer and
the Max. diameter of trimmed hole shall be less than 1.2 times bolt diameter.
When the trimming work is carried out, in order to avoid the iron chips from
falling into the gaps of plates, the bolts around the holes shall be tightened
before trimming so that there is no gaps between plates. Reaming holes by gas
cutting is strictly prohibited. At site it is quite normal that reaming is done
by gas cutting, so special supervision shall be executed.
When installing the HSB, the
friction surfaces shall be dry and clean. Working in the rain is strictly prohibited.
When tightening the HSB, the
torque can be only applied on the nut. The bolt can be tightened only if the
space is not enough.
The tightening process of HSB
is divided into pre-tightening and final tightening. For big jointing, the tightening
process is divided into pre-tightening, re-tightening and final tightening.
The value of pre-tightening is 50% of constructing torque and the value of re-tightening
is equal to the value of pre-tightening. In order to avoid from skipping, the
nut shall be marked with color after the HSB is pre-tightened or re-tightened.
The nut shall be marked with other color after the HSB is final-tightened.
When the HSB is applied with
pre-tightening, re-tightening and final tightening, the bolts at connection
shall be tightened in a certain order, generally the tightening order is from
center to outer.
The pre-tightening, re-tightening
and final tightening shall be done in the same day, and it is not allowed to
carry out the final tightening in the next day. There are some installation
companies that carry out the final tightening one day, one week or one month
after the re-tightening is finished. Therefore the installation companies and
supervision companies shall be pay much attention on it.
and de-rusting for HSB connection
After the tightening check of
HSB connection is accepted, the gaps of main structures and between connection
plates shall be immediately sealed with silicone adhesive (the silicone adhesive
shall be appropriate for metal) in order to prevent water from getting into
the connection set surface and inner space of steel structure to cause the corrosion.
The parent body, connection plates
and HSB shall be immediately painted with antirust paint to prevent from corrosion
after the HSB connection is accepted.
of Construction Report
The construction report is the
proof for inspecting the HSB connection installation and completion, so various
data shall be detail recorded during construction in order to document the final
technical documents and fulfill the obligation.
The followings shall be included
into the complete construction report:
resistance factor of HSB connection plate joint line
coefficient for various batch HSB, including the certificate and data sheet
coefficient of HSB which was retested
constructing torque for various batch HSB at various locations
time, weather, name of chief engineer for construction and name of project manager,
The HSB connection set and connection plate are the important part of steel
structure. During construction, the installation quality can not meet the specified
requirement due to not complying with the construction specification. Therefore,
the installation company has to pay much attention on the HSB installation in
order to avoid the big economic damage raised from construction fault. Herein
the torque of HSB, reliability of torque wrench, quality of bolt, torque coefficient
of bolt set and treatment for connection surface shall be emphasized so that
the equipment installation period and installation quality will not be affected
by the re-working.