Issues with sliding doors

[Adsibob]

I think it depends what aesthetic one is trying to achieve. If, for example, one wants to have continuity between the inside and the outside, for large summer parties (and since lockdown for all year socially distanced gatherings < 6!), then having a wide opening with a flush threshold and trying to match the external finished floor covering to the inside finished floor covering, as well as the threshold material, it can work really rather well. But that requires enough space for furniture not to get in the way, etc. It won't work in every layout so it won't be worth going to all that trouble in every situation. It also depends what the external environment and view is like. We ended up only having 60% of our rear elevation covered with sliding doors

[PeterW]

Ok so for those that want to see a pretty horrendous - but acceptable - load graph then is is from a job we did but did not use the steel in the end as it was designed without the ridge as a single steel. 
 

This is the sort of detail I get from the SE - note it’s in three sections which is the inputs, the graphs and the notes. 
 

First off, this is a beam that is 305 x 165 - no small beam. At 46kg/m it would weigh in over 350kgs. It is purely a ridge beam, so no real additional loads. 
 

What’s worth noting is the Deflection Limits. Fully loaded, it’s Span/200. That means if you want a 4.5m span that only deflects 5mm max, you need to tell your engineer you want Span/900..!!  That is the bit most people get wrong or miss, as it’s a standard value unless you change it..!!

 

Now look at the graphs. 
 

 

The bottom is the deflection graph - at 3.75m it would be deflecting over 15mm. 
 

Now finally look at the notes. Note the comments about assumptions made, and the deflection being within the tolerance ..!!

 

 

This is a beam that is 305mm deep, with a decent profile and yet a 37.5mm deflection would have been acceptable ..?? That’s 1 1/2” in old money ... 

 

So maybe a few people need to talk to their engineers and explain their requirements and a lot of these door issues wouldn’t occur. 

[Gus Potter]

Interesting thread this. Good technical points made by Peter,Craig et al.

 

To add my thoughts, partly technical, but mainly to open up design options you can consider that are cost effective, that won't lead to problems with your mastic etc on the finishes, sticky door issues, glass that fails due to adverse unforseen loads etc and the subsequent arguments. Don't forget that these large glazed openings cost a lot, the glass, slim frames etc.

 

A bit of technical stuff.

 

The formula for deflection of a steel beam is 5* w* L^4 /384 EI. For the keen, often domestic steel beam design ignores what is called shear deflection but for timber beams this needs to be accounted for as it is significant. To explain the formula for typical domestic steel beams. E is Young's modulus.. a property of the steel which does not change with beam size. w is the load per metre run of the beam. I is what is called the second moment of area and L is the length.

 

Now you can see that if E, I and w all stay the same then if you have a beam 3.0m long then L is to the power of four i.e. 3*3*3*3 = 81 units, if you have a four metre beam we have 4*4*4*4 = 256 Now 256 divided by 81 = 3.16. So you get over 300% more deflection on a four metre beam than one that is 3.0m long all other things being equal. The thing to draw from this is that beam deflections are very sensetive to length. Extrapolate this to an 6-8m opening and you get this exponential deflection which plays havoc with your doors.

 

The next parameter you can vary is the second moment of area "I". To get your head around this the formula for a rectangular beam is I = b*d^3 / 12 where b is the width and d is the depth. You can see here that if you have a rectangular beam 200mm deep beam d^3 (cubed) is 200*200*200 = 8*10^6 mm^3 (mm cubed) but if you have a same width but 300mm deep it is 300*300*300 = 27*10^6mm^3. So buy increasing the depth of the beam by 100mm you reduce the deflection by 27/8 ~ 300%. The way a steel I beam works is that you cut out the sides and place more steel in the flanges. This give a much more efficient shape so by moving the material to the top and bottom flanges you get more "I" for your buck!

 

Now for a big glazed opening design the starting point is how much deflection will be ok over the head of the doors. There are general structural recommendations in the codes that go along the lines of beam span / 360 but these are mainly to do with the other elements of the building. If you have an effective clear span of 8.0m that is 8000mm / 360 = 22mm. That is going to jamb your doors, break the mastic seal (at times you will notice the bend over the opening) on the outside and probably damage any wall paper inside.

 

It's easy to fall into the trap where you see the deflection as being say 22 mm, so you put in 25mm of say compriband.. but if you compress compriband or similar down to 3.0mm it will start to extert load on your doors.. it's good (squashy) but not that good! You also need to make sure that when the load is not there that the compriband will recover and not leave a gap.

 

The next thing (Craig, Peter etc have touched on this I think in the past) is that there is a difference in how you install bifold doors and true sliding doors. A sliding door head can be installed with a bit of a gap over the head as they now tend to be all bottom supported, but bifold door heads need tighter packing at the head. In other words bifolds are less forgiving in terms of beam deflection. If you think about it. When the bifolds are open there is a lot of glass weight hanging out from the building so the mechanism at the head of the doors needs to be held firmly in place so that over time they still perform, the tolerances are tighter and less compatible with the structure.

 

As promised.

 

If you have read this far then if you have a single storey extension with a large opening you may have a flat roof above. Here you may be able to use the upstand on the flat roof to accomodate a deeper beam thus reducing any downstand in the extension. If you have a two storey house with large bifolds etc below you can start to look at turning the upper floor external wall into a big truss. This in the right circumstances can allow you almost take the doors right up to the ceiling! The secret is that there comes a point where a big steel beam is no use / economic if you have some height above to do something.

 

If you have a modern house, say with an "L" shaped roof" and go in the attic you may see lots of thin prefabricated trusses. But at the "L" bit you may see some sturdy looking trusses.. girder trusses. You can apply the same principle to creating a large glazed opening on the ground floor.

 

If I was looking at designing a 3.5m plus opening for glazing I would use start by saying.. I want no more than 6 - 8mm deflection at concept design stage under say snow loading or roof access. I would then look at the type of construction.. masonry, timber frame. Timber creeps over time so that has to be accounted for. I would also look to see how much the founds may settle.. if there is uneven loading.. differential settlement. Once you get a handle on the "feel" of things you are on your way to getting a problem free solution.

 

This all may sound expensive as you need an SE, experienced designer that can look holistically at this but it may only cost a little more for the extra design input. Ideally it may end up that you save money!

[Gus Potter]

Thanks volcane, dp and Peter for reading. When I was doing the formula bit I thought readers would loose the will to live!

[Adsibob]

On 08/05/2021 at 10:06, craig said:

we advised to remove all the PU foam that was used above and replace with Compriband.

My sliding door papers has come back following site survey and I'm asked to sign off on a contract note that specifies the following for the "external weather seal":

"silicone ARBO range or compriband 7-12"

 

Presumably I should specify Compriband, but is 7-12 the right type? Didn't realise there were different types.

[PeterW]

Arbo is just a brand - they do low and high modulus silicone. What deflection has the SE specced on the steel and what have they allowed for on the window design 

[jack]

21 minutes ago, Adsibob said:

My sliding door papers has come back following site survey and I'm asked to sign off on a contract note that specifies the following for the "external weather seal":

"silicone ARBO range or compriband 7-12"

 

Presumably I should specify Compriband, but is 7-12 the right type? Didn't realise there were different types.

 

7-12 relates to the size (basically the range of gaps widths its designed for, I believe). You choose the size to suit the specific gaps that need sealing.

[craig]

47 minutes ago, Adsibob said:

but is 7-12 the right type? 

Depends what the actual gap is. The first number (15 for example) is usually the width of the compriband, the second number of expansion (from and too) - in this instance expanding from 7mm to 12mm.

 

The compriband manufacturer will only warranty for that expansion and not above that.

 

Important to note, this will be fine for windows if they have a 10mm tolerance. Head of slider “may” be different.

Edited May 11, 2021 by craig

[craig]

38 minutes ago, PeterW said:

What deflection has the SE specced on the steel and what have they allowed for on the window design

Also this.

[Mandana]

Hello everyone. Thanks for all your replies, and apologies for the radio silence. 
 

We’ve been in touch with a construction law solicitor and will go that route if we don’t hear back from the company within the timelines we’ve asked them to. 
 

@craig are you an inspector? Where are you based? 

[craig]

4 minutes ago, Mandana said:

are you an inspector? Where are you based? 

 

Professional, experienced but not an inspector. I'm based in Scotland, were are you located? I could supply my expertise or recommend a 3rd party that could give you expertise from a building and installation aspect.

Edited May 26, 2021 by craig

[Mandana]

13 minutes ago, craig said:

 

Professional, experienced but not an inspector. I'm based in Scotland, were are you located? I could supply my expertise or recommend a 3rd party that could give you expertise from a building and installation aspect.

Thanks Craig. We are based in the South East. 

[craig]

I'll drop you a PM.

[Mandana]

Thanks Craig. I’ve replied now. 

[MAB]

I split the steel 'goal post' wide opening for my folding/sliding doors in two with a central brick clad steel pillar and two sets of identical doors (mirror image) to avoid the deflection problems I have seen very wide openings suffer from (it is also a double story extension above). In practice it works well as most times it is only one set of doors that we open, apart from on very hot days or for family gatherings, etc. Smaller folding door panels i find are also an advantage for less weight and bulk for stacking up each side when fully open.

 

A slight disadvantage when all our doors are opened up wide and the room briefly unattended has been the occasional pigeon flying inside and making a mess where it has landed....and a local tame fox which once casually wandered in and out after sniffing out food our cat had left in his bowl in the kitchen ! ?

Edited May 26, 2021 by MAB

[Mandana]

On 26/05/2021 at 15:31, craig said:

I'll drop you a PM.

Hi Craig

Did you receive my message? 

[Jilly]

I imagine you end up with a house full of insects ??

[craig]

57 minutes ago, Mandana said:

Hi Craig

Did you receive my message? 

Sorry, I got caught up with a few things. I did see it, I’ll pop his number on to you.

[Mandana]

59 minutes ago, craig said:

Sorry, I got caught up with a few things. I did see it, I’ll pop his number on to you.

Great, thanks Craig ?