Structural Ridge Rafter Thrust 8

[cap4000]

The IBC code is clear as long as there is a structural ridge beam with posts no thrust will occur at the exterior walls. Therefore no heel connection design is required. Is this because of plate action of the plywood. Any tips will be greatly appreciated. When I place the snow load perpendicular along the slope of the rafter and not along the horizontal thrust forces occur. This is easily verified with Ram Advanse.

 

[woodman88]

JAE - If you supported a horizontal beam (without any connections) with a roller at each end, the beam would not move until a lateral force was applied to it. If you supported the same beam placed at a slope by a roller at each end. The beam would roll off the upper bearing.
Now you start connecting the beam to a structure and you have many difference means of restraining this force. Most of them are not calculated in designing the structure but are still there to resist the slipping of the beam off of the supports.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[msquared48]

Woodman:

You said that...

"If you supported a horizontal beam (without any connections) with a roller at each end, the beam would not move until a lateral force was applied to it. If you supported the same beam placed at a slope by a roller at each end. The beam would roll off the upper bearing."

Yes, only because the rafter is inclined to the roller. Basic statics.

However, if the rafter is not inclined to the roller, as in flat bearing - requiring flat notching of the joists - this will not be the case. The only thrust seen will be from the deflection of the ridge beam, or rotation of the joist end seats due to their deflection. This is not accounting for the restraint of any roof diaphragm which will limit the thrust to the center part of the wall and span it out to the ends of the diaphragm.

This becomes the driving argument for notching the rafters.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[cap4000]

Here are a couple of links for the static calc's.

http://std.kku.ac.th/4630408404/E book/Design Wood of structures ASD/77169_02.pdf

http://mohandes.net/eng/lds/index.php?pg=sloads

 

[woodman88]

msquared48 - You are correct if both ends are supported on a flat surface. I dislike notching the upper bearing in this manner and most contractors (a least for the sloped rafters) I have dealt with also. So I always look at the restraining this lateral force from a sloped bearing condition at the upper support.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[cap4000]

See the link below pages 247 and 283 of 434 for both rafter and hip beam design with thrust.

http://www.pathnet.org/sp.asp?id=1442

 

[JAE]

Well the flat sloped beam with no lateral thrust is ideal - granted. But so is the roller-roller example where the centroid of the sloped beam is above the "roller" supports below and thus you have small force couples at the supports that initiate an unbalanced lateral load.

But the original post above suggests a "non-roller" connection support at the top (ridge beam with posts) and that is also key here.

With an adequate connection at the top, with a "roller" top-of-wall bearing at the bottom of the slope, there's no lateral thrust at the top of the wall as the ridge beam connection takes any and all "thrust" out of the sloped beam.

 

[woodman88]

I think the word "thrust" in the OP is mis-used? If you look at a sloping beam the vertical reaction breaks down into a perpendicular and parallel to the slope of the beam reactions. The horizontal design method, as it is typically used, does not account for the parallel reaction in the beam design. This is not a thrust force, but it is transfered into the beam at the lower reaction and transfered out of the beam at the upper reaction, when only the vertical loads and reactions are considered.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[woodman88]

Well after rereading my last three posts I do not seem to be explaining myself too good today. But I will give it one more try.
If you look at a sloping beam from the beam being viewed horizontal. The live, snow, dead loads are being applied perpendicular and parallel to the beam. If the supporting reactions were only perpendicular to the beam than this parallel force would “thrust” the beam off of the supports. But the supporting reactions are not only perpendicular to the beam, but are perpendicular and parallel to the beam supports. The wall sees the perpendicular and parallel (thrust) reactions from the beam as only a parallel to the wall force. The wall sees no perpendicular (thrust) to the wall force from the beam.
I hope this is a little bit better explaination. If not, I will at least not try to do any better today.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[BAretired]

The statics of this problem are straightforward. The load is a gravity load. If the end reactions are vertical, i.e. hinge at the ridge and horizontal roller at the wall, the moment is correctly calculated as wL²/8 where L is the horizontal span. There is no horizontal reaction, but the rafter has an axial force of R*sin[θ] where R is the reaction and theta is the slope of the rafter.

If you resolve the load into components parallel and perpendicular to the rafter, exactly the same result is obtained.

The statement: "The ASCE 7-05 section 7.4 "assumes" the load to act on the horizontal projection of the surface which is typically used but technically incorrect." is in fact, incorrect.

BA

 

[JAE]

I agree with BAretired.

 

[woodman88]

There are two methods that the OP is asking about IMHO; 1) sloping beam method and 2) horizontal projection method. Both methods have no horizontal reaction at the bearings.

The first method breaks the loads into parallel and perpendicular components to the beam and is statically correct. The second method just looks at the vertical loads and is not statically correct. This is because the first method shows where the axial force in the beam comes from and goes. The second method does not. But the second method gives one the same answer for the sizing of the beam as the first method with less calculations, so it is more commonly used.

If any other method is being discussed here I would like to know what the method is and where to read up about it.

Thank you.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[cap4000]

When I model the rafter with Ram Advanse the load "perpendicular" to the beam with a roller at the low end and a pin connection at the high end of the rafter it results in an Fx reaction at the ridge. You can reverse the pin and roller and it still comes out with an Fx reaction.

 

[haynewp]

You need the load perpendicular to the beam and the load along the slope in the model if you are doing it that way. The result should be global vertical reactions only and no global horizontal reaction.

 

[WillisV]

BAretired is correct.

 

[woodman88]

WillisV - You state that BAretired is correct. Are you referring to the statement "The statement: "The ASCE 7-05 section 7.4 "assumes" the load to act on the horizontal projection of the surface which is typically used but technically incorrect." is in fact, incorrect. "
If you are than please show me or tell me where to go to see the "horizontal projection method" means of determining the parallel axial force in and out of the beam.
I know the force is there and can prove with the “sloping beam method" or by modifying the "horizontal projection method" to include it. But how is it shown within the "horizontal projection method"? Because if it can not be shown within the "horizontal projection method" then the method is technically incorrect.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[BAretired]

cap4000,

You may be confusing local and global systems.

If 'Fx' represents axial force in the local 'x' direction, then your result is correct. There is an axial force in the rafter varying from a compression at the low end to a tension at the high end.

In the global 'x' direction, Fx = 0 throughout.


BA

 

[cap4000]

The fact that there is an axial load on an incline beam results in an Fx. This is similar to an upper chord of a triangular truss at the lower support. For there to be shear the load has to be resolved into a 90 degree transverse component. The nails into the ridge, the structural ridge not sagging and the sheathing all restrain this Fx force.

 

[BAretired]

woodman88,

A vertical reaction at each end of the rafter may be resolved into parallel and normal components. The shear is R*cos[θ] and the axial is R*sin[θ] at the ends. Axial is compression at one end and tension at the other.


BA

 

[BAretired]

cap4000.
If you draw a free body diagram of either support, you see that the horizontal components of the axial and shear in the member cancel each other resulting in a vertical reaction.

BA

 

[msquared48]

I'll throw another thought into the mix here...

When there is not a ridge beam, you need collar ties to keep the walls from spreading, the lower on the rafters the less spreading.

When you have a ridge beam, you do not need the collar ties.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com