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Topic Title: Structural engineer - drilling holes in floor joists - UFH
Topic Summary: Structural engineer - drilling holes in floor joists - UFH
Created On: 15 October 2013 01:14 PM
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 15 October 2013 01:14 PM
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Blackadderofwickford

Posts: 2
Joined: 25 July 2008

I'm looking to drill lots of 20mm holes in my floor joists c24 and tr26 timber (47x175), at 200mm spacing's along the 3.5m span (middle height of 175). 1 noggin, 400mm spacing, 11 joists total.

My question is how do I prove / what are the calc's for drilling the middle of a joist. I need to evidence to building control this will not weaken the structure to a level that is un acceptable.

This may seam silly, its for my under floor heating system. The reason is because the room concerned has split floor type, part suspended, part solid. As a result to keep the thermals more equal I want to run pipes up and down the room from solid to suspended to solid, etc. Combined with biscuit mix between the joists over the pipes this will help the room warm up more evenly, opposed to one end being hot other cold, or the need for two separate circuits of pipework and associated controls.

Thanks in anticipation.
 18 October 2013 04:24 PM
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jarathoon

Posts: 1040
Joined: 05 September 2004

Given the long silence this probably doesn't need to be said, but just in case it does need to be said, here is my advice...

Discuss your drawing and designs fully with a qualified and experienced structural engineer before you attempt to go any further with your building plans: let them advise you and carry out any and all structural calculations for you and physically inspect what they need to inspect.

From the last time I looked structural engineers are quite good value for money, given the numbers of lives they must have saved over the years. Better value than lawyers for instance.

James Arathoon

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James Arathoon
 20 October 2013 12:00 AM
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Zuiko

Posts: 521
Joined: 14 September 2010

This is quite a simple formulation.

In effect, what you will be doing is reducing the cross-sectional area of the joists and introducing a stress concentrator, the drilled hole.

The stress the joist is under is:

Stress in joist = (Applied force on joist / effective cross-sectional area of joist) x stress concentration factor due to hole.

The stress concentration factor depends on the ratio of the diameter of the drilled hole to the height of the joist:

20mm / 175mm = 0.1

You then look up this in your engineers handbook and find a concentration factor of about 3. Factoring the decrease in cross sectional area, the stress the joist is increased by a factor of approximately 4 per unit of load.

A drilled joist that is under a force of 25% design load in this case is effectively 100% loaded, so is not safe; you will need to ensure that the design load is well below 25% of the force carrying abilty of the undrilled joist.


Cheers
W

Edited: 20 October 2013 at 02:18 PM by Zuiko
 20 October 2013 09:49 AM
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Zuiko

Posts: 521
Joined: 14 September 2010

Also note that the joists will be subject to bending which may be quite complex. You don't want your maximum bending moment to be where you drill the hole. Reducing the x-section will effect the stiffness of the joist and this will work against the strength of the joist so you will get a total stress of:

Total Stress = (stress due to bending) + (stress due to applied load ) + (concentration factor due to hole)


Can you attach some sort of tray-work to the joists of the suspended floor instead?

Edited: 20 October 2013 at 02:26 PM by Zuiko
 21 October 2013 08:53 AM
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Blackadderofwickford

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Thankyou for the responses, very much appreciated.

Looks like I will have to conceed and run pipes left to right across the room, with the joists opposed to across them, up and down the room.

Once its all up to temp, it will all remain constant anyway, it was only a concideration for the warm up / cool down rates, trying to keep it more uniform throughout the room.

This means I will just have to notch the ends of the joists to loop the pipes from one void to the next. (I will follow the guidance zones from TRADA)
 21 October 2013 12:01 PM
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Avatar for OMS.
OMS

Posts: 19463
Joined: 23 March 2004

What are you planning for the "solid" end of the room. Have you considered rasing the floor by some thickness and placing the UFH in that zone (probably a combination of heat spreader plates and cemientitious screed.

Where's the insulation layer being placed ?

For what it's worth, my rough calcs echo above - a row of holes as you suggest derates that joist by about 75%

Regards

OMS

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Failure is always an option
 21 October 2013 01:14 PM
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Zuiko

Posts: 521
Joined: 14 September 2010

Notching the joists will have a similar effect - a reduction of CSA and introduction of a stress concentrator.

cheers
W
 21 October 2013 05:05 PM
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OMS

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Having had another look at this, actually, for 25mm diameter holes, not closer together than 75mm, and within the middle of the joist, the derating is effectively zero. The effect of putting those holes in the first and last quarter span of the beam only derates by about 10%.

Notching the end of the beam close to the bearing point by 25mm would also have almost no effect on load bearing (ie less than 10%)

Regards

OMS

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Failure is always an option
 21 October 2013 05:44 PM
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jarathoon

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I would have thought that if beams were to start rotting they will most likely rot in from the ends, where condensation/water runs down walls etc.

Does the possibility that wood can rot given the right circumstances have any bearing on the issue?

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James Arathoon
 21 October 2013 06:59 PM
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Zuiko

Posts: 521
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I'm not so sure about that OMS.

The joist can only be as strong as its weakest part.

Disregarding things that are uncontrolled, like wood grain etc, the weakest part is where the cross-sectional area is smallest and/or where there are stress concentrators, like drilled holes.

Reducing the CSA of the joist by drilling it will derate it, and not by a negligible amount:

http://www.matsceng.ohio-state...ncentration_Notes.gif

You can see for a drilled hole, the stress concentration factor approaches 3 for a ratio of 0.1.

Notches are not too good either.

Cheers
W
 21 October 2013 07:23 PM
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Zuiko

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this looks useful, and also gives links to the relevent British Standards

http://niceic.com/Uploads/File3777.pdf
 22 October 2013 09:26 PM
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cblackha

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One thing you could consider is to strengthen the joists you need to drill by bolting sections of steel plate or angle along their lengths.
 23 October 2013 06:29 PM
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OMS

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I went back to basics and determined that the joist is a simple beam supported at either end. The greatest bending moment occurs mid span, but a 25mm hole in the geometric centre of the joist of that size is in a zone of almost zero tension or compression.

I took the top and bottom 50mm of the joist as chords in compression and assumed the middle section was acting in shear to seperate the chords - drilling through the centre doesn't have much effect at all - if it's on centre line (effectively in the neutral axis)

Regards

OMS

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Failure is always an option
 28 October 2013 02:52 AM
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kengreen

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I have always understood that underfloor heating gets turned off permanently after one or two winters' use.

The problem apparently is that it results in hot-feet/cold head.

Ken Green
 30 January 2014 07:39 AM
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gloriasakins

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Joined: 30 January 2014

A right-angle drill is a power drill that features a chuck that is oriented at a 90-degree angle compared to the rest of the tool. A chuck is the clamp that holds the bit or other tool on the drill. It is always suggested by structural engineering firms as this type of drill is most commonly used by plumbers and electricians because of its ability to work in tight wall spaces, though many trades benefit from this drill design.
Link removed/
 21 February 2014 05:15 AM
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peterpadams

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Joined: 21 February 2014

Joist construction should be long dispatched to handle the load. Suggestion by a strutural engineer could be that you should handle the load by drilling 35 mm holes on the side diagonally to center and not in the middle.
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