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
- Start date
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.
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.
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
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
woodman88
Structural
- Oct 23, 2009
- 1,020
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.
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.
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!
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!
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
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
jimodore
Structural
- Jul 26, 2012
- 6
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.
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.
woodman88
Structural
- Oct 23, 2009
- 1,020
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.
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 said:
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
woodman88
Structural
- Oct 23, 2009
- 1,020
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.
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,
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
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
woodman88
Structural
- Oct 23, 2009
- 1,020
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.
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.
- Thread starter
- #77
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.
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.
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
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
- Thread starter
- #79
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.
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
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