Purlins in shear

blasted · 7 Jul 2009

I'm considering undertaking a loft conversion and as such i would have to remove a binder in order for stair access. The purlin above the binder is not big enough for the roof load on its own, the binder assists in preventing purlin deflection.

In order for me to be able to remove the binder i need to strengthen the existing purlin. I have a calculation for a 300x90 Kerto beam which is strong enough to take the entire roof load. However, for ease, i dont want to remove the exisitng purlin. I would prefer to bolt it alongside the existing purlin (like a splint) in accordance with the manufacturers bolting instructions, this would prevent deflection.

I'm a little unsure of the existing purlins strength in shear, For ease again i would prefer not to open up the block work alongside the existing purlin. I did consider using a heavy duty simpson hanger to go around both purlins, but the masonry where it would bolt into isnt very good. it was mentioned that the existing timber may be strong enough on its own for shear, but the new pulrin would prevent deflection.

The existing pulrin is 225x60 and spans 5.35m with 100mm bearing at each end, onto dense blockwork.
Any ideas?

Regards Blasted

Deluks · 8 Jul 2009

blasted said:
the binder assists in preventing purlin deflection.

No it doesn't. Are sure it's not a strut? (a vertical or angled section from purlin to a loadbearing point such as the top of a wall)

When you sat 'existing masonry isn't very good' how will it be able to take the loading on the purlin end? Unless you mean tricky to bolt into, in which case a resin fixing would probably suffice.

Goldberg · 8 Jul 2009

I don't understand your concern about shear stress on the existing purlin when your motivation for keeping it is not strength, but convenience.

And when you say "it was mentioned that the existing timber may be strong enough on its own for shear", who did the mentioning?

blasted · 8 Jul 2009

To elaborate a little more, with picture at bottom

The roof load sits on the purlin; the purlin on its own is not strong enough for the entire load. Therefore some of that load is transferred to a binder below by vertical struts/stringers. The binders are also attached to the ceiling joists to prevent them deflecting. Both the purlin and binder bear on the existing block work. There are no load bearing walls underneath. There is a stud wall in the middle of the house that the ceiling joists overlap on, but this does not factor into the design.

To make the job easy to undertake, I would leave the existing purlin in place, so I don’t have to support the roof and mess about removing the old purlin. I would simply be bolting the kerto to the inside face of the purlin. This would also be in contact with the rafters above. The new kerto however would not be bearing onto the block work. This means that the only timber taking the vertical loading of the roof and transferring it to the block work would be the existing (225x60) purlin. It is this part of the structure that would be in shear. Therefore I would like to know how I would work out the shear value for that end. From a brief calculation the end reactions on the purlins are 11.2kN.

Thanks

Roof Construction Picture

Goldberg · 8 Jul 2009

Tricky one. I'm slightly curious about two things:

1. Who did the calculation for the Kerto, and why did they not calculate the shear load on the purlin(s) according to your planned deployment?

2. What happens to the strength of the Kerto if you drill holes in it for bolting to the purlin?

You say that it's easier to leave the existing purlin(s) in place, which of course makes sense, but exactly how much harder is it to remove it/them? Could you install each Kerto in a new position nearby, and have it take up the load before you remove the corresponding purlin?

A couple of other points:

3. Is the work that you're undertaking being done with LABC approval?

4. Why [on Earth] is the existing purlin not oriented with its long axis parallel to the direction of the load?

noseall · 8 Jul 2009

That roof looks fairly steep so i guess a lot of the load is going back down to the plate.

How do you plan on introducing the Kerto beam into the loft?

The masonry looks fine to me.

Are the existing joists adequate?

blasted · 8 Jul 2009

1. Who did the calculation for the Kerto, and why did they not calculate the shear load on the purlin(s) according to your planned deployment?

It was done on the structural software that they use at the distibuter's, they can only design from scratch, plus they provide the calcs to back up the design.

2. What happens to the strength of the Kerto if you drill holes in it for bolting to the purlin?

Drilling and bolting is fine as long as its within the manufacturers specifications. Typically two rows at 600 centres, with the rows staggered.

You say that it's easier to leave the existing purlin(s) in place, which of course makes sense, but exactly how much harder is it to remove it/them? Could you install each Kerto in a new position nearby, and have it take up the load before you remove the corresponding purlin?

That is a possibility, but it would mean chopping into the blockwork, which also means it would be very difficult to get them into the space. To get them in i will need to make a hole in the side of the wall and slot them in, which can only be done in the middle and not where the purlins are. I will also be slotting in the posi joist floor which will be running at 90 degrees to the existing ceiling joists and hung by masonry hangers from the gable walls.

A couple of other points:

3. Is the work that you're undertaking being done with LABC approval?

Once I have found an adequate solution to the problem, drawings will be submitted for proper approval.

4. Why [on Earth] is the existing purlin not oriented with its long axis parallel to the direction of the load?

Your guess is as good as mine, it was built 20 odd years ago, well before i got here.

The masonry around the back of the purlin is a bit shoddy, they used half bricks to fill in the space. I certainly wouldnt like to be drilling into it. The rafters are birdsmouthed onto the wallplates. At the end of the day im a competent diy'er with an engineering background, im not a builder, so simple is allways the best approach, and help ios allways appreciated. Its got to better than the usual board the loft out, for a gym type questions.

^woody^ · 8 Jul 2009

Either put this new beam on hangers. Or, bolt a post to the wall to form a bearing

Goldberg · 8 Jul 2009

Well, taking everything into account, I know what I'd do if it were my roof:-

1. Install the Kertos, or equivalent flitch beams, or whatever, just above the old purlins.
Naturally each one would be at right angles to the rafters, i.e. not in the bizarre orientation of the existing purlin.

2. Brace the new 'purlins' using an A frame at each end, on a floating platform resting on the joists.

3. Remove the vertical struts and cut out the old purlins

4. Make new bearing surfaces on the gable ends and brick around the new beams.

5. Remove the bracing.

TBH I'm not convinced that those existing struts are doing anything whatsoever. The purlins will deflect horizontally, if at all, so I don't believe the struts are stopping deflection.

blasted · 9 Jul 2009

Thanks for the advice chaps.

Ideally I would like to follow the plan Goldberg suggested, but getting the beams up there is a problem on its own.

regards

Blasted

Goldberg · 9 Jul 2009

How much clearance do you have between your gable wall and the next house?

blasted · 9 Jul 2009

There is no room on one side of the house; the other side has reasonable access, assuming the neighbour lets me drive down their drive. To insert the posi joists and Kerto, I was going to put a hole in wall, below the roof ridge, roughly just above the proposed floor level. As all parts would only be the length of the distance between the internal gable walls faces, I could easily move them around. To put the kertos in with the extra 100mm bearing at each end, I would have to knock holes in at the side of the existing purlins and then slot through. How I would get a 75 kilo 5.55m beam up there and slot it sideways through a hole in the wall is another thing I’m unsure of.

At the end of the day this is being done by a DIY’er and a very limited budget, so roof off is just not an option.

Goldberg · 9 Jul 2009

I recommend that you insert the Kertos from the side that has access, not the other side.

blasted said:
As all parts would only be the length of the distance between the internal gable walls faces, I could easily move them around.

This seems unwise.

blasted said:
To put the kertos in with the extra 100mm bearing at each end, I would have to knock holes in at the side of the existing purlins and then slot through.

That's more like it!

blasted said:
How I would get a 75 kilo 5.55m beam up there and slot it sideways through a hole in the wall is another thing I’m unsure of.

I don't see an alternative to this kind of thing:

blasted · 9 Jul 2009

Thanks for your help,

Its going to be fun regardless of how I do it.

Goldberg · 9 Jul 2009

I agree, and I'm quite envious. Any chance of more photos as the project progresses?

diynot

If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.

Select the supplier or trade you require, enter your location to begin your search.

Are you a trade or supplier? You can create your listing free at DIYnot Local

Purlins in shear

Goldberg

Goldberg

Goldberg

Goldberg

Goldberg

Goldberg

Similar threads