Steel post for hip point??

[Rad1o]

We are extending our house with a above garage and loft extension.
We got planning permission.
Our loft currently has a set of rafters around the roof joists and 2 floor binders across the short length.
The extension will be along the long distance of the roof.
The structural engineer has now drawn plans to remove the rafters and put a steel post under the hips of the roof.
The steel posts will be pretty much in the middle of the room.
Is there another way to support the hips but not using a post in the room?

 

[^woody^]

There is an argument that hip rafters are not load bearing.

Without knowing his assumptions or seeing what the engineer has done, its hard to comment on why he has done it

 

[RonnyRaygun]

It doesn't sound like an ideal solution. There will almost certainly be an alternative, but it may well cost you a lot more in engineer's and construction charges.

Can you attach the drawings for us to have a look?

Post no. 666 haha

 

[tony1851]

Is there another way to support the hips but not using a post in the room?

That is just stupid; is he an engineer or a cowboy? As RR said, post the drawings.

 

[tony1851]

There is an argument that hip rafters are not load bearing.

If they are not load-bearing (ie don't carry any weight) why do we have them? Just wondering.

 

[^woody^]

To fix the rafters to!

 

[RonnyRaygun]

We had this discussion in a thread a few weeks ago...

I pointed out then that a hip rafter is not like a ridge board. Because the common rafters are directly opposite each other, they support each other if the ridge board is removed. Gravity acts towards the ground, putting the commons into compression, as long as they are tied at their bases to stop spread. Without ties, you have to have a ridge beam.
Jack rafters, supported by the hip, are different. They are not directly opposite each other (in any one plane) and therefore cannot be supported by each other and cannot support the hip rafter. So the hip must support the jack rafters and must be load bearing.

The top of the hip has to be supported by the first pair of common rafters...wonder how often they are designed for that compressive load...

The lateral loads must be taken by the ridge board, so in that respect even the ridge board is structural.

Take away the ridge board and first set of commons and everything falls over...unless...both the base of the hip and the jack rafters are secured in position...
Then they form a kind of truss in which the jacks are supporting the hip, and in return the hip is supporting the jacks. Not what should be happening though, as then there will be serious thrust into the walls which could cause all kinds of trouble...

Anyone agree or disagree?

 

[^woody^]

I've seen very good arguments from both points of view, but nothing conclusive

Here's my take on it

The jack rafters can't drop unles the hip does, likewise the hip can't drop becuase the jack rafters are working together to hold it up. Neither can function without the other, but load wise the load goes down the rafters - the bending load (hip) is transferred to tension and compression in the jack rafters

Its a relationship similar to a truss - the web needs a top and bottom chord (rafter and tie) so as to be supported and to function. Likewise, the top and bottom chords need the web to be supported and to function. But its the web which takes the load

 

[tony1851]

RR; while you were writing you post, I was sketching out an imaginary hip roof with no hip rafter, but with the tops of the jack rafters simply nailed together (quite how I wouldn't know).
Obviously the pairs of raters would be unstable and would topple over. The fact that they don't when fixed to a hip rafter suggests logically that the hip must be supporting some load from the rafters, but how much? Who knows?

Another fallacy I've heard of, common with building inspectors, is that the hip rafter 'pushes out' on the wall plate with considerable thrust and has to be tied back to stop the roof somehow pusing out at the lower corner.

There are many old wartime photographs which show bomb damage to hipped roof houses, where the corner of the wall is demolished, but the roof maintains its shape. My guess is that the downward load of the roof is fairly evenly distributed around the walls.

Untitled

• tony1851
• 4 Sep 2012
• 1

 

[RonnyRaygun]

Another fallacy I've heard of, common with building inspectors, is that the hip rafter 'pushes out' on the wall plate with considerable thrust and has to be tied back to stop the roof somehow pusing out at the lower corner.

Yeah, logically there can't be much outward thrust without the ridge board dropping...unless the hip magically manages to extend itself...

You're catching me up on both posts and thanks...two kids, one just over two, and one of three months have put paid to my regular postings!

 

[tony1851]

I've seen very good arguments from both points of view, but nothing conclusive

Here's my take on it

The jack rafters can't drop unles the hip does, likewise the hip can't drop becuase the jack rafters are working together to hold it up. Neither can function without the other, but load wise the load goes down the rafters - the bending load (hip) is transferred to tension and compression in the jack rafters

A reasonable hypothesis, and to add, I would suggest that the hip angle is also critical.
In theory, it could be almost horizontal and act like a beam (tension and compression) or very steeply pitched and act almost like a column (compression only). But then if it's nearly vertical, presumably it will be carrying very little load from the rafters as their own loads will also be not far off vertical.... this makes my head hurt.

 

[RonnyRaygun]

I wonder if a 3D frame analysis would give a difinitive answer

 

[freddiemercurystwin]

Well what you waiting for then?

 

[RonnyRaygun]

Well what you waiting for then?

Ok, will start lunchtime

 

[RonnyRaygun]

I wonder if a 3D frame analysis would give a difinitive answer

And the answer is...

Next to no moment in the hip member - in fact the moment changed between sagging and hogging along the hip between each pair of adjoining jack rafters. But it's tiny; less than 10% of the moment in the jack rafters.

So the hip is very nearly non load bearing, but not quite...

Clearly designing for half of the jack rafter load is way way too conservative. In fact, anything that is deep enough to allow a full butt of the cut edge of the jack rafter is going to be plenty strong enough.

There is also almost no displacement at the end of the hip members...as long as the common rafters are tied, the hips can't move out.

So it seems that roofers have had it right all along, and traditional hips are not undersized...and engineers are often overspeccing when designing hips on new properties. Whether this is down to the designer, checking engineer, or the building inspector is another matter...

This is working on the assumption that I've set up the model correctly...not easy!