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.
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Structural Ridge Rafter Thrust 8
- Thread starter cap4000
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msquared48
Structural
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
Mike McCann
MMC Engineering
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.
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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.
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.
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msquared48
Structural
And there must be a tension tie to each rafter across the top of the ridge beam.
Mike McCann
MMC Engineering
Mike McCann
MMC Engineering
- Thread starter
- #8
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.
msquared48
Structural
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
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
- Thread starter
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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.
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.
msquared48
Structural
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
Mike McCann
MMC Engineering
msquared48
Structural
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
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
cap4000 said:
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
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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.
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.
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.
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woodman88
Structural
- Oct 23, 2009
- 1,020
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.
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.
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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.
If the upper ridge beam deflects downward, then I can see a second order thrust force at the top of the wall.
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