Calculating load on a central beam in a residential deck 1

[Big_Daddy]

This is going to be very elementary for most of you structural guys. In my defense, I have a mechanical degree and I work for a structural fabricator as a project manager. I haven't done any true engineering in about 5 years - so I am a bit rusty. Fair warning, I will have follow up questions to this question.

My boss has requested that I design a free-standing deck for his house. For simplistic sakes, the deck is 40' x 25'. He wants to support this deck with only using 6 columns: 1 at each corner and 1 in the center of the 25' span on each end (the 40' width runs parallel to the house). this means that my largest beam member will be the center beam spanning 40' and carrying 12'-6" of tributary loading.

Load requirements for his area are 40/10. Because I know my boss, I am bumping the live load to 50 and will calculate the dead based on material used including secondary beam points.

For secondary beams, spaced 4' on center and spanning 12'-6", I have determined that a W8x15 will sifficiently carry this load. The end reactions of these beams are 1345 lbs (each end).

For the central beam, how do I need to calculate it:
1. Base the loading on the 50 psf live load and then add in the weight of the secondary beams every 4' feet on center?
2. Just use double the reaction load of the w8x15 every 4' on center? Double since there will be W8x15 off to each side every 4' on center.

Each way gives a wildly different answer. Like a W18x40 vs a W24x104.

 

[jhnblgr]

Your boss needs to hire a professional licensed engineer.

The engineer will draw up a layout and design based on that. It is unlikely they will design the deck free standing unless that is absolutely required.

Then they would design the joists which tie into the beams. The beams would transfer the loads to columns then to footings.

The beam likely would be continuous but uplift could occur so that needs to be checked.

The columns need to be sized and the footings also.

The deck should be checked for lateral stability.

What is the ground snow load should be checked against the live load.

Also if any heavy loads such are hot tubs should be thought about.

This work needs to be done by a person competent and knowledgeable.

 

[jerseyshore]

Your boss needs to hire a professional licensed engineer.

The engineer will draw up a layout and design based on that. It is unlikely they will design the deck free standing unless that is absolutely required.

Then they would design the joists which tie into the beams. The beams would transfer the loads to columns then to footings.

The beam likely would be continuous but uplift could occur so that needs to be checked.

The columns need to be sized and the footings also.

The deck should be checked for lateral stability.

What is the ground snow load should be checked against the live load.

Also if any heavy loads such are hot tubs should be thought about.

This work needs to be done by a person competent and knowledgeable.

 

[sdz]

You're a mechanical engineer and out of practice. Get a structural engineer involved. Remember if it fails it's your neck on the block.

 

[Eng16080]

Load requirements for his area are 40/10. Because I know my boss, I am bumping the live load to 50 and will calculate the dead based on material used including secondary beam points.

You may know your boss but do you know ASCE 7? Decks are required to support a minimum live load of 1.5 times the live load of the area served (meaning the house), which would most likely be 60 psf.

I don't often see steel beams used for exterior decks, but if you do that, make sure they're galvanized. Most likely you'll be adding more support to eliminate the steel after getting a price for that.

Your boss needs to hire a professional licensed engineer.

This is the most important thing. Doing this work for your boss in an engineering field that you're both not competent in is a lose lose. For one thing, since it's your boss, you likely won't feel comfortable challenging his design concept, whereas if I was brought in on such a project, I would immediately ask:

1. Why are there so few support columns? Can this be changed? I would further explain that while there is certainly a solution, it's likely significantly more expensive than an alternative with more columns. People will fight all day long to eliminate a single column, but this changes rather quickly when the price becomes apparent.
2. Since this is not connected to the house, lateral bracing is a concern. I would point this out and go over some options.

 

[Big_Daddy]

I 100% agree that he needs to hire a licensed engineer (PE). I have even told him that. My understanding is that he will have to get this design signed off on in order to receive a permit.
For whatever reason, he feels that me doing the ground work will lesson the chareg he has to pay to his PE. His words - not mine.
However, I did do a lot of research since I made this post. Once I started looking further into it, all of this came back to me. I actually looked at my previous pst and thought, "dumbass".

 

[Eng16080]

For whatever reason, he feels that me doing the ground work will lesson the chareg he has to pay to his PE. His words - not mine.

Most often, when I'm presented with a potential project like this, where the client already did a small amount of the design (which they often think is all of the design) or where they want to be really involved, I have two approaches (with the first being most common):

1. Turn down the project.
2. Charge more than I normally would, probably a lot more.

I don't mean to sound arrogant or anything, but often you don't know what you don't know. It would be no better if I was going to hire you to provide some kind of mechanical engineering design and I thought I might save some of the fee by doing some of the design work myself.

My understanding is that he will have to get this design signed off on in order to receive a permit.

While there are definitely some ethically compromised engineers out there who would likely stamp a design without properly reviewing it, if it was me, to get it "signed off," I would likely be redoing the whole thing. Recognizing that this can be difficult for the client to understand when they feel they already have an adequate design, I often choose to avoid the frustrations in this situation by observing option #1 above.

I don't mean to rain on the parade, but this is honest advice. I've seen your exact situation play out many many times. I wish you the best.

 

[XR250]

You may know your boss but do you know ASCE 7? Decks are required to support a minimum live load of 1.5 times the live load of the area served (meaning the house), which would most likely be 60 psf.

Depends. IRC does not say this IIRC.

 

[XR250]

One other thing to mention, hiring an experienced PE can also save money as they can be more efficient with the design.
I get plans from contractors frequently where I tell them up-front that I can easily save them more than my fee in their materials and labor without sacrificing perceived performance.
They don't mind paying my relatively high fees.

 

[Eng16080]

Depends. IRC does not say this IIRC.

That's fair, although I consider it good practice to design decks for the higher loading in ASCE7. Also, if for some reason the design didn't meet all the criteria necessary to fall under the IRC code, then I suppose IBC would control, which would require the higher loading anyway.

 

[Big_Daddy]

I appreciate all the advise and comments. Please understand that I am doing what I have been instructed to do. I have exhausted the fact that structural is not my strong point - to my boss that is. Boss for now - hopefully making some changes soon.

 

[XR250]

Each way gives a wildly different answer. Like a W18x40 vs a W24x104.

Honestly, if this is how it is working out, you should not be doing this design.

 

[Phil1934]

You will find beam needs to be sized larger than structural capacity to keep deflection acceptable

 

[Eng16080]

Please understand that I am doing what I have been instructed to do.

This is a tough thing to deal with for sure. Your boss sounds like a dumbass. I think there's the real possibility that you provide a questionable design (perhaps even disclaiming that it's a preliminary design and you don't really know what you're doing) and your boss takes it and builds it. Then it becomes a big mess. At that point, you would be obligated to report this to the local building department, IMO.

 

[Ron247]

I don't know how much actual structural design experience you have, but I know mechanical engineering teaches different courses than CE does. If you are relying heavily on your old "strength of materials" course, you are at a huge disadvantage. While it is possibly the single most important course for CEs, it is followed by several other courses that I don't think you take at all. You are at a HUGE disadvantage. If your boss thinks any engineering 4-year graduate can design a deck, he is mistaken. But if he does, let him hire a lawn care service or appliance installer to build the deck. It is about the same ratio of "What you Need to What you Got."

I was a builder before I went to college, stayed and got a Master in CE centered around structures and upon graduation, I would not have been able to "appropriately design the deck". I could take a good stab at it, but would have no real idea if I covered all the bases or not, that is what the 4 years of working under a licensed engineer gives you. Lateral stability and Foundations? I doubt your covered those in ME.

Is the walking surface wood or concrete for the deck?

 

[kissymoose]

He's gonna spend so much more money on framing to make that 40' span work than he would on putting a couple more footings in.

Big_Daddy
1. Base the loading on the 50 psf live load and then add in the weight of the secondary beams every 4' feet on center?
2. Just use double the reaction load of the w8x15 every 4' on center? Double since there will be W8x15 off to each side every 4' on center.

Your math must be wrong on the second one, those should yield the same result.
Also, that 18x40 may meet strength requirements, and code deflection requirements, but an 1-1/2" deflection at the middle could still be unfavorable.

Materials, soil conditions, lateral loading, possible snow, possible uplift, actual loading from intended use, connections connections connections. There's a lot more to this that a good engineer would bring to the table.