Building a deck: can you shift the load points? 3

[MiniMe4Eng]

Hi guys

I am trying to understand if this solution is acceptable
Here is the story: I was trying to help a friend to rebuild a deck and he wanted to reuse the existing piers (concrete footings). Another fried who is an architect came up with a new design but that requires us to shift the points where the load is transferred from the joists to the beams posts and deck supports

Below you have the proposed design that I am trying to validate with your help

The reasons for the below design are:
-he wants to place the stairs adjacent to the wall
-he wants to use the deck posts as rail posts as well
-this is replacing an old deck and I do not want to dig new holes
At the bottom I have 4 beams 2x8 (a pair on each side sitting on the deck supports)
Just below the deck I have two 2x8 beams parallel to the house, one of them sitting under the joists, the other sitting at the same level with the joists

Some measurements: the left side left post distance to the deck support is 13", the right side post (closer to the house) distance to the deck support that is next to the house is 21"
The deck is 44" high, 10' wide along the house, 8' deep (perpendicular to the house). The piers are dug down to below the frost line (48") and they are 10" in diameter.

I need to understand if this is an acceptable design I just need to know if there could be other potential issues

 

[msquared48]

Lateral stability for one in the 10 foot direction at least.

Mike McCann, PE, SE (WA)

 

[MiniMe4Eng]

I think you are right
Though how is that different from having the posts installed in the traditional way ?

 

[EngineeringEric]

You have a few issues, outside of engineering number crunching. Are you engineering this? designing it? or building it?

Your beamto post connection is resulting in cross grain tension, you need to notch it in, eliminates the need ability to act as a rail post (may). you need to brace it. you may want to tie it to the house, at which point don't forget the tension ties.

 

[MiniMe4Eng]

The posts are 4x4 was notched on both sides and slided in between the two 2x8 that make the bottom beam
Those 2x8s are sadwiched and fit in the deck support which can take a 4x4 in

So the notched end of the post is 1/2 inch wide
This will be bolted and braces will be installed between beams and posts

 

[MiniMe4Eng]

We braced and notched the posts and we plan to use carriage bolts as well

Quick question: I am being told that it is not OK to use 2x8s in this design because they are not intended for point load.
Is that correct ?

 

[Tmoose]

I don't see the advantage of placing those "beams" at ground level. I'd move them up into the deck to help deal with the cantilever directly, with posts in the original location.

http://www.awc.org/pdf/codes-standards/publications/dca/AWC-DCA62012-DeckGuide-1405.pdf

 

[MiniMe4Eng]

I needed to be able to close the storage area underneath.
If I install them at the top of the beams it won't be the same thing

 

[EngineeringEric]

The last thing you said makes me think you are in over your head. "2x8 are not intended for a point load...." Are they engineered?

Now i will not judge you for trying a unique deck and learn something. But the deck as shown in sketch up is unstable. a 1/2" notch is not sufficient for any load capacity. Carriage bolts are typically not allowed for deck construction.

Read AWC DCA-6

 

[MiniMe4Eng]

The quote belongs to a contractor and it sounded strange to me. As you can see my degree was in Electronics and Telecommunications so I needed a second opinion about this (nonsense or not )
The beams are not engineered, they are of the shelf beams (Lowes)
The colored elements have been added to the structure
Deck connectors were used everywhere where possible.

 

[SwinnyGG]

This deck is already built?

You need to take a hard look at the process for designing structures which can injure or kill people- there's a reason that codes exist and structural engineers get paid a lot of money.

The desire to use a space for storage doesn't override the necessity to engineer a safe structure.

The desire for a structure to look good doesn't override the necessity for that structure to be safe.

If this deck is already built, I hope for your sake that it's on your own property. By having anything to do with this deck design, you have exposed yourself to a significant level of liability should things go sideways.

 

[MiniMe4Eng]

Years ago I built a fence using post spikes.
95% of the contractors who saw that design said it is not going to last a winter or a single storm
It is still there doing just fine!

I would like to hear more educated opinions, more than "I am telling you that this design is wrong"

 

[SwinnyGG]

There are several valid reasons above why this design is not a great approach.. You seem to be ignoring them.

The tone of your posts seem to indicate that you came here looking for permission to use the design you had already 'validated'... That would only have happened if the design was engineered well. It isn't, which is why you're getting so much negative feedback.

The issues already directly called out by other posters beyond 'I am telling you this design is wrong":

-wood members subject to cross-grain tension
-use of carriage bolts which are not code compliant in many areas and bad mechanically
-bearing interfaces between components not sized for their loads
-railing connections not designed for their service loads
-insufficient bracing to provide lateral stability of the structure

We haven't even dug into other very common issues with existing decks that often make them unsafe- such as the specifics of the attachment of the ledger board to the house.

No one on this board is going to stop you from building whatever you want- but don't expect the people here that design structures for a living to sign off on your arrangement because you built a fence once and it didn't fall over.

The odds of a failure of your fence causing serious injury or death to a third party, and a subsequent lawsuit costing you dearly, are much, much, much lower than with this unconventional deck design. 'I built something once and it worked fine' is a reasonable approach to building a fence- it is not a reasonable approach to building a structure which is expensive and has potentially dangerous consequences of failure.

If you value your friendship with whoever you're doing this work for, not to mention your own long term financial well being, build this deck precisely to code and have it inspected and certified as such.

You are exposing yourself to significant financial peril should something go wrong with this structure, even if that 'something' is out of your control.

Should any part of this structure fail and cause an injury of any kind, you are completely exposed to the consequences as you are, in effect, practicing engineering without a license.

 

[MiniMe4Eng]

First of all I am not seeking approval here, but a second opinion
I can work with concrete numbers and examples, I can't work with "trust me, I am an engineer" because if you say that you are not. Engineers do not trust anything else but numbers

To your points

-wood members subject to cross-grain tension -which components exactly are you referring to ?
-use of carriage bolts which are not code compliant in many areas and bad mechanically -no bolts anymore
-bearing interfaces between components not sized for their loads -which components exactly are you referring to ?
-railing connections not designed for their service loads
-insufficient bracing to provide lateral stability of the structure -this was addressed in the second picture

 

[EngineeringEric]

None of us said trust me, we said we have serious concerns that you should evaluate with real numbers. you didn't hire me so i am not going to run them for you. Next time you do something you don't understand ask nicely, don't claim we are not engineers because we will not validate a poor design.


-wood members subject to cross-grain tension -which components exactly are you referring to ?
Every beam that is bolted to the face of a post. You have to know what cross grain tension is to respect it... trust me respect it

-use of carriage bolts which are not code compliant in many areas and bad mechanically -no bolts anymore
No you need bolts, not carriage bolts

-bearing interfaces between components not sized for their loads -which components exactly are you referring to ?
the base of the 4x4 doesn't work. The beam to 4x4 doesn't work. i believe your bottom beams don't work due to lack of weak axis bracing.... so maybe your decking work, thats about all i see that works

-railing connections not designed for their service loads
i can tell you from looking at it those posts cannot handle 200#@42". try to transfer that moment through a 3.5"x3.5" and then into a force couple of 3" bolts

-insufficient bracing to provide lateral stability of the structure -this was addressed in the second picture
There was an attempt. This is not enough. what about the base of the post brace? how are the braces connected? what lateral force are you designing it for? how does this lateral force get into the foundation?


This is all what we do on a daily basis without even thinking. I bet you have a skill set where you don't even think about rewiring a two-way switch, same with me when it comes to loads.