20150119-tightening bolts

[Type text] How to Tighten a Bolt Correctly Data Date 07/04/2015

of 14 Allan S Phillips QA/QC Coordinator

Abstract

The site based SMP construction industry at large, take items manufactured by others, locate two holes that correctly line up fit a bolt through the holes and finally tightening up the bolt. This paper has been written for those directly concerned with tightening the bolts- HS Bolts in the steel construction industry. Commencing with the most important aspect, following The Engineers Instructions, the paper discusses when and where to apply Snug Tightening, the equipment needed to apply tension tightening, and most importantly this paper sets out to demystify engineering terms, and bring together the Engineers Code requirements with Practical application. Follow the guiding information and you will never have to ask, how tight do you want the bolt, or will you?

Index

Item Description Page 1 The Engineers Instructions

2 Overstressing

3 The Yield Point

4 Types of Connections

5 Snug Tightening

6 Application in the Field

7 Bolt Types

8 Washer Types

9 Tension tightening using the Part Turn Method

10 The Part Turn Method of Tightening Nuts

11 Marking the Socket

12 Calibrated Equipment

13 Keeping a Good Record of your work

14 The Engineers Inspection

15 Inspection of Snug Tightened Joints

16 Inspection of Tension Tightened Joints

17 Questions and Answers on the Material so Far

18 Photo of Skidmore Wilhelm Bolt Testing Unit

19 Photo of Two types of Squirter Direct Tension Indicator Washers

20 Tension Tightening using the Squirter DTI method

21 Operator Control

22 Skidmore Wilhelm Bolt Tester

23 Coronet Washers

24 Is there a right way for your job (refer to Item 1)

25 Further Reading



This paper has been written for the riggers and fitters who actually tighten the bolts.

The Engineers Instructions In order to simplify and explain how to tighten a bolt correctly, we firstly need to understand that at all times we are required to follow the Engineers Instructions, whenever the tightening procedure is spelt out on the drawings and specifications. The first section of this paper deals with the snug tightening of Gr8.8 High Strength Structural Bolts shown on the drawings or specifications as Gr8.8/S. We then go on to discuss what is known as tension tightening Gr8.8/TB or Gr8.8/TF I will explain what these symbols mean further on. No matter what the bolt type, or the type of industry you are in, the Engineers requirements are the same

We do not want the nut to come loose on the bolted joint and we do not want to over stretch (overstress) the bolt by over tightening the nut.

Overstressing So let’s spend a few minutes by discussing over stretching (over stressing) is the more correct term to use. When the nut is tightened on a steel bolt we actually stretch the bolt, to act like a spring. When a steel bolt is stretched and released the bolt will return to the original length, so long as we do not stretch the bolt past the yield point. Quite simply, the yield point is where the steel changes from having elastic properties to having plastic properties. I cannot overstress just how important it is for you to understand this basic principle, if you are to take a firm grasp on the principles of bolted connections in structures. So I have prepared a simple sketch to illustrate two properties of steel, the elastic range and the plastic range.

Yield Point



The yield point is where the straight line ends and the curved section begins.

Now we show the same sketch this time with units The x axis shows the extension of the steel in mm - we call the extension (delta) It is simply how much the bolt length increases as we apply the load. The Y axis is the stress (force/area) we apply to the bolt. Stress can be explained as the result of the force applied to the area of the bolt. The units for stress are Mega Pascals *** *** One Pascal is the pressure created when the force of one newton is applied to one sq. meter) One Mega Pascal is 1 Million Pascals

Now don’t give in at this stage – we need to introduce the concept of stress at an early stage as that’s what we are trying to control whenever we work on a structure, especially the bolted connections. But if the numbers confuse you don’t worry it is the description of stress, and how we deal with it, that we are concerned with here. Types of Connections One more concept the engineer has to deal with (and you need to be aware of) is the type of connection he is designing for. Let’s make it simple and say there are two types of joints the first one is where the force is applied along the length of the bolt ( tension connections) and the second type, where the force is applied across the shank of the bolt ( shear connections). The flowing two photographs will make this very important concept a lot clearer.



The most common tension connection in structures is the connection of the portal rafter to the column. The load on the connection when erecting the steelwork is the weight of the rafter itself, the rafter is trying to rotate the connection and open the plates at the top. This opening is resisted by the pre-tension in the bolts that was created when we tension tightened the nut. Take another look at the sketch and notice that the load is acting vertically downwards, at the same time as the cantilever is trying to open the joint at the top. So the rafter end plate is bearing down on the bolt, and the bolt Is bearing down on the column flange. That’s where we developed the term tension bearing Gr8.8/TB as the bolt is in tension and bearing at the same time.



The second photograph shows a bracing tube connected to the bracing cleat on a column. The force from the bracing tube is trying to shear the bolts and no matter how tight you make the nut the shear force is still there. There is no point to pre tensioning the bolt as the force is being applied across the shank of the bolt, and not along the shank of the bolt. So if we snug tightened the nut by applying a man’s force on a podger or give it a few seconds of the rattle gun, after seeing that the plates are clean and line up properly -that should do it-

we call this snug tightening.

A little more about cleaning and checking the fit up follows later on. So to recap what we have dealt with so far.

1) The Engineer will show you what type of connection it is by calling up the connection type either directly on the erection drawing, or in the notes section on the erection drawing, or in the specification.

2) The snug joints do not require pre-tension tightening of the nut

3) Tension tightening does require pre-tension tightening of the nut



Application in the Field Let’s now introduce a few more items for you to deal with and let’s start with:-

Identifying the bolt types you will come across

How to clean and bring the connecting plates together

Which side of the connection should you place the nut, why and where we use washers.

The correct bolting procedure to follow when erecting steel work is; Sung tighten all bolts first, then check vertical alignments with a surveyor then grout the base plates then pre-tension the bolts that require it.

When to apply part turning of the nut

How to mark the connection ( and how not to apply art work)

How to be sure you are using calibrated equipment for checking the part turn

Where does part turn sit on the Yield curve

How much pre tension does each joint type require?

What is stress reversal and how does it affect the joint I am working on.

Making sure you keep a good record of your work

What the Engineer wants to see when we walks round inspecting his design.

Inspection of snug joints

Inspection of Pre tensioned joints

Bolt types We will be dealing with two only types of bolts, the High Strength Gr 8.8 bolt and the Commercial bolt Gr 4.6 bolt -the numbers refer to the type of material the bolt is made of. The first digit refers to the Ultimate Tensile strength (the highest number on our Stress/Strain Diagram) Then second digit refers to the percentage of the UTS we use to locate the Yield Point. Example a Grade 8.8 bolt has a UTS of 800 MPa and a Yield point of 800x.8=640 MPa The Gr 4.6 Bolt has UTS of 400 MPa and a yield point of 240 MPa The very first thing you should look for whenever you open a box of bolts is the Gr markings on the Bolt plus read the stress ratings on the box itself.



The second most important thing to look for is the coating on the bolt. More on this subject later But if the Engineer has called up Galvanised Bolts - use Galvanised and not zinc plated. Yes it is the same coating material (zinc) but galvanised bolts have a much thicker coating and are not polished. Prepare the connection while it is still on the ground. Take some time to clean the connecting plates and look to see if there is any damage to the mating surfaces. Be sure to check that they look flat (place a steel rule across the surface) so that you will not have to apply any force just to achieve a good mating surface before tightening all the bolts. On the subject of cleaning – it is sad to see so much steelwork going up minus a wash. What’s hard about washing the steel say an hour before lift time?

Washers There are two rules to follow here. The first is that whenever practical place the washer under the nut and not the bolt head. By placing the washer under the rotating part we restrict the amount of galling that could occur. Galling is when the edge of the nut cuts in to the steel. And that leads us to the second rule, that whenever galling could occur always have the rotating part on a cleat and not the Primary steel member itself. We can always repair or replace a damaged cleat, not easy to replace the main member !!!!. Always keep in mind that if you need to use a tension wrench/gun be sure that you have enough room to get the gun socket in squarely on the nut. So having the nut face outward away from the steel is usually the easiest way, plus when you mark the connection and sign off, it should be easy to see from the ground, the marks not hidden away and difficult to see. You can rotate the bolt instead of the nut but this is to be avoided as a general practise.

Take a good look at the erection markings on the drawing to be sure you will erect the stick steel On the correct side of the connection cleats. Look for the note on the erection plan, failing to find a note, look for where the steel is marked usually with a North stamp. Bring the stick steel in on the correct side of the connecting plate and insert all the bolts. Start the snug tightening exactly the same as you do when you replace a head gasket on an engine –that is work from the inside out – from the stiff part to the flexible part gradually drawing the two plies together without overstressing a bolt. Try to resist using the rattle gun with one or two bolts only as this can easily take a bolt past the yield point where a bolt failure is likely. The bolted connection was not designed to straighten and

flatten out the steel plates; we have boilermakers to do that, not riggers with loud rattle guns As always erect the braced bays first and when the structure has been snug tightened call for the first alignment check to be carried out. Now that you have a basic structure connected and aligned you can continue to erect off the strong point knowing that you are not forcing or mis-aligning the structure as you proceed.

Tension tightening – using the part turn method.

Before discussing the part turn method let’s take a quick look at other methods of tension tightening Always remembering that in this case the stress is along the shank of the bolt and rotating the nut will stretch the bolt. What we need to control is how much we should stretch the bolt so as to remain under the yield point. You would have heard of or seen squirter washers or coronet washers both have indentations on one face that are squashed during the nut tightening process so that we can see when to stop tightening the nut. In the case of the squirter’s as the name suggests they squirt a coloured gel on to the connection at the correct extension of the bolt. With a similar aim, when using coronet washers, we use a feeler gauge to again check to see when the bolt has stretched the correct amount to remain within the elastic range – i.e., below the yield point.



Although either of these two systems can be used quite economically on structures, a further method has been adopted in the structural industry and proven to work when applied judiciously, the Part Turn method.

The Part Turn Method of tightening Nuts Simply explained, we have a bolt with a pitch of thread of say 2mm when we rotate the nut one rotation we will have stretched the bolt 2mm. If we rotate the nut say ½ a rotation we will have stretched the bolt 1 mm and so on. You will be provided with a chart showing just how much to rotate the nut after snug tightening all the bolts in the connection. The variable is the length of the bolt. A long bolt requires more rotation of the nut to achieve the same tension as a short bolt (temporary torsion). The part turn method starts by marking the nut and bolt head Using a fine felt pen with indelible ink mark a straight line starting from the centre of the end of the bolt then across the face of the nut and continue to mark right up the plate being connected

Marking the socket Let’s say you are to rotate the nut by 1/3rd . Using a heavy duty socket, mark the outside of the socket with two lines 120 deg apart. When you place the socket on the nut, use the felt pen and mark the steel plate with a line say 20-30mm long as your reference point. Then slowly and uniformly use the impact wrench (rattle gun) to rotate the nut 120 deg or 1/3 rd of a turn. You will need to use a ring spanner (or similar) on the opposite side (usually the bolt head) to actually stop it from rotating once you use the impact wrench or rattle gun if you are more familiar with that term. Again start from the stiffer part of the joint and work outwards. When finished mark the connection with the type of Bolt and Joint, your initials and date Just like this Gr8.8/TB ASP 14/12/2014 Mark the same on the erection drawing so as to keep a record of your work And to make it easier for the inspector to verify your work. Now a word from the wise (don’t get lazy and mark up all the bolts after you have rotated them Using the rattle gun.) That’s what we call artwork, and it is a practise that has serious consequences when discovered.

Calibrated Equipment For the part turn, the only equipment we use is the tension wrench or rattle gun. Because we rely on the settings available on the gun to apply a constant torque to the socket we need to have the gun calibrated at regular intervals. It is very easy to over trigger the use of a high torque m/c and put the bolt in to the plastic range. There is a practise where the inspector uses a calibrated torque wrench to take readings off the structural connections and compare those readings with a test connection in the shop. There is much to be understood when calling up for this inspection method, the basic rule to be understood is that Torque is not Tension. So if using a torque wrench for checking be sure you fully understand what we know as the “K” factor and all the variables behind it. Where does Part Turn sit on the Yield Curve? Arguably the most important lesson here is to be sure that you have a good understanding of the Australian Structures Code AS4100 as it is our governing code when dealing with bolted connections.



AS4100 Structures Code sets the limit of Yield that may be used depending on the type of connection that we are dealing with. The part turn method (tension tightening) aims to use up to 90% of the Yield point, retaining 10% as a safety factor against going plastic. The allowable yield itself is reduced when a stress reversal connection has been designed These connections are common under heavy rotating equipment having a moving load where the beam is in alternatively compression and tension throughout its life. The yield point then is limited to 45% of the Ultimate stress with a similar reduction in all of the connecting components.

Keeping a good record of your work. Always keep a good record of the bolt type, where you used snug connections, where you pre-tensioned The bolts. Mark your work on the actual connection by writing down the bolt type, either snug or tension Gr8.8/S or Gr8.8/TB for pre tensioned bolts in bearing, date your work and initial it. I use a spread sheet Set up for this purpose, a note book would also help, so does a photo. I always mark up the structural erection drawing as this will confirm that the Engineers directions have been followed.

Engineers Inspection The Structural Engineer carries with him a heavy responsibility as his signing off on the final design drawings Means that the structure has been designed to a professional standard set by the Engineering Institutes And that with reasonable Superintendence of the works the structure will be fit for purpose. To assist in achieving a safe plant for commissioning and operations the Engineer needs to know that all reasonable care has been taken in following his instructions. His walk around inspection is made that much easier when he can see that the rigger Has understood his instructions and carried them out, hence the marking of each joint on the steel itself.

Inspection of Snug Tightened Joints Following the guidelines in AS4100 the structural inspector is required to check for snug on a limited basis. The accepted approach to carrying out a limited inspection is to try to undo the nut using the same Force and podger bar as was used to tighten it. It is very quick and easy to do. When satisfied that the bolts are snug tightened, that the plies of the connection are in full contact, the Inspector can then place his initials and date Alongside the riggers and on the drawings and check sheets.

Inspection of Tension Tightened Joints. More time and effort is required here as the Inspectors role is to ascertain the tension in the bolt without overstressing the bolt itself. AS4100 suggests that a calibrated torque wrench may be used to check the torque Then the tension calculated by reference to charts provided for that purpose.



Questions and Answers on the Material so far Q1 What is Proof Load? A1 The proof load is the end point of the straight line on the stress strain curve that Q2 What is the Elastic Limit? A2 The elastic limit is located just past the Proof Load and is where the plastic curve has begun Just past the straight section. Q3 Why and where do we use hardened washers? A3 Hardened washers are used whenever we tension bolts; they are used to protect the steel from galling

(digging in of the nut sharp corner) Q4 Can a torque wrench be used to measure tension in a tightened bolt. A4 The short answer is no. Torque is a measure of resistance to rotation, Tension is linear stress. Q5 What does the code say about using Torque Wrenches to tighten nuts A5 Australian Standard AS4100 Steel Structures Code does not allow Torque Wrenches to be used

When tightening nuts

Q6 What does the code say about using Torque Wrenches to inspect the tightness of nuts A5 Australian Standard AS4100 Steel Structures Code states that Torque Wrenches shall not be used Other than to carry out simple checks to see if the nuts are loose. Q7 What is the difference between A325 Bolts and GR8.8HS A7 You should always check with the Engineer that the bolts you have been given are the correct ones When the steel is made overseas and the bolts supplied with the steel you often see unfamiliar markings

On the head of the bolt, you should always have the bolt spec verified before using them. The same comment goes for the nuts and the washers. The washers can either be soft or hardened. Have the spec checked before you use the bolt

Q8 What does the Bolt mark A325M 8S stand for A8 A325M is an International Standard Bolt ASTM A 325 is the more correct designation. RCSC the Research

Council on Structural Connections issues an excellent reference for tightening bolts. Q9 Why and where do we use Hardened washers and what do they look like A9 Hardened washers are identified usually with lobes (soft washers are completely round) Q10 Is there a way to easily identify the different types of bolts nuts and washers A10 Yes, the bolt manufacturers issue wall charts for this purpose. Ajax Fasteners and others provide Illustrated Bolt, Nut marking charts.



Squirter DTI

Squirter DTI Direct Tension Indicator M 24 Bolt 5 Coloured Gel Pads

Squirter DTI Direct Tension Indicator M36 Bolt with 8 Coloured Gel Pads



Skidmore Wilhelm Bolt Tester

This unit shown is bolted to a steel column just as you would in the Field

The unit can also be set up in the Shop to check the tension wrenches daily, twice daily in many places where

Temperature and moisture conditions vary.



Tension tightening – using the Squirter DTI method DTI = Direct Tension Indicator Simply explained it is a structural grade washer with indentations on one side filled with a coloured gel. Directly opposite on the other side of the washer are raised bumps. A plain washer is placed between the Squirter and the nut. Tighten the nut and we start to flatten out the raised bumps. When the bolt starts to stretch, the gap between the flat washer and the side of the squirter washer with bumps gets smaller, plus the coloured gel starts to squirt out, where you can see it. (see attached sketch). Squirter’s provide a simple way to measure the extension of the bolt. They are tension indicators and require calibration before use. At the beginning of the tension indicator exercise, we calibrate the amount the bumps are to be squashed. We do this using a feeler gauge placed between the bumps up to the bolt shank. The feeler gauge (fg) thickness is known and we insert the fg right up to the bolt shank, if we can. Try fitting the fg in 5 places, if it enters all five places or even 4 the nut requires a little more rotation to squash the bump just a little more. When you can insert the fg in one only place out of 5 places, bingo the bolt has been stretched the correct amount to produce a spring with the correct amount of tension. Now notice the amount of coloured gel that has been squashed out. In a bolt with 5 bumps we should see 4 gel blots. From the calibration exercise we go straight to the job site that has snug tightened bolted connections with Squirter’s already installed. We then tighten the nuts until we see the same amount of gel. The manufacturers have calculated the size of the bump and equated the compression required to the minimum tension required as set down in AS4100.

Operator Control Now there is a requirement for operator control whenever tightening a bolt. One item is the speed that the nut is to be rotated, and the second item is to recognise that a long bolt when tightened actually has torsion induced whereas a short bolt does not. Torsion relaxes over time. As an example a M20 x 150mm long bolt when tensioned (has induced torsion) and will relax over a period of 12 hours to the point where it requires re-tightening to maintain tension. Torsion relaxes with temperature. A bolt tightened at 2pm in 50 deg C heat will relax during the night and could easily be loose at 7am in the morning. So a retighten check is always necessary and a good industry practise. Skidmore Wilhelm Bolt Tester The Skidmore units are used to calibrate tension wrenches used in tightening bolts. They do so by firstly registering the tension on a gauge when tensioning a test bolt. The test bolt is connected to a piston that compresses oil in the body chamber, the pressure in the oil is registered in tension on the special purpose special calibrated gauge. The way in which the unit operates is quite simple. For each bolt size you are going to use in the field, select the corresponding test bolt and install it in the skidmore. Turn the nut, tension the test bolt, and check on the gauge That you have the correct reading.



For work complying with AS4100 and that’s most if not all projects in Australia, the Test Bolts are shop set to the minimum bolt tension as set out in AS4100 The Minimum Bolt tension for a GR 8.8 Bolt are:- M20 Bolt is 145Kn M24 Bolt is 210Kn M30 Bolt is 335 Kn M36 Bolt is 490 Kn So the Skidmore is set accordingly, also the front plate sleeve is selected according to the bolt diameter to be tested Once the m/c is set we slip on the Squirter with the silicone gel towards the plate, then a flat washer next to the raised bumps on the squirter then finally the nut. Using a pre calibrated tension wrench we calibrate the gap by rotating the nut until the required Minimum Tension is reached on the gauge. At this point we check the gap and also check the gel quantity and pattern. 4 out of 5 blots is ideal with 7 out of 8 for the larger washers. Refer to the Squirter instructions for more precise details. Coronet Washers They are used and calibrated using a skidmore in exactly the same way as a Squirter minus the coloured gel.

Is there a right way for your job Yes there is, follow the Engineers Instructions

Seriously it is an easily asked question but one that should be answered by way of direction to you before you start the job. On your next project ask the question well ahead of the time you are going to start, as it may take some time for the Engineer to obtain all the answers. The Engineer has to discuss and obtain instruction from the vendors in many cases, then he has to read and follow the project design drawings, specifications, project standards etc. It takes time to provide you with a detailed answer.

Further Reading AS4100 is the reference code that you should read, be at least familiar with the section dealing with Bolting Bolt supply companies also have excellent reading material. Writers Notes The foregoing material was prepared to assist those whose job it is to tighten bolts on steel structures that may not have a technical background.

20150119-tightening bolts

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