I have a 24' load bearing wall. The only thing bearing on the wall are the truss/roof assembly. Trusses are 24" o.c. and span 20'. The snow load is 55psf. I would like to remove 10' of the wall in order to add an addition. Can someone help me with the necessary steps to determine the beam size if I used a metal beam or a wood beam? Most of my work is in the civil/site area so I am a little weak in the structural department. Thanks.
Sizing a beam...help with calcs.
- Thread starter Jambruins
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Surpized nobody has responded by now. I was going to last friday but I had a fire drill come up.
I drew a picture to help me explain.
The highlighted area is the portion of the roof which has its loads transferred directly to the wall portion you are removing. This is assuming you have a symmetrical roof. Either flat or you have the peak along the center.
To calculate the snow load you have to multiply the shaded area by the snow load. So that would be 10*10*55 = 5500 lbs. This load is evenly distributed along the beam supporting the opening. Therefore it would need to be converted to a distributed load 5500#/10'= 550 lb-ft.
Next you need to calculate the moment associated with the load. Since we are dealing with a distributed load the equation for the max moment is (wl^2)/8. w is the distributed load and l is the span. (550*(10^2))/8 = 6875 Ft-lb.
The last number is what you will use to size the beam. I don't work with wood structures so I will size it for steel. Since its steel I am not going to bother with a shear calculation. If it were wood I would do the shear check.
Next, with the moment and span length I can use table 3-6 in the AISC manual (13th ed) to select a beam. I would select a wide flange because they are easier to work with. I'm looking at page 3-77. On the left hand side of the chart is the design span. The top of the columns show the beam selection. The chart shows available moment capacity for each beam and the moment capacity depending on the span. The smallest beam avalable is a W8x10. This should be sufficient. For a span of 10' this beam has a capacity of 17.5 kip-ft (ASD). Which is more than is required. The beam is considered non-compact but this should not be a problem since the loads are not big. This means that the mode of failure will more likely be some type of localized buckling if you were to max out the beam.
There are other beams available that are shallower. The w8x10 is 7 7/8" deep. You could use a W6 shape but the beam is not shown in the beam selection charts. This would mean that you would have to calculate the beam's moment capacity by hand.
Couple things to note. You can reduce the snow load depending on the pitch of the roof. That snow load is huge. The beam also needs to carry the dead load of the roof and any live loads on top of the snow load. I don't think it would be a problem since the beams capacity is more than twice what you need.
I drew a picture to help me explain.
The highlighted area is the portion of the roof which has its loads transferred directly to the wall portion you are removing. This is assuming you have a symmetrical roof. Either flat or you have the peak along the center.
To calculate the snow load you have to multiply the shaded area by the snow load. So that would be 10*10*55 = 5500 lbs. This load is evenly distributed along the beam supporting the opening. Therefore it would need to be converted to a distributed load 5500#/10'= 550 lb-ft.
Next you need to calculate the moment associated with the load. Since we are dealing with a distributed load the equation for the max moment is (wl^2)/8. w is the distributed load and l is the span. (550*(10^2))/8 = 6875 Ft-lb.
The last number is what you will use to size the beam. I don't work with wood structures so I will size it for steel. Since its steel I am not going to bother with a shear calculation. If it were wood I would do the shear check.
Next, with the moment and span length I can use table 3-6 in the AISC manual (13th ed) to select a beam. I would select a wide flange because they are easier to work with. I'm looking at page 3-77. On the left hand side of the chart is the design span. The top of the columns show the beam selection. The chart shows available moment capacity for each beam and the moment capacity depending on the span. The smallest beam avalable is a W8x10. This should be sufficient. For a span of 10' this beam has a capacity of 17.5 kip-ft (ASD). Which is more than is required. The beam is considered non-compact but this should not be a problem since the loads are not big. This means that the mode of failure will more likely be some type of localized buckling if you were to max out the beam.
There are other beams available that are shallower. The w8x10 is 7 7/8" deep. You could use a W6 shape but the beam is not shown in the beam selection charts. This would mean that you would have to calculate the beam's moment capacity by hand.
Couple things to note. You can reduce the snow load depending on the pitch of the roof. That snow load is huge. The beam also needs to carry the dead load of the roof and any live loads on top of the snow load. I don't think it would be a problem since the beams capacity is more than twice what you need.
I am hesitant to reply to this post for many reasons. These are just my opinions and maybe I am being overly cautious. I do not deal with wood structures that often so maybe my concerns are off base.
Firstly, although there is a fine group of people here at EB, I feel a person competent in this area should be directly responsible for this modification in person, not through the internet on a forum. What you are explaining may not be exactly the situation.
I feel we have to make too many assumptions. Roof dead loads, attic and roof live loads, effects to the diaphragm, etc. Even dimensional constraints are not given for beam selection.
What about lateral loads? Is seismic a concern? Wind loading? What about uneven snow loads due to items on the roof we are unaware of.
Will the new structure load these beam at all?
All that said, I am not sure I would consider this beam fully laterally supported with all of these unknowns, and I would check deflections.
Good luck!
Firstly, although there is a fine group of people here at EB, I feel a person competent in this area should be directly responsible for this modification in person, not through the internet on a forum. What you are explaining may not be exactly the situation.
I feel we have to make too many assumptions. Roof dead loads, attic and roof live loads, effects to the diaphragm, etc. Even dimensional constraints are not given for beam selection.
What about lateral loads? Is seismic a concern? Wind loading? What about uneven snow loads due to items on the roof we are unaware of.
Will the new structure load these beam at all?
All that said, I am not sure I would consider this beam fully laterally supported with all of these unknowns, and I would check deflections.
Good luck!
You should really only be practicing in your area of expertice. This sounds like a fairly simple and straight forward problem. The fact that you require assistance on how to size a steel beam (a very basic structural task), means you should probably hire an qualified consultant.
I'm not trying to berate you here, but structural engineering is highly regulated due to the life-safety issues. What requirements does your state have regarding stamping structural drawings? Some states require an SE, others just a PE. Does your insurance cover you for this type of design?
There are a lot of other factors to consider. A load bearing wall is likely also part of the lateral system. I wouldn't go cutting into it unless you were sure the remainder of the wall was okay to transfer the lateral load. Are you in a seismic zone or is it just wind load? What is the wall construction? It would be inappropriate for any engineer to provide a design based on the limited amount of information you've given.
I'm not trying to berate you here, but structural engineering is highly regulated due to the life-safety issues. What requirements does your state have regarding stamping structural drawings? Some states require an SE, others just a PE. Does your insurance cover you for this type of design?
There are a lot of other factors to consider. A load bearing wall is likely also part of the lateral system. I wouldn't go cutting into it unless you were sure the remainder of the wall was okay to transfer the lateral load. Are you in a seismic zone or is it just wind load? What is the wall construction? It would be inappropriate for any engineer to provide a design based on the limited amount of information you've given.
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