Tie bar end bolt torque 2

[shaurya241100]

HI Guys,

Hope someone might have experienced this things in the recent past and may be able to provide some information.

We are trying to connect spandrel walls of a arch masonry railway viaduct using steel tie bars. I am just thinking what should be the criteria for torque which shall be applied at each end of the tie bars where it is connected with end plate/washer plate. Should the torque be converted into pressure (how) and in that case should it be greater than the strength of the masonry in compression?
Regards,

 

[ishvaaag]

Normally you will want to impart some particular force at precise points. So you will select the threadbar, and assuming one end doesn't move, you can measure the elongation in the bar by how much a tick mar at the surface of the bar just made straigth till the maximum elongation after applying torque enough. So you will have a longitudinal strain Delta/L and from such strain a stress sigma=E·epsilon, then you will have your stress and multiplying by the section the force imparted. Take it backwards and you can specify how much the tick mark must advance from the surface of the nut.

All above, as usual for threadbar, in elastic regime. Also, when planning the force, account by steel relaxation. Respect to the introduction of the force, you may need to build a reinforced concrete socket or use a steel plate to distribute the load to the masonry.

From what above I would say that thinking in torque terms is less relevant for threadbar than bolts.

 

[shaurya241100]

Thanks Ishvaaag,

Attached is the tie bar I am using to connect spandrel walls and not sure actually what value of torque should we specify for the contractors.

One of the generally used convention is to apply a torque equal to 0.15% of ultimate tensile strength but not very sure if this is the right approach.

 

[desertfox]

hi shaurya241100

With large fasteners in mechanical engineering bolt elongation is sometimes measured, knowing the elongation allows the fastener pre-load to be calculated which can then be used to calculate the pressure under the nut and consequently on the washer or in your case the washer plate.
The value of fastener pre-load is determined by the amount of external load the stud or bolt is liable to see in service,
So in your case whatever the external load you expect the studs to carry, the pre-load sholud be greater than this.

This site might help but bear in mind I am looking at this from a mechanical point of view.

http://www.roymech.co.uk/Useful_Tables/Screws/Preloading.html

desertfox

 

[shaurya241100]

Hi Desertfox,

Many thanks for your response.
Regards,

 

[desertfox]

hi shaurya241100

Your welcome

desertfox

 

[ishvaaag]

What diwydag says...

"Dywidag couplers and hex nuts develop the full ultimate load of the Threadbar. Slippage of the couple under stress in controlled by torquing opposing Threadbars together or by using lock nuts. The magnitude of the torque required varies with the allowable slip and threadbar size."


WHEN couplers are used they need be tightened to a torque

"Lock nuts should be used at each end of the coupler when the threadbar is used in reinforced concrete applications designed in accordance with BS 8110. These lock nuts should be torqued to a predetermined value to prevent cracking of the structural concrete at the coupler when the joint comes under load. See table on Page 3 for torque values."

http://www.dywidag-systems.com/uploads/media/DSI-UK_GEWI-Steel_High_Yield_Threadbar_System_UK_01.pdf

See Section 5 Page 2 at the following document for a relationship torque-tension for dywidag bars

http://www.americancommercial.com/uploads/media/DSI_Ground_Support_DYWIDAG_Threadbar_us_01.pdf

The provider should be able to help, send them an email.

 

[shaurya241100]

Thanks Guys,
I am still not clear about the basic principal for tightening boolts to a particular torque. A simple example is if I have to connect spandrel walls of a brick masonry arch bridge to stop them moving out and use grip bar anchor system, to what torque should the locking nuts be tigtened or should there be any torque at all be used for lock nuts?

Suppose the spandrel walls are 1.5m hight, retaining ballast and tie anchors are used at a spacing of 5m.
Thanks

 

[desertfox]

Hi shaurya241100

I have uploaded a file which might help, the first part calculates the torque req to achieve a pre-load of 1000N and it is only a very approximate formula, the second part shows how to calculate bolt stretch to achieve the same pre-load.
Now a word of caution I have calculated the pre-load to match the external load of a 1000N, you should calculate your external load on the walls then allow for a safety factor and set a higher pre-load for your studs.
Using torque values for tightening is subject to an error of about +/- 25% I think you might be better using bolt stretch were torque settings are not req.

desertfox

 

[shaurya241100]

HI Desertfox,

That's what I was looking for and this is fabulous help, many thanks. Just one thing; it seems to be an empirical formaula as dimensioanl equality is not there.
Thanks and regards

 

[desertfox]

Hi shaurya241100

It is an empirical formula its very approximate if you go to this page on the sites I previously posted you will see it scroll down and look for the title "intial tension in bolt"

http://www.roymech.co.uk/Useful_Tables/Screws/Preloading.html

A more accurate formula is underneath it, remember also you might have to allow for expansion and contraction of both the brick walls and the stud itself, so when you set your preload make sure if the walls grow outwards your tie rods don't get over stressed.


desertfox

 

[shaurya241100]

Many thanks Desretfox for excellent help on this issue.
Regards

 

[desertfox]

hi shaurya241100

Your welcome

desertfox

 

[dhengr]

shaurya241100:

Several of the above posts give you good info. on some of the systems available for what you are trying to do, including allowable forces/rod, and installation instructions, etc. It seems to me you are way over working the nut torque/rod stress issue given the problem you are trying to address. You need to know the lateral force you are trying to resist or liable to need to resist. That will determine the rod size, washer plate size, spacing, need for a whaler beam, etc. You need to know the working strength of the brick wall system, including some estimate of the mortar strength and bond strength. If you space the rods at 5', will the brick wall span btwn. the rods or do you need some sort of a continuous whaler beam? If you use discreet rods, bearing and bond strength of the brick and bending of the plate will determine the plate washer size. Then you tighten the nuts until, and this is the hard part, just before you start to see the brick moving under the washer plate, then put a lock nut on the rod end. But, this real problem has nothing to do with nut torque.

If you are trying to keep the walls from spreading wrt each other under the superimposed lateral load from above, you must determine this load/foot. That determines rod size and spacing, and washer plate size. You might grout behind the washer plates, tighten the nuts, medium tight, and you’re done. Pay attention to how you install the rods under the road bed so you don’t have to take 6" of sag out of them with the nut tightening process. If you are trying to pull the walls back together against a passive lateral load, then you need to know the stress in the rods, but you don’t seem to indicate that is the problem. That’s quite another project.

Good Luck.

 

[shaurya241100]

Dhengr,

You have been able to understand like desertfox what I need to do in this particular case. The size requirements for the washer plate is less than 200 square and the size being provided by the contractor is 400 dia. washer plate so the masonry strength is going to be okay as it will be speread over a larger than design rquirement size washer plate. Grouting is being done on washer plate and double lock nuts and this torque as you rightly say is to prevent spandrel wall moving out of their current alignment and not to pull them together. The lateral forces is about 150kN that need to be resisted and this is being done by spacing 42 dia grip bars at 5m centre to centre. I have worked out a torque requirement of in the region of 500 Nm which I think is slightly on the higher side but does it really matter as long as the tie bars have that much strength to withstand this forces and masonry has enough strength.
Regards
Shaurya241100

 

[dhengr]

shaurya241100:

Check the thickness of the washer plates, given the larger dia.; you may want to spread that load over the larger area. Whatever torque you think you want, it should stop before you cause localized movement of the masonry, which means localized bond failure in the joints, not wanted. 5m (16.5') seems like a pretty long spacing. 150kN (33.71kips) rod force sounds fine for a 42mm dia. rod, but if my math and conversions are right, that’s 33.7k/16.5' = 2 kips/ft. lateral force to each rod. Again, probably no problem wrt the rod, but will the unreinforced masonry spanning 16.5' and can you tolerate the tension on the faces of the masonry wall, inside or out. Compression should be no problem as long as the mortar joints are good. Think about straightness of the rods on installation, because straightening out sag, after the fact, will allow a lot of lateral movement of the wall, just what you don’t want.

 

[dhengr]

You might want to consider a 2 or 3" I.D., light walled, pipe, which can be checked for straightness after installation. Then install your rod and grout, or some protective mateYou might want to consider a 2 or 3" I.D., light walled, pipe, which can be checked for straightness after installation. Then install your rod and grout, or some protective material, for long term corrosion protection.rial, for long term corrosion protection.

 

[cntw1953]

Did anyone mentioned bond beams, or whalers, which may help if applicable.

 

[shaurya241100]

Thanks guys for useful tips. there is a pvc pipe which I have incorporated in my design of 100mm dia which will house the inner length of the tie rods. This core thorugh which the tie rod is passing will be then grouted and end would be plugged. Behind the washer plate high strength mortar will cover the nuts and washer plate. As the plate will be resting against the injected mortar and would be much stiffer than the brick masonry, I think the masonry won't be subjected to much pressure as long as the grout is strong enough in this case 7 days strength more than 42 MpA.

The pvc pipe covering the inner 90% length of the tie bar will ensure that tie bars are stright and as they bar will be joined using high strength coupler I think it will be straight enough over its length.
Thanks guys once again.