Precautions for High Strength Bolt Connection Construction

By Qu lijuan , Li yimin, Qian yachen

www.bulkcn.com

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.

1. Constructing 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:

Tc=1.05k·Pc·d
　　　　Tc—constructing torque(N·m)；
　　　　k—mean value of torque coefficient for HSB connection set ；
　　　　Pc—construction pretension of HSB (kN)，see table 1 ；
　　　　d—diameter of HSB screw rod (mm)；

Table 1 Big Hexagon HSB Construction Pretension Pc(kN)

Bolt performance rate	Bolt nominal diameter
Bolt performance rate	M12	M16	M20	(M22)	M24	M27	M30
8.8S	45	75	120	150	170	225	275
10.9S	60	110	170	210	250	320	390

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.

2. Torque Wrench

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% .

l The torque wrench can be used only if it is qualified. The torque deviation of torque wrench for calibration shall be no more than ±3%.

l It is 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.

3. 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.

For example:

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.

4. Requirement 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 friction surface.

5. Other 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.

6. Painting 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.

7. Content 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:

a. the skip resistance factor of HSB connection plate joint line

b. torque coefficient for various batch HSB, including the certificate and data sheet etc;

c. torque coefficient of HSB which was retested

d. actual constructing torque for various batch HSB at various locations

e. date, time, weather, name of chief engineer for construction and name of project manager, etc.

Summary

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.

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