Carport analysis

[CEmonkee]

Hi - I have a question about a carport I'm analyzing. Please see the attached .jpg file.

The carport profile and ridge line will match the existing house. On one end the glulam beams will frame into the wall of the existing house. On the opposite end two 6x8 posts will be used (indicated with the red squares in the plan view). This side of the carport will be sheathed as shown. The front and back of the carport will be open.

For the wind loads parallel to the ridge (wind normal to the sheathed wall), I used the wind pressures in ASCE Figure 6-2. When analyzing the posts, I was going to assume that the posts would take half of the total shear load to each side. Is this a reasonable assumption?

Thanks for your help!

 

[Ron]

For the parallel condition...Yes. For perpendicular, No. The leading face will typically accept more..usually in a 2/3 to 1/3 split, depending on how you analyze.

 

[CEmonkee]

Hi Ron - thanks for your reply, I appreciate it. So if the total load to one side was 1000#, I would consider the post on that side to get 500# of load, and the rest would distribute to the wall at the other end of the GLB?

For the perpendicular direction (in which the front and back are open), I was going to analyze it as an open structure and use ASCE Figure 6-18B. Does that sound correct?

 

[msquared48]

If there is no more wind exposure to the existing residence by adding the carport, then nothing has changed in the wind direction shown and no additional lateral capacity is needed in that direction due to the addition of the carport.

However, in the wind direction transverse to that shown on the sketch, the exposure is increased, so the two posts would have to take 1/2 of the wind load to the carport and the house the rest. Each post should be designed as a pole footing and see 1/4 of the wind load.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[CEmonkee]

Hi Mike - there is no additional wind exposure, so as far as the house is concerned, nothing has changed in that direction. But the posts on the sheathed end would have to be designed to withstand the wind load in that direction (I was going to consider each post taking half of the wind load to each side).

For the transverse direction (perpendicular to the ridge), the two posts together would take half of the additional wind load (the other half would go to the house)... So you would consider the load splitting evenly between the posts in that direction?

 

[msquared48]

Oh, you are correct if the endwall is sheathed, but half of the lateral the lateral force is still transmitted directly to the house. The posts just transfer the wind force to the carport roof diaphragm (which goes to the house) and the foundation.

Yes, so long as they are linked across by a beam or truss and it is adequately attached to the top of the posts.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[CEmonkee]

Hi Mike - I think the posts are tied together pretty well... parallel to the ridge they are connected to the end wall of the house with GLB's; in the other direction they are tied together with the trusses.

Thanks for your input - it's very much appreciated!!

 

[MaddEngineer]

What about quatering winds... I think you will realize an increase of 42% on the existing house (sin 45 = 0.71, 0.71x2 Faces = 1.42) due to the exposure on the open sides.

 

[CEmonkee]

Hi MaddEngineer - thanks for your reply.

I was going to use the ASCE All Heights method and assume a partially enclosed structure. Can you elaborate a bit more on the quartering loads? I don't see anything in the ASCE that addresses that, unless I'm missing something.

ASCE paragraph 6.5.12.3 has a statement that one-story buildings less than or equal to 30 ft in height framed with light-frame construction need only be designed for Load Case 1 and Load Case 3 in Fig. 6-9.

Thanks.

 

[CEmonkee]

Another consideration...

When analyzing the posts, I was (very) conservatively assuming a cantilever with a point load at the end to get my bending moment. However the top of the post is actually "supported" by the beam that frames into the house, so I don't think it's a true cantilever situation and the moment would be much lower than what I'm coming up with.

But I'm not sure how to reduce the moment to reflect the actual condition... one thing I was thinking of was assuming a deflection at the end of the post, then back calculating the load that would produce it. Does that sound like a good way to go about it, or does anyone have other ideas?

Thanks!

 

[MaddEngineer]

You are only reviewing wind loads normal to the face of the structure (which is shielding the house), which is usually fine with an enclosed structure. However, becasue you have openings on two sides if the prevailing wind is comming at 45 degrees you will get a vectorial component on the two exposed sides. The summation will actually be an increase at most 42%, but could be less depending on how stiff the posts are.

 

[Cap07]

CEmonkee,

Regarding your last question, I agree with you - if the beam at the top of the post is framing into the front wall of the house, I believe you would get a little more support there than if you had a free cantilevered end.

I'm not really sure how you should take that additional support into account though. I'd be interested in seeing what others think.

 

[CEmonkee]

Hi Cap07 - the more I think about it, the wind load parallel to the ridge isn't really going to be the critical load as far as the posts are concerned. I was planning on looking at a dead load + wind load combination and analyze the post as a beam-column, but since the beam on top of the post frames into the house I don't think the wind will stress the post out too much in bending - the load path for that will be through the beam and into the existing house.

However, the other direction (perpendicular to the ridge) is another matter. For that direction, I'm going to assume that the posts will take out all of the wind loading. The contractor is planning on sheathing the wall between the posts, and there is come concrete there that he'll be bolting the sill plate to, but I'm not sure how much concrete thickness I have, so I'm going to neglect the shearwall and see if the posts are good for the shear load on that side.

 

[BAretired]

The wall between the posts must resist direct wind on the sheathing, so the bottom plate requires lateral support from the foundation. If the foundation is not adequate, it should be replaced.

For wind normal to the ridge line, the sheathed wall will be much stiffer than the posts and should be designed and constructed as a shear wall.

BA

 

[MaddEngineer]

CEmonkee,

You have to make sure that your building complies with the definition of an enclosed or partially enclosed building. It may actually be an open building I believe the criteria is less than 20% of the wall can be open. So if you have a 16' long opening the balance of the wall would have to be 64' long to comply with the definition of a partially enclosed building.

Also section 6.5.6.4.2 Low Rise Buildings, States that MWFRS should be designed for maximum winds in any direction, not just orthogonal, although you can utilize orthogonal projection. Case 2 figure 6-9 seems to be a black box approach to quatering loads taking 75% simultaneously.

 

[CEmonkee]

Hi MaddEngineer - The definition for an open building says "a building having each wall at least 80% open." The carport is completely enclosed on one end, the other end is next to an existing house... so I don't believe it meets the criteria since only two walls are 80% open.

BAretired - good point, I specified the shearwall construction on the drawing.

 

[MaddEngineer]

CEmonkee,

But does the building fit the acid test for a partially enclosed building, which is the main section 6.5.12, that subsection 6.5.12.3 is part of... it appears that there is a buidling type that would be considered partially open (i.e., a building that is not fully enclosed, partially enclosed or fully open)

Any other opinions out there. It is understood that these building types are principally for internal pressure analysis. However, I think it may be very liberal not to consider the effects due to the large openings which does create additional wind exposure.