Joe_Carrick

Structural Member Sizing

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I've been working on a way to calculate and display the required sizes of Rafters, Joists and Beams depending on loads, spans and material.  The idea is to analyze the Plan Framing Members as they exist and determine what would be required to meet that layout.  Chief does not prevent a Plan from being impossible - framing members to small for the spans and loads - so this system would assist in getting a more realistic structural design.

 

There are a couple of different ways to analyze and select a size:

 

1.  Simply find the smallest size that will work with set spacing

     a.  limited to specified structure depth

     b.  increasing width if requred

     c.  increasing both width and depth

2.  Decrease the spacing and try the same (a,b,c)

     a.  spacing limited to 12", 16", 24" for rafters and joists.

3.  Change the material stress grade and use one of the above.

 

In your opinion, which of the above would be most desirable?

 

Assuming that #1 is the first step and didn't get a satisfactory result, would #2 and/or #3 need to be automatic or would a manual selection of those criteria be acceptable?

 

If none of the above result in sizes that fit within the design then it would require the structural layout to be modified by the user.

 

 

 

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The main criteria that I use is deflection.  Not just code L over whatever, but the actual deflection measured at points along the structural member under the design load.  Some programs will allow you to just move the mouse along the graph to check deflection anywhere on the member.  How a member actually performs in a meaningful way in a structure is more related to what can be seen external to the member such as how far it will sag or creep, not the forces that are internal to the member itself.  This approach generally produces a very conservative design, but there are generally no problems down the road with a professional engineers calculations.

 

Depth is the first thing that makes a meaningful difference in performance.  Second would be either the member spacing or the lumber grade / type of engineered member, kind of have to play with these two in order to decide the best approach.  Third, and that is a very distant third would be width, but sometimes you just are not given any other options than to increase the width no mater how poor a solution that may be.

 

I take it you are working on a macro to these purposes?    What I would like to see is a macro for calculating wind loads based on area.  Gave that a try a few versions back and to be honest just got completely frustrated with the whole process.  Hope everything works out well for you. 

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Rod,

 

I am using a macro and I use all 3 criteria (Shear, Bending & Deflection) to determine the sizes. I agree that Deflection is often the critical element - but there are times when Shear or Bending govern.

 

The problems I have right now is that Chief doesn't provide all the information needed so I am limited to uniformly loaded simple span members.  I've asked for more intelligent framing members with more complex loading and support conditions.  I don't know if CA will ever get to that point but it's what I would need to have.

 

For wind load calculations, we would need to be able to get the total exposed wall and roof (horizontal projection) area of each elevation.  This could be done by using the Wall & Roof Labels (each would need to be identified for the specific elevation) and a macro to collect those.  When you were working on this we didn't have "Wall Labels" so it would have been almost impossible.  I believe it is possible now - but it would still be difficult without additional wall attributes surface_area & centroid_height.  Those things could probably be calculated by using Floor Elevation Data extracted from Rooms - but it wouldn't be 100% accurate in all cases.

 

My work is not usually dependent on Wind Loads.  Seismic is almost always the governing lateral load for me and of course that requires a very comprehensive vertical load analysis.

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Yes, Bending and Shear are very important, but the programs themsleves can potentially give you a passing grade when the member may not actually perform well at all given the design conditions and loading.

 

So far, in our area wind is generally the controling factor, but Seismic is important as well.  Yes, there were no labels available at that time, and even worse the macros would not work in a cross section elevation view.  I could get some things to work by doing a CAD Detail from View and then placing that on the floor plan, but it just got to be more than I wanted to deal with and on top of that it was a very tedious process.

 

Anything I can help you with don't hesitate to ask.

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Rod,

 

It's really been a long time since I needed to do a lateral analysis for wind. Can you summarize the calculation and where forces need to be applied and/or resisted?  My recollection is the total wind load at any given elevation is just the load from everything above that level.

 

  Shear at top of walls.

  Shear at base of walls.

  Overturning/HoldDown requirements use basically a "Cantilever Moment" calculation.

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Wind usually governs here too.

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Really Perry?  You're practically sitting on top of the San Andreas.  I would think seismic would be critical. 

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Wind loads and Uplift governs here in Michigan too. Not too many earthquakes in this area. Really like this idea Joe.

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These questions are for everyone.....

 

What psf wind loads do you have to design for in your local?

What is your local?

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San Andreas is really only capable of a 7.5 or 8, at least that's what they say. Cities around here has up the wind loads within the last 5 years or so.

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Only a 7.5 or 8 - not a 9.0 ;)

 

But I do see that you are in the special Los Angeles to the Salton Sea (Flat-Lander) wind zone - and most of your projects are 1-2 stories so the weight isn't as much which accounts for lower seismic forces.

 

We don't have the same wind loads down here in San Diego County and winds up in the San Bernardino mountains are lower.

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saw a new report just last week about it.

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Big-Rigs on the 10 Freeway - really scary when it blows.  I've see a couple go over sideways.  I also remember many years ago having my windshield pitted so badly on the 215 just north of San Bernardino that I had to get it replaced at Park City the next day (January) because the snow just stuck and it was impossible to see.

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If we are going to do any kind of detailed span/sizing information etc. ourselves, we usually use this program if it's outside of the tables available in the code.

 

http://www.strucalc.com/

 

Blessings,

 

kevin

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Kevin,

 

I agree that StruCalc is a good way to go for final engineering and printed calculations.  What it doesn't do is take the loads and framing member layout directly from the Chief Plan.  If there was a direct export to StruCalc from a Chief Plan and a Import back into the Plan it would be great. 

 

What I'm trying to do is simply provide some basic preliminary sizing so the Plan doesn't have to be changed later.  If the preliminary sizing is done directly within Chief then there's less chance that the results from detailed analysis like StruCalc or from an Engineer will result in problems.

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Until Chief is willing to try and adhere / follow the International Building Code with regards to framing a little more closely.... and allow access to a complete dbx.........it is a mute point.

 

For instance...... Garage Door Headers and required nailing as an example...... Energy Code Complance with Rafters on top of Ceiling Joist ..... and the list goes on....

 

kevin

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Until Chief is willing to try and adhere / follow the International Building Code with regards to framing a little more closely.... and allow access to a complete dbx.........it is a mute point.

 

For instance...... Garage Door Headers and required nailing as an example...... Energy Code Complance with Rafters on top of Ceiling Joist ..... and the list goes on....

 

kevin

 

Not only a complete dbx but also attributes that can be accessed to provide the needed information as well as allowing attributes to be modified resulting in modifications to the Plan.

 

OTOH, we should be careful about "Code Compliance" in the software as it would be very possible to overly constrain what could be done.  A lot of the IBC is unnecessarily limiting - exceptions are allowed with engineering documentation and backup.  There are new products and systems appearing all the time that surpass what the IBC requires.

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Joe

 

The folks at StruCalc use Chief themselves, or at least they used to the last time I spoke with them.  You might try giving them a call to see if there is a posibility of having some type of interoperability between Chief and StruCalc.  Too bad we already know the answer on Chief's end regarding plug-ins.  Hopefully that will change.

 

Woodworks has a plug-in  that used to provide a limited form of interoperability between it and Revit.  I have not upgraded to that program ever since they went down that road.  The problem was that the interoperablity did not extend to the entire program, just one module, and it only worked for studs in Revit at the time.  BTW the price of the program went way way up, imagine that from the leader of free and open interoperabilty.

 

If I am following your statements regarding wind loads that is basically it.  Why it is getting to be more important to me is now that the construction industry is getting a bit busier it is even more important to have the design as close as possible in order to streamline the design process.  During the bubble we saw turnaround time with engineers go from a couple of weeks to a month or more.  Hope we do not see that again, but I just want to make sure everything is as close to a go as I can before sending it out.

 

Another thing that comes into play at times is the floor diaphram and or the retaining wall and the anchorage to it.

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My area in the thumb of Michigan 90 mph wind load. Ground snow load 35 lbs. 

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I'm in Massachusetts where I am in the state wind is 110 and snow is 25-30 lbs. The whole state from one side to the other goes from 25-65 lbs.

I grew up in San Bernardino right near where the 210 and 215 meet. I've seen some crazy wind there as well Joe. Trucks do blow over coming down the Cajon pass more often than one would think.

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@ Jay on Cape: You have hurricane areas too, which adds to horizontal loads. I have done 14 Extra Mart gas stations on east coast and 1/2 in your state. Last project Building official had issue with truss manufacturer and loads due to it being hurricane area. All worked out but it was a "NAIL BITER".

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@ Jay on Cape: You have hurricane areas too, which adds to horizontal loads. I have done 14 Extra Mart gas stations on east coast and 1/2 in your state. Last project Building official had issue with truss manufacturer and loads due to it being hurricane area. All worked out but it was a "NAIL BITER".

What town? If it's the coastal area I could probably guess which one.....

It's not too bad in a 110 wind zone. Most of the cookie cutter houses I draw for a production builder I can get away with using the 110 checklist so it doesn't have to go to an engineer. Anything that has to go to an engineer here could take 4-12 weeks and if you don't know the engineer you could get the one that say's " I know they've been building Cape Cod homes for 400 years but I can't get the dormer to work without a structural ridge"........

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@ Jay on Cape: You have hurricane areas too, which adds to horizontal loads. I have done 14 Extra Mart gas stations on east coast and 1/2 in your state. Last project Building official had issue with truss manufacturer and loads due to it being hurricane area. All worked out but it was a "NAIL BITER".

Next project in MA try to use National Lumber. They are a large lumber supplier / truss manufacture / turnkey framing & project management company. I was an EWP designer with them for a couple years and they are who I deal with now for all of our framing, roof trusses and pre-cut EWP floor systems.

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We have a set wind load load design based on the municipality and it ranges from 90-120 mph, roof snow load is 20 psf.

 

As joist design goes, I always use 2x10 lumber so I set that in my plan default for every design and second floor ceiling are always 2x8 with strong backs.

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@ Jay: These projects were panelized construction method and TY for the lead. You have some very nice homes on your website.

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