Lateral support

[LiamJones]

Can a bungalow with external walls of 17m x 7m, with no load bearing internal walls, provide adequate lateral support to the 17m walls by posi-trusses spanning the 7m? Or are butress walls required along the 17m walls?

 

Thanks,

Liam

[markc]

With a truss you can span pretty much anything, but the truss/joist depth increases significantly as spans increase, trying to keep them shallow just introduces bounce … or more bounce.

having re read the OP, 17m walls are fine but the floor will require some design input regarding lateral bracing.

Edited September 11, 2023 by markc

[kandgmitchell]

If this is new work then check out Part A of the Building Regulations. To fall into the simple guidance a wall should not be longer than 12m between buttressing walls and the enclosed space should not be more than 70m2. That is not to say your proposal will not be OK but it will be up to you to prove it is - usually via a structural engineer.

[JohnMo]

We have a wall longer than 12m, ICF build and the structural engineer required additional structural steel in the wall half way along. Would think your wind loading would have an effect on any needs also.  Would think this is all stuff your structural engineer would sort out.

[saveasteading]

17m is a long way unsupported. Are there not division walls internally?

[LiamJones]

Hi, thanks for the replies.

 

I initially posted this with the idea of using roof trusses to span the width, supported only by external walls. I was then curious if I could make do without any internal solid walls, but not sure about the lateral support requirement. 
 

I’ve since instructed a structural engineer who’s drawing up some plans for a couple of load bearing internal walls (to provide lateral restraint too) and steel ridge beam. I’ll likely go with this approach.

[Gus Potter]

1 hour ago, LiamJones said:

I’ve since instructed a structural engineer

Good plan. In theory we can design a house with a trussed rafter roof with a big open plan empty space and then infill with non load bearing walls.

 

But we need to balance that against the cost.

 

However at times we may want to design a house where we can adapt it later easily. We pay a little more to create a big space that can be easily altered.. much like how we approach an office development.

 

Would be interested to see what you SE says.. maybe they will give you two options.. a big box that you infill and a box with racking walls Then you can compare the cost difference?

 

You mention a ridge beam.. are you doing something like a 1.5 storey and not a bungalow after all?

[LiamJones]

8 hours ago, Gus Potter said:

Would be interested to see what you SE says

 

You mention a ridge beam.. are you doing something like a 1.5 storey and not a bungalow after all?

I’ll certainly report back.

 

yes, 1.5 story with 3 bedrooms upstairs.

[Gus Potter]

On 04/10/2023 at 07:57, LiamJones said:

I’ll certainly report back.

 

yes, 1.5 story with 3 bedrooms upstairs.

Yes do as as there are loads of folk on here that can give you loads of advise, help and ideas not least.

[Temp]

Long walls also need expansion joints. Think that's slso in Part A somewhere. BCO said we needed one.

[LiamJones]

Here’s what the structural engineer has come back with. 
 

9692 structural markups.pdf

[Temp]

Ok so he is proposing a structural ridge beam instead of a ridge board. These work by effectively "hanging" the roof from the ridge beam. The ridge beam is typically supported on steel posts, chimneys and gable end walls. The rafters need to be well secured to the ridge beam, possibly with metal straps connecting the rafters one side with the rafters on the other side.

 

This solution is commonly used for "one and a half" story houses where you cannot use prefabricated trusses or triangulate the rafters using joists because the joists would be at the wrong height. Eg not in a floor or ceiling.

 

Make sure your builder understands the concept and follows drawings.

 

In the UK most structural engineers reach for steel beams in this situation. In other countries they seem more willing to use timber beams. 

 

The cost of the steel work is one of the disadvantages of 1.5 storey houses but then you save on brick work over a 2 storey.

 

[ProDave]

Our house is built with ridge beams like that, in our case Kerto beams (think overgrown plywood) which are smaller for a given strength than say a glulam beam.

 

It makes for a very open roof space, one of the better design choices we made.

[JohnMo]

This what we did but we used posi rafters, the ridge bean is something like 450 X 200mm. Ridge beam is about 16m long in two parts

[Gus Potter]

23 hours ago, LiamJones said:

Here’s what the structural engineer has come back with. 

Can see the general thrust of the design, some steels etc.

 

I have not studied in detail but pick out one element.. the roof. You have flitch beams at the roof hips. That is going to be more expensive once you detail that up and realise how difficult that is for a normal builder to put together and build properly. The following is pretty detailed but I hope it gives you food for though in the sense that you need to ask a lot of questions and try and look at the design holistically In other words if we buy a motor bike in parts the sum of the parts will cost more than just buying.. ready to ride away!

 

Below is screen shot of my structural model of a roof I got involved in and improved / value engineered. It's possibly more complex than yours but there is a lot in it that may help you get your head round what is coming down the pipe. The first screen shot is of the overall roof structure.

 

I can see that on your roof plan at the bottom you have some heavier, treble rafters.. I'm guessing this is for a big roof opening / dormer? At he bottom left of the model there is a dormer, a bit smaller than yours but the concept is the same.

 

The main thing about this model is that all the parts are off the shelf, nothing hard to source and a decent builder will be familar with working with the different components and know how they are supposed to be installed. The aim is to go for the simple stupid. Lastly all roofs are different ( the layout and support positions below also drives the roof model) when we get into the fun part of self building so don't take my way as the best way for your project.

 

 

 

Below is how I swapped out the originally hip flitch beams for solid timbers and the hips.. they are offset in the vertical direction but I gave the builder the info they need to offset, rather than just guessing. It's actually quite easy to do once you get your head round the roof angles and if you can do a compound cut (a competant joiner/ chippy should be able to do this easily). I know as I used to be on the tools before I was an SE)

 

 

In the above there are short horizontal timbers with long threaded rods (from Jewson ect). These tie the hips together and back into the main roof. You can see there is no connection at the ends of the hips to the ridge area aNd may wonder how the vertical downwards loads are resisted.. The reason for this is that in practice with the steel between the timbers the connection is so complex it almost stops working. Ask you SE how the are going to detail the flitch beam to ridge connection. Would be interested to see how they do it. For me I'm sharing what I know and how I try and simplify so in the round we stand the best chance of getting it built the we want without costing a fortune.

 

 

The above is a screen shot looking up from below. Here I cantilevered the ridge beam so we have a simple connection between the supporting post and the ridge beam. Let's break down big problems and make them into simple things that can be tackled one at a time on self builds.

 

The ridge beam has an big steel angle shop welded onto the end.. now we have big ledge to support the hips.. the chippies / joiners can't miss that landing area, even if they cock it up a bit there is some slack in the design.

 

In summary the two images above show how I tie the roof together and support the vertical loads by decoupling the hard things to do on site..

 

Below are some screenshots to show how the model gets translated into a "paper drawing" for the builder. Incedentally if I'm doing something like this I always give the builder the 3D view and say.. I want it to look just like that! It's also great for self builders (and me) as its helps you visualise what needs done.. and it is great fun!

 

 

 

 

 

 

Below there are a few things / ways where you can save money and some food for thought.

 

When we are working with steels supporting roofs the steels themselves are usually not that heavily loaded to the extent that the steel material wil start to fail. Normally as steel is "bendy" it's the deflection that govens the design. This often means that the connections can be made more cheaply in a domestic application. You may see some SE's asking for a "full strength connection. This can  involve an expensive welding process and beam end preparation. If the connection forces are relatively low then we can often use what is called a division plate with a bog standard 6.0mm fillet weld. The plate is cut from a bog standard flat bar.. off the shelf.

Below shows a cranked beam form the model with a division plate shown as the black bit between the beams. Here we just use standard 6.0mm fillet welds to join it all together. Cheep and chearfull. I did check the division plates would not stick out the roof by the way!.

 

In the above you can see a timber wall plate bolted to the top of the steels. It can also often be shot fired.. ask your build what preferance they have. In this case all the steels came predrilled to make it idiot proof.. I said if there is a hole it needs a bolt.

 

The red bits are plywood gusset plates nailed and glued to the sides of the rafters, there is a timber packer behind these gusset plates you can't see. The reason for this is partly geometric  and partly govenernd buy the Architectural design. But in essence this worked on this job to transfer the roof loads down onto the steels while also tying everything together. The L shape brackets are just Simpson or similar stuff off the shelf.

 

Hope this post helps inspire..