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Structural Ridge Rafter Thrust
8

Structural Ridge Rafter Thrust

Structural Ridge Rafter Thrust

(OP)
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.

RE: Structural Ridge Rafter Thrust

It limits the deflection of the ridge and mitigates the thrust.
The plate action of the plywood helps as well.

RE: Structural Ridge Rafter Thrust

Some thrust and associated lateral deflection will occur , but it is very limited due to the diaphragm action of the roof sheathing.

Mike McCann
MMC Engineering
http://mmcengineering.tripod.com

RE: Structural Ridge Rafter Thrust

Quote:

When I place the snow load perpendicular along the slope of the rafter and not along the horizontal thrust forces occur.

Snow load always is vertical since that is gravity's general direction.

If you have a sloped beam (supported at the high end by the ridge beam and at the low end by the wall) there will be no lateral thrust since all loads and all reactions are vertical.

RE: Structural Ridge Rafter Thrust

Cap:
If you plumb cut a rafter at a ridge board, the primary reaction at that point is a horiz. force; thus there must be an equal and opposite thrust at the walls, or ceiling jsts. must pick this up, plus a vert. reaction which is equal to half the load on the roof over that 16" wall length. If you have a real ridge beam it is assumed that it takes 1/4 of the bldg. width (load) from both sides as a vert. load in bending. And to the extent that it can deflect it will tend to produce a thrust out at the walls, but much smaller than in the former case. With a ridge beam, a top bearing rafter or one hung with rafter framing hangers the reactions imparted are primarily vertical, thus not much (no) thrust. The plywood sheathing diaphragm (a shear diaphragm) also helps reduce this thrust action, even from the ridge beam deflection, if it is well nailed and blocked and properly nailed to a good shear diaphragm at the gable ends. You can see this thrust affect on a long, low pitch, bldg. where the middle of the bldg. will have the walls leaning out a bit at the top, while they are better supported near the gable ends so there isn’t much outward movement. This wall top deflection is in direct proportion to the ridge beam deflection vs. the roof shear diaphragm stiffness (deflection). You do the trig.

When you do your calcs. remember that roof L.L’s. are given on a horiz. projection, so you should convert the roof D.L. to a horiz. projection also, then use half the bldg. width as the beam span length, and calc. the bending moment. Don’t forget any overhangs. The horiz. reaction (thrust) at the ridge board is gotten by taking moments about the center of the wall and assuming no vert. reaction is possible at a ridge board. The vert. reaction at the wall is based on half the bldg. width, plus the overhang. You can convert all the loads to perpendicular and axial loads on the longer sloped length beam, and everything should come out in the wash, assuming all your geometric (trig.) and load conversions are proper. For normally dimensioned roofs we just don’t bother with this. Think a little about this problem as the roof pitch changes: at the one extreme, zero pitch, the rafter is like a floor joist, with no thrust; at low pitches the rafter is still just a floor jst. (a beam) and the thrusts are very high; the rafter becomes a beam-column as the roof pitch increases; and finally the rafters and roof sheathing act more like studs and sheathing, or a deep beam, as the pitch approaches vertical.

RE: Structural Ridge Rafter Thrust

JAE,
Yes, except that when the ridge beam deflects, the geometry changes, forcing the top of the wall out slightly.

RE: Structural Ridge Rafter Thrust

And there must be a tension tie to each rafter across the top of the ridge beam.

Mike McCann
MMC Engineering
http://mmcengineering.tripod.com

RE: Structural Ridge Rafter Thrust

(OP)

JAE

Breyers book Design of Wood Structures has a "sloping beam method" in which there is a thrust force shown. 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.

RE: Structural Ridge Rafter Thrust

Why is it incorrect? It is as correct as it can be.

BA

RE: Structural Ridge Rafter Thrust

Case in point here...

If you were to mount a beam inclined with a downslope to the left on rollers and hold it in place while a snow load was put on it, it would not move right or left once the load was in place. However, if the snow melts and slides, the beam would move to the right.

So, I would deduce that there is no lateral force from the snow load while in a static position, only in a dynamic state.

The beam deflecting will cause lateral distortion to the top of the wall through the joists. However, if the joists are flat bearing (notched flat), the only way it can transmit a lateral force is by horizontal friction on the top plate of the wall, frequently done with hurricane anchors. If a tension strap is placed over the top of the beam, this force is never generated, but the distortion, and subsequent spreading force, due to any ridge beam deflection will still exist. Nothing you can do to totally eliminate that.

Mike McCann
MMC Engineering
http://mmcengineering.tripod.com

RE: Structural Ridge Rafter Thrust

(OP)
Mike

To simplify this a bit think of a hip roof case. A hip beam is a sloping ridge beam. Since its inclined and the jack rafters are framed into the hip beam there will be two components one vertical and one horizontal on the hip. Therefore ceiling joists should be placed in both directions to keep the wall corner from spreading out. Most textbooks with loaded inclined beam have a thrust force. The difference is there is no plywood sheathing.

RE: Structural Ridge Rafter Thrust

cap - this is a case not like your hip - there is a separately supported ridge beam at the top that will not move laterally.

RE: Structural Ridge Rafter Thrust

Frequently too, with the hip, any lateral thrust, if present, is usually taken into the wall top plate and shear walls below, so there is very little lateral movement, only a lateral thrust.

Mike McCann
MMC Engineering
http://mmcengineering.tripod.com

RE: Structural Ridge Rafter Thrust

More thoughts on that...

If the hips are connected to the end of a ridge beam and an attempt is made to develop a moment connection of the hips to the ridge, there is definitely going to be a lateral thrust at the base of the hip.

If the hips just frame to a posted or collar tied ridgebeam , or the ridge beam cantilevered from a post, the thrust will be minimal.

Mike McCann
MMC Engineering
http://mmcengineering.tripod.com

RE: Structural Ridge Rafter Thrust

Quote (cap4000)


Breyers book Design of Wood Structures has a "sloping beam method" in which there is a thrust force shown. 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.

At the risk of repeating myself, I reiterate that the load acting on the horizontal projection of the surface is the correct load to be used. What do you mean that it is technically incorrect?

It is absolutely correct and I would be surprised to learn that a textbook written by a knowledgable person would say otherwise.

BA

RE: Structural Ridge Rafter Thrust

(OP)
Mike,

I agree with you. The rafters when properly nailed to the ridge beam provide a lot of restraint against rafter thrust. 8-8d nails toe nailed, 4 on each side provides about 160 pounds of restraint per inch of penetration into the ridge. I am an expert witness on a 50 foot long single pitch vaulted roof collapse and have pictures showing the nails partially withdrawn from the ridge prior to the collapse.

RE: Structural Ridge Rafter Thrust

Wow... It took all this time to finally get to the actual problem. And you say... “I am an expert witness on a 50 foot long single pitch vaulted roof collapse and have pictures showing the nails partially withdrawn from the ridge prior to the collapse.” The 50' long wall certainly can bow out at the top over that length, despite the roof sheathing diaphragm. This movement is due to a thrust at the top of the exterior wall. This thrust is actually trying to resist the vertical movement in the joint at the plumb cut at the top end of the rafter. Is the “ridge?” at the top end of the rafter actually more like a ledger beam, in this instance? In any case, the 8d toe-nails are not a very good connection and are often not installed very well. Either rafter end splitting or improper penetration into the ledger beam, and very poor pull out resistance. This joint settles a little under roof loading reactions; this geometry change pushes the wall out; unloading, as likely as not tends to pull the toe-nails out a little; now they are bending across a gap, weakening the joint further. The next loading bends the nails a bit, kinda snugging up the plumb cut joint and pushes the wall out a bit more, because the plumb cut joint has moved a bit lower. And, pretty soon you have a roof failure. The connection at the plumb cut and ledger beam has to prevent any vert. settlement of the rafters (at least limit it) or you will end up with a significant thrust at the exterior wall.

RE: Structural Ridge Rafter Thrust

BAretired – What my Third Edition of Breyers book Design of Wood Structures actually states is that the sloping beam method is “theoretically correct” (see section 2.5, page 26). This is due to the book (and most engineers) not completing the horizontal plane calculations. Which if you looked at the bearings as being sloped with the beam, 1) The vertical reaction would need to be changed into a perpendicular and parallel to the beam reactions. 2) Not adding the compression/tension force of the parallel reaction into the beam for the horizontal plane calculations or checking the shear at the bearing for the perpendicular reaction (which is less than the vertical so is not necessary if it works) in place of the vertical reaction.
If you did this complete analysis you would have the same answers for both methods. But with wood there is typically no reason to do these additional calculations. But it is a good idea to be aware of them.

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.

RE: Structural Ridge Rafter Thrust

(OP)
Garth

Thanks for clearing up the BA disconnect. Breyers 4th and 5th edition has the same simple static calc's.

RE: Structural Ridge Rafter Thrust

cap4000 - so where is this thrust coming from? The loads are vertical. The ridge beam at the top of the roof is assumed perfectly rigid. The sloping rafter has vertical load applied - nothing in the horizontal direction. I see zero horizontal thrust at the top of the wall.

If the upper ridge beam deflects downward, then I can see a second order thrust force at the top of the wall.

RE: Structural Ridge Rafter Thrust

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.

RE: Structural Ridge Rafter Thrust

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

RE: Structural Ridge Rafter Thrust

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.

RE: Structural Ridge Rafter Thrust

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.

RE: Structural Ridge Rafter Thrust

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.

RE: Structural Ridge Rafter Thrust

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.

RE: Structural Ridge Rafter Thrust

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*sinθ 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

RE: Structural Ridge Rafter Thrust

I agree with BAretired.

RE: Structural Ridge Rafter Thrust

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.

RE: Structural Ridge Rafter Thrust

(OP)
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.

RE: Structural Ridge Rafter Thrust

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.

RE: Structural Ridge Rafter Thrust

BAretired is correct.

RE: Structural Ridge Rafter Thrust

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.

RE: Structural Ridge Rafter Thrust

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

RE: Structural Ridge Rafter Thrust

(OP)
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.

RE: Structural Ridge Rafter Thrust

woodman88,

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

BA

RE: Structural Ridge Rafter Thrust

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

RE: Structural Ridge Rafter Thrust

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

RE: Structural Ridge Rafter Thrust

BAretired - I know this and agree with it. The point I am trying to make is that the "horizontal projection method" does not calculate this within the method.
It is there and you can calculate it as you have done. But because it is not in the method the method is technically incorrect in that it does not do all the calculations required.
So I must agree with Breyers book Design of Wood Structures statement and the fact that the method is "....typically used but 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.

RE: Structural Ridge Rafter Thrust

woodman88,
I wasn't even aware that it was called "the horizontal projection method". It is just plain statics and it can be done by either method.

I disagree that either method is technically incorrect simply because it does not perform all of the calculations. You might want to say it is incomplete, but why would you say in is incorrect?

BA

RE: Structural Ridge Rafter Thrust

This thread is very fun. However I think the OP needs to supply some FBD w/loading of the different analyzed scenarios in order to adequate discuss this. Pictures would be very nice too. It seems to me that everyone here are very experienced and know exactly what to do but are not connecting at the same points.

Also from earlier post on the pending investigation, I personally have lots of pictures of partially withdrawn nailed members that have NOT failed to collapse. So, not saying that this isn't the cause but it certainly doesn't necessarily follow that that it was the cause of the collapse based on the information given... Just saying.
Further, in a mono slope situation a lot of lateral/horizontal deflection could occur at the ridge which typically would be balanced and developed to the eave walls in a typical gable.

In short, my $.02 on this: Upper support on the rafter in the vertical direction should contain the rafter "thrust" to negligible as long as:
1) the ideal rafter does not deflect too much
2) the ridge support does not deflect too much vertically
3) lateral/horizontal forces do not enter into the model(snow is a vertical load globally) and no horizontal deflections of the supporting frame were taking place.
4) the connection at the heel can be idealized as a horizontal roller.
5) the connection at the ridge can be idealized as a pin.
Since we don't have any specifics on the situation It is safe to say: "there might be thrust and there might not".

______________
MAP

RE: Structural Ridge Rafter Thrust

BAretired - Because you should say "something" to made people aware that there may be potential issues with assuming it is completely correct?

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.

RE: Structural Ridge Rafter Thrust

Quote (woodman88)

BAretired - Because you should say "something" to made people aware that there may be potential issues with assuming it is completely correct?
It is completely correct for the assumptions made, i.e. hinge at ridge and horizontal roller at wall. The load and reactions are vertical and may be resolved into components parallel and perpendicular to the rafter or, for that matter, any other directions desired.

BA

RE: Structural Ridge Rafter Thrust

Hello to all from the UK where the more frequent variation of this problem is the horizontal spreading of the top of masonry walls supporting sloping rafters without horizontal ceiling joists. The movement results in cracks in the horizontal mortar joints at the top of the walls.

So, here is my two pence woth:

If the support from a wall or posts to the bottom of the rafters effectively allows free horizontal movement, then the reactions there will be vertical, whilst at the ridge the reactions will have vertical and horizontal components. An effective diaphragm in the plane of the roof would transmit the horizontal load component to the gables, reducing or eliminating the horizontal component at the ridge.

However, this (pin and roller supports) model is unlikely to be the realistic in practice if the wall or posts resist horizontal movement to any degree. Timber posts may readily accommodate the movement by flexing whereas masonry walls tend to suffer horizontal cracks in the upper mortar bed joints - but there will be some restraint foces in both cases. This does not detract from using this modelling for the sizing / design of the timber sections.

A more useful model to illustrate the potential for horizontal restraint forces at the wall / post top is to consider the rafter as pinned at both ends, with the posts / wall either pinned or fixed at their feet. Such modelling gives an upperbound limit on what forces might be generated if the joint between the rafters and the wall/posts resists movement to some degree. As noted above, an effective roof diaphragm will reduce or eliminate such hoizontal reaction forces. This modelling approach is not advocated for the rafter sizing because of the extra complexity in the analysis and the uncertainty around the degree of restraint at the top of the posts / wall.

Footnotes:
1. Other mechanisms for generating horizontal forces at the post/wall to rafter connection include:
a) pressure / suction on the elevation (depending on the solidity)
b) horizontal components of wind loads on the roof
c) the spreading of the soffit of the rafter under vertical load due to tensile strains.
2. Memo to self: I must keep things short and sweet......





RE: Structural Ridge Rafter Thrust

BA:

I agree with you on your last post, however, I do not feel that the model stated is "absolutely" correct here, as the "roller" is not frictionless in reality at the wall.

There is a connection that is required by code at the ends of the rafters, and this does allow a lateral force, not only to be resisted (wind), but also to be generated, as in the small amount of thrust generated in the deflections of the components.

Mike McCann
MMC Engineering
http://mmcengineering.tripod.com

RE: Structural Ridge Rafter Thrust


Quote (jimodore)

If the support from a wall or posts to the bottom of the rafters effectively allows free horizontal movement, then the reactions there will be vertical, whilst at the ridge the reactions will have vertical and horizontal components. An effective diaphragm in the plane of the roof would transmit the horizontal load component to the gables, reducing or eliminating the horizontal component at the ridge.

Elementary statics prevents having gravity load, a vertical reaction at the wall and horizontal components at the ridge. That is just plain wrong. The ridge reaction must also be vertical.

If the walls resist lateral movement, then I agree that if the ridge beam deflects even slightly, there will be horizontal forces. But the horizontal force is the same at the ridge as it is at the wall.

BA

RE: Structural Ridge Rafter Thrust

New Postmsquared48 (Structural)
30 Jul 12 16:07
BA:

Quote (msquared)

I agree with you on your last post, however, I do not feel that the model stated is "absolutely" correct here, as the "roller" is not frictionless in reality at the wall.

There is a connection that is required by code at the ends of the rafters, and this does allow a lateral force, not only to be resisted (wind), but also to be generated, as in the small amount of thrust generated in the deflections of the components.

I am not suggesting the model is absolutely correct, Mike. But the first post made it clear that we are considering no lateral force on the wall. All of my comments have been based on that assumption.

For a wood stud wall, I would assume that is substantially correct. For a masonry wall, probably not.

If there is a ridge board but no ridge beam, then as you mentioned earlier, collar ties would be needed.

If there is no ridge beam and no collar ties, the roof can still carry snow load as a folded plate but that entails careful detailing of the roof diaphragm.


BA

RE: Structural Ridge Rafter Thrust

jimodore - You did not mention deflection as a cause for horizontal reactions in a pinned pinned rafter/beam. You know, the shortest distance between two points is a straight line. Force the line to curve and the distance between the two points must get smaller or the line longer. So to keep the two points in the same location and not allow the line to get longer the line must be pulled back to a straight position. Creating a horizontal reaction at the pinned 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.

RE: Structural Ridge Rafter Thrust

Quote (jimodore)

Hello to all from the UK where the more frequent variation of this problem is the horizontal spreading of the top of masonry walls supporting sloping rafters without horizontal ceiling joists. The movement results in cracks in the horizontal mortar joints at the top of the walls.
Why would that be, jimodore?
Would it not be more correct to say that the cracks occurred at the bottom of the walls where the moment is maximum?

Quote (jimodore)

A more useful model to illustrate the potential for horizontal restraint forces at the wall / post top is to consider the rafter as pinned at both ends, with the posts / wall either pinned or fixed at their feet. Such modelling gives an upperbound limit on what forces might be generated if the joint between the rafters and the wall/posts resists movement to some degree. As noted above, an effective roof diaphragm will reduce or eliminate such hoizontal reaction forces. This modelling approach is not advocated for the rafter sizing because of the extra complexity in the analysis and the uncertainty around the degree of restraint at the top of the posts / wall.

A much more useful model is to assume that the wall has no resistance to horizontal force and that the rafter is required to take the full moment without benefit of horizontal resistance from the wall.

Quote (woodman88)


jimodore - You did not mention deflection as a cause for horizontal reactions in a pinned pinned rafter/beam. You know, the shortest distance between two points is a straight line. Force the line to curve and the distance between the two points must get smaller or the line longer. So to keep the two points in the same location and not allow the line to get longer the line must be pulled back to a straight position. Creating a horizontal reaction at the pinned supports.

Why would he mention that, woodman88? A pinned/pinned situation is not representative of a real structure.

Perhaps the best thing that could happen to this thread is that it be removed from Eng-Tips because it demonstrates that practicing engineers do not understand the true nature of the structures they are designing.

Alternatively, perhaps it should remain in Eng-Tips so that young engineers could study the problem at length and determine whose brand of baloney they are prepared to buy.

BA

RE: Structural Ridge Rafter Thrust

BAretired - Actually the pinned pinned bearing condition is one of my pet peeves in wood structures. It can occur in two basic conditions. The first is at scissor trusses at the gable end of a building. The second is with scissor trusses (or roof/ceiling rafters to a ridge beam) at the midspan of a exterior wall with perpendicular interior non-bearing walls connecting to them.
In both conditions the scissor trusses want to push the exterior wall away from the perpendicular walls (either the gable end or the interior non-bearing walls) or have their bearing slide out over the exterior wall. Usually in the most extreme cases this is only 1/4” to 1/2” at each end. This rarely occurs at the gable ends as the roof diaphragm usually will resist this horizontal deflection, but I doubt if it is being calculated for this. But I have heard (no doubt only because of my 15 years in the wood truss industry) complaints about this and the answer is that it is not going to move much now so patch the wall.
It is a rare situation that only occurs with a sloped ceiling mainly in custom homes and unless you had had the problem you probably do not care. But I wished that more engineers or building designers doing these type of structures would as least take a closer look at it.

As for you comment "Perhaps the best thing that could happen to this thread is that it be removed from Eng-Tips because it demonstrates that practicing engineers do not understand the true nature of the structures they are designing." I worry more about the structures retired engineers have done.

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.

RE: Structural Ridge Rafter Thrust

Woodman88,
Your concern about structures designed by retired engineers may be valid. I have seen a lot of bad design over the years. But this thread is a clear indication to me that some of the current engineers do not understand simple statics. To me, that is a huge concern.

BA

RE: Structural Ridge Rafter Thrust

Cap - That link proves BA's point there is NO Fx reaction (Fx=Ha=0lbs) for Gravity loads (dead and snow) only for wind load is there a Fx reaction that is not equal to 0.

EIT
www.HowToEngineer.com

RE: Structural Ridge Rafter Thrust

@RFreund - yup, statics, works every time.

RE: Structural Ridge Rafter Thrust

I think the main point that is being missed by most of the comments here is that the reason the sloping beam/rafter does not move is because we are assuming the bearings are/will be horizontal. This assumptionn of horizontal bearing may be valid in 99% or more of the situations and when one specifies a pre-made sloping hanger, a Simpson hanger for instance, the axial force parallel to the rafter/beam has been considered for us. Whether by the hanger designer or the testing.
The huge concern to me, is that with so many of the comments here are assuming a horizontal bearing. But continuing to merely to restate "no horizontal reaction" and not adding that this is only because of the assumed horizontal bearing.

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.

RE: Structural Ridge Rafter Thrust

So for the sake of the argument:

What is the horizontal "thrust" of a rafter at a 45deg angle spanning 10 horizontal with a 20plf snow load (projected horizontally) and supported at the top by a pinned connection and a semi-pinned OR roller in the horizontal only? Assume no wind or seismic loading and dead load is non existant. Oh yeah, it should be a 2x12 rafter #2 DFL.

Values only! Do your work using statics only. Fancy schmancy software packaged answers not accepted and will result in an "F". We have enough commments already. Put your money where your mouth is. FEA matrix analysis will earn you extra credit plus you would have to do this if you assume a pinned connection at the bottom.

______________
MAP

RE: Structural Ridge Rafter Thrust

I do not believe we are assuming the bearings are horizontal. They might be, but do not have to be.

I am assuming the wall is incapable of resisting a horizontal force, i.e. hinged top and bottom. For a wood stud wall that is not far from the truth. For a concrete or masonry wall, perhaps not. If the wall is capable of resisting only a vertical force, there can be no horizontal reaction at the low end of the rafter. The connection between rafter and wall must be designed to accommodate the reaction.

The connection at the ridge beam may be a joist hanger with sloping seat, but if so, the nails must be sufficient to safely sustain the axial tension at that end.

BA

RE: Structural Ridge Rafter Thrust

are you asking what type of supports we should use in this home work problem?

"supported at the top by a pinned connection and a semi-pinned OR roller in the horizontal only?"

RE: Structural Ridge Rafter Thrust

focuseng,

Total snow load 200#.
Vertical reaction each end 100#.
Horizontal reaction each end 0.
Axial force -71# at wall, 71# at ridge.
Shear force 71# each end.

BA

RE: Structural Ridge Rafter Thrust

I think the walls of most conventional stick frame homes are capable of resisting some thrust loads.
The top plates are typically doubled and the interior partitions are usually tied into the exterior walls and even the drywall partitions have some shear capacity. I think you essentially have the top double plate spanning between or continuously across the partitions.

RE: Structural Ridge Rafter Thrust

Toad,
I interpreted this to read:

"supported at the top by a pinned connection and at the bottom by a horizontal roller"

BA

RE: Structural Ridge Rafter Thrust

Quote (ToadJones)

I think the walls of most conventional stick frame homes are capable of resisting some thrust loads.
The top plates are typically doubled and the interior partitions are usually tied into the exterior walls and even the drywall partitions have some shear capacity. I think you essentially have the top double plate spanning between or continuously across the partitions.

Who said it was a stick frame home? It could be an open storage shed.

In any case, horizontal reactions would not likely be considered in the design of a stick frame home.

BA

RE: Structural Ridge Rafter Thrust

BA:
Thank you for your answer. I assume your member stress are local and the reactions are global.

Others:
I just want people to stop talking about it an show actual working example of what they are talking about. All the posted reference material is great but they do not actually show the resolved forces as many here are discussing. Belly up!

Toad:
I am not trying to duplicate the OP problem or exact situation. Mainly because the OP has not provided any details for us to actually work off of (HINT HINT). Make it a flippin' steel WF and put the snow load in SI units for all I care. This is Just a quick, easy, 5 min problem which should show how this static problem is done. You may scoff and say "I don't need to do that" but it is clear that people are NOT talking about the same thing here. Until this is resolved it is pointless! I am not interested in duscussing the merits of the support of the walls of a stud framed house being idealized as anything. I am only interested in the merits of the Problem as posed which is about as good as the OP gave us. Now I am waiting for the opposing camp to reply. I put in the option for your horizontal roller OR semi roller as a mild amusement to myself to see what somebody may do. I would clearly not make a good College TA.

______________
MAP

RE: Structural Ridge Rafter Thrust

cap4000 - Per your OP, the parallel axial force in the beam you are seeing in the Ram Advanse is not a horizontal thrust to the wall. It does exist but the horizontal component of it is cancled by the horizontal component of the perpedicular force to the beam. See attached pdf.

As for the "sloping beam method is “theoretically correct” " and "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." statements.
I agree with them and I am sad that other engineers do not. But anybody can have an opinion. Go read up on them (if you want) and form your own opinion.

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.

RE: Structural Ridge Rafter Thrust

I was honestly just confused as to the support conditions that were to be used.

I agree...this whole discussion is bizarre.

As I skimmed through the posts, I was thinking the whole time that the debate was over using the sloping beam method versus the horizontal projection method and whether or not the dead load and horizontal loads in the codes were being properly added together!

RE: Structural Ridge Rafter Thrust

Toad
Agreed, it is bizarre. The whole thing is Bizarre.
From the OP:
"The IBC code is clear as long as there is a structural ridge beam with posts no thrust will occur at the exterior walls." I am pretty sure this is not "clear" from the IBC and I would ask for the exact code reference where this statement is made.

Further: "Therefore no heel connection design is required." I cannot imagine a more strange deduction from the first statement. The IBC in fact has just such prescriptive requirements... not "design requirement" but a connection requirement nonetheless. I am assuming a wood structure here for the prescriptive method. I don't believe this is addressed in steel or conc sections. And if you follow the engineering method then you must have your own engineering judgement which excludes the deduction or foreknowledge of the first "Code concept".

One more: "This is easily verified with Ram Advanse." Well, lets have the model and we shall see. This is not verified the way I would model it. If you apply any perpendicular load to a sloping member then of course you will get horizontal forces. NOT THRUST FORCES BUT LATERAL FORCES. But the fact is that snow load is a gravity load in a global FBD and not a perpendicular member load. Yes, convert to local coordinate loading and apply perpendicular load but then covert back to global reaction forces after the stress check is done. As my Grammy always said, Garbage in -Garbage out.

Woodman:
I am not trying to belittle or discredit the method you are proposing. What I did want, is a concrete example to compare the two. What I believe is happening is a very thorough miscommunication. It would be nice to see this using the problem I proposed so we can all sleep tonight. I think the OP has modeled this problem wrong. I honestly think an simple example would help, not just "here, do this" statements.

______________
MAP

RE: Structural Ridge Rafter Thrust

BA (& others): You are correct - under dead and snow vertical loads there will only be a vertical reaction at the ridge beam. I agree that a horizontal reaction there will only occur when there is a horizontal component to the applied load (i.e. wind) or when the supporting wall or posts resist any displacement.

Woodman88: I should have included the deflection of the ridge beam as another factor that could give rise to horizontal reactions at the wall/post to rafter connection - where the supporting wall or posts resist the resulting displacement.

BA: the cracks occur at the top of a masonry wall supporting the sloping rafters arise there because there is little dead weight at that level. At the bottom of the wall, there is greater dead weight, the buttressing by return walls is more effective and the higher level cracking has already occurred.

I would note that in England (but not in Scotland) roof boarding may not be present to provide diaphragm action and the ridge member is often only a nominal board. So, in a duopitch roof under vertical load, the top of the rafters on one side are supported by the tops of the rafters on the other side - giving rise to significant horizontal thrusts at the bottoms of the rafters that will lead to problems unless horizontal ceiling ties are present.

The point I was trying to focus on was that horizontal forces can develop at the bottom support to sloping rafters from a number of factors (including wind) and unless the wall or posts can accommodate those forces there may be problems. Those forces can be reduced or eliminated by the use of some or all of: ridge beams, roof diaphragms or bracing and horizontal ties.



RE: Structural Ridge Rafter Thrust

focuseng - I was not proposing a method. I was just trying to explain an existing method. Unfortunately I did not realize that some of the people I was talking to had never heard of the method.

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.

RE: Structural Ridge Rafter Thrust

Quote (woodman88)

focuseng - I was just trying to explain an existing method. Unfortunately I did not realize that some of the people I was talking to had never heard of the method.

That is not a fair comment. The sloping beam method and the horizontal projection method, as you call them, are known to anyone who has studied statics even if not by name. The fact that the two "methods" produce identical results means they are both correct.

BA

RE: Structural Ridge Rafter Thrust

BAretired - Maybe you should read the "horizontal projection method" in Breyers book Design of Wood Structures and see if it meets all the same critical as your horizontal projection method. Because the “horizontal projection method" I know is a quick and easy way to size a rafter without doing all the static calculations. It is like using 22/7 for pi (which I did alot before I got a calculator that had a pi bottom) in equations that you only needed two significant numbers in the answer. You got a correct answer then but if you needed six significant numbers you got a wrong answer.

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.

RE: Structural Ridge Rafter Thrust

woodman88,
I do not have Breyer's book "Design of Wood Structures" and I do not intend to purchase it simply to settle a minor dispute with you. You have already admitted that the two methods under discussion produce precisely the same result, so it is not like using an approximation for pi. I honestly don't know what you are arguing about.

BA

RE: Structural Ridge Rafter Thrust

BAretired - I like the idea of arguing instead of trying to explain something. It would probably take less thinking. I will ponder (but not think) about doing it.

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.

RE: Structural Ridge Rafter Thrust

Okay, woodman. How's the weather in Phoenix these days?

BA

RE: Structural Ridge Rafter Thrust

(OP)
EIT

To continue with your calc's can you look at this link and switch the wind load to a snow load condition. The supports are a pin and a roller which yields an Fx. Thanks for your input.

http://lms.cee.carleton.ca/notes/3203/SDBeamsAndFr...

RE: Structural Ridge Rafter Thrust

Cap -

Maybe I'm misunderstanding but I don't think you can switch the wind and snow load condition because the wind only has force components normal to the inclined member. The snow load must act 'globally' downward (toward the center of the earth) thus it wall have components both perpendicular and parallel to the inclined member.
I believe the reason that RAM shows and Fx reaction is because there will be deflection at the ridge (unless the ridge is infinitely stiff or has an infinite amount of tension applied to it, both not likely). Thus with this deflection there will be a force at the wall. The other situation I believe that dheng or woodman is referring to is that if you plumb cut the rafter so that the end face of the rafter is vertical against the ridge beam (i.e. the rafter does not sit on top of the ridge beam) there is the possibility that the rafter wants to slide down the ridge beam (pushing against the ridge beam) and thus pushing out the wall out. I could be wrong in this last paragraph and dheng or woodman would need to further explain.

EIT
www.HowToEngineer.com

RE: Structural Ridge Rafter Thrust

(OP)
I am only looking at a "sloping beam method" indicated in Breyers book where the snow load is perpendicular. Take a look at pg 16/81 in the link below. Thanks again. Ram Advanse requires a pin and a roller condition for the beam to be stable as shown in my previous link.

http://std.kku.ac.th/4630408404/E%20book/Design%20...

RE: Structural Ridge Rafter Thrust

Yes I agree with this method and I believe it was this example I used in my attachment above to show that the Fx reaction = 0. There is a reaction along the axis of the beam (axial load) and a reaction perpendicular to the member (shear load) but when you combine these forces with the support reactions (vertical and horizontal) you will find that there is no Fx (horizontal) reaction at the support. Only vertical (due to the fact that the perpendicular load (shear) component 'cancels out' the parallel load (axial) component in the horizontal direction (x).

EIT
www.HowToEngineer.com

RE: Structural Ridge Rafter Thrust

(OP)
Understood. But there is a 30kn Fx reaction in this link from wind perpendicular to the beam. This is verified with Ram Advanse with a pin and roller support condition. Snow or wind lets say it the same magnitude.

http://lms.cee.carleton.ca/notes/3203/SDBeamsAndFr...

RE: Structural Ridge Rafter Thrust

If we are talking about applying wind to the inclined member then yes I agree there will be a horizontal reaction (sorry if this is what you were talking about the whole time). Because in the example shown in the link the wind only acts perpendicular to the inclined member and there is no axial component.

EIT
www.HowToEngineer.com

RE: Structural Ridge Rafter Thrust

(OP)
While its not indicated in the diagram the calculated axial force is 4.2 kips in tension it for wind or snow loading perpendicular to the beam.

RE: Structural Ridge Rafter Thrust

Sum of Moments = 0
Sum Vertical Forces = 0
Sum of Horizontal Forces = 0

Six of one = 1/2 dozen of another (in terms of quantity, unless dealing with a Baker in which case 6 of one will in fact equal 6.5 of other, but in either case I would suggest the sum of the vertical forces is still zero whether the Baker has eaten the 13th roll or not).

RE: Structural Ridge Rafter Thrust

Toad: Can I have a doughnut?
Cap4000: Can I suggest to throw out the Ram analysis on this? Do it by hand. In 5 minutes or less using either method you will have your answer which, if you like, submit all the calcs for verification here. There will be zero thrust from snow and you will have a lateral force component from wind. 2 separate calculations -very simple -very straight forward. That is unless your supports are different from a global x/y cardinal direction or you are doing something with them to make them settle or move etc. but even so that can be dealt with.

______________
MAP

RE: Structural Ridge Rafter Thrust

cap4000 - To use the "sloping beam method" per Figure 2.6b (your attachment page 16 of 81) you can not ignore the axial (parallel to the beam) component of the snow. The wind condition not having a axial (parallel to the beam) component gives a horizontal reaction. In the "sloping beam method" the 1242 lb parallel to the beam axial force shown has a vertical and horizontal component. The 2484 lb perpendicular to the beam force also has a vertical and horizontal component. These horizontal components, when added together, equal zero.
And because these horizontal components equal zero we can ignore them and use the horizontal plane method. Which does not break the snow load into parallel and axial components.

BAretired - It has been HOT and HUMID here in the Phoenix area. I apologize for letting it get to me to the point that I left the “working engineering methods” that is the purpose of this forum to wander down the paths (and being less than polite about it) of the theoretically and technical explanations which are not part of this forum.

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.

RE: Structural Ridge Rafter Thrust

suffice it to say that the confusion arising from using FEM software is that the user might not be aware the the thrust reactions that show up in the results are a second order in nature

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