Crazy cantilevered steel deck

[DoubleStud]

I found this video on youtube. What do you think of this deck? The guy went to the end of the deck and jumped a little bit, you can clearly see the deck moves up and down. At minute 3:55 he shows the structural detail.

 

[phamENG]

also...how shallow is that cantilevered roof? is this snow country?

 

[Ron247]

The video is from the "Build Show Network" which has a website. While the deck is in the Big MO, it was presented nationally. While I have several reservations, I would have to do a more intense review before I would feel comfortable implying someone else did wrong. or was careless. As I stated before, at this stage, I question serviceability and longevity as opposed to stress.

There have been many situations in my professional engineering life where I wished I could "Unsee Something". This is one.

 

[Ron247]

also...how shallow is that cantilevered roof? is this snow country?

I noticed the roof too and felt it looked spindly and yes they have snow in MO. I did not mention it because the deck was being critiqued so hard, I hated to pick on its little brother too. the roof sticks out about 2' more than the deck.

 

[XR250]

Maybe a call to the local AHJ might be in order? Let them sort it out at least to see if a PE was on the job.

 

[Ron247]

Again, I need to stop looking at the video. Just noticed the 150 lb presenter may be an architect. They are allowed to do "engineering incidental to architecture". There is his website. So at least, there was at least one professional involved.

 

[Greenalleycat]

Yes he's an architect, he is very prominent
I have no idea how a pure cantilever deck made from structural steel could be considered 'engineering indidental to architecture'

 

[XR250]

OK, who is going to ping him?
Maybe this would work on the moon - a place where he actually may weigh 150lbs!

 

[Greenalleycat]

Maybe one of us internationals should do it - can't get an out of jurisdiction complaint when we're not in the country anyway!

 

[XR250]

He has a contact form on his website. Might be worth just pasting a link to this discussion in there and sending it.

 

[Ron247]

Yes he's an architect, he is very prominent
I have no idea how a pure cantilever deck made from structural steel could be considered 'engineering indidental to architecture'

In the state I live in, engineers can "do architecture incidental to engineering" AND architects can do "engineering incidental to architecture". Neither one has a definition or examples of what the mean by the sayings.

I am not sure about other states, but the difference is, who determines it was done wrong and who disciplines who. In my state, the architects complain to the engineering board for us doing architecture wrong. The Eng board punishes us. Even for small design infractions, you can lose your Eng license for 6 months. The Arch board is VERY lenient to Archs for engineering infractions of any magnitude.

 

[azcats]

So I got bored...

Caveats:
Ignored tapers
60 psf LL
20 psf DL (plus self weight)

No members are 'overstressed' per-se. 8x8 are in the high 80's. 12x4 is in the low 80's. I don't trust my risa skills enough with cantilevered members to really report on the double angle.

Deflection over 2" at the center of the deck at the cantilever.

Not much rotation of the 12x4 - 0.12° at the center.

A static 300# load at the center of the cantilever yields a vertical deflection of 0.15"

 

[Ron247]

A static 300# load at the center of the cantilever yields a vertical deflection of 0.15"

Are you using a 300# load because the presenter has an evil twin?

The 20 psf DL may be high in addition to structure weight. I think it is only one of those 3/4" plastic deck board installs based on his drawings.

What were the reactions at the base plate?
I am curious if the original design used a bunch of 2D calcs, rather than a 3D model. Also curious what fixities were used at the double angles.

Am I correct the angles may be thinner than noted on the drawings? That would explain the bounce more. To me, they looked thinner than 3/8" in the video but it may be an optional delusion.

 

[human909]

A static 300# load at the center of the cantilever yields a vertical deflection of 0.15"

Which is a decent amount beyond my local codes recommendation of maximum allowable deflections. Which was already evident when the guy in the video jumped.

I learnt my lesson fairly early on having designed a simply supported steel walkway. (I had actually designed it with an intermediate support but the guys on site didn't provide it, there was plenty of capacity so I didn't kick up a fuss at the time. However it was bouncy as hell. And IMO and other not suitable for a brand new industrial site.**) For a residential build the acceptable deflections and vibrations should be far tighter than an industrial walkway.

**I could design everything super light for industrial walkways. But the cost savings are barely worth it compared to user experience. If it feels bouncy and flimsy then the client doesn't get the impression of a high quality job, even if everything is well withing strength requirements.

 

[Tomfh]

It’ll deflect more than that anyway as the RHS wall and connections will flex

 

[azcats]

I left everything fixed.

The big reaction is about 25 kip-ft (ASD). Thrust force of about 6 kips.

Some other weird internal moments with the fixity. I just tried releasing the torsion at the baseplate and it didn't impact much.

Regarding the angle, I don't know that they make a 7x4 that's less than 3/8" thick.

 

[azcats]

It’ll deflect more than that anyway as the RHS wall and connections will flex

100% on this. There's a lot of other flex in this system. Wonder if it's significant enough for fatigue to start cracking welds.

 

[Enhineyero]

The weight of the L178x102x9.5 (SW=0.198kN/m) spaced at 610mm is 0.65kPa alone, add to it the weight of wood deck, and railing system distributed over the whole deck(just to simplify) you'll get significantly more then DL=0.5kPa and I am pretty sure balconies are LL=4.8kPa
So no, it doesn't check out. but like others have said there was no engineer involved.

DL = 0.198 / 0.61 = 0.32kPa, deck is probably around 0.2kPa. Its probably a touch higher than 0.5kPa but not far off.

Where I practice LL of 2kPa is sufficient for a balcony in a self contained dwelling such as this. Balcony LL of 4-5kPa is for commercial type building or a multi-occupancy building, thats like 3-4 large sized person per square meter - possible in bar/disco but unlikely in a residential setting (unless you are really intimate or arguing over something, like the load capacity of the deck you are standing on).

Personally I'm not too worried about the steel structure, the probability built-in the material and design load factors are probably enough to take care of any calculated over-stress.

Rotation of the footing is more concerning as it may actually lead to a collapse.

Hmmm. So as licensed engineers who are aware of a design that is being constructed that doesn't meet code and poses a risk to public safety and welfare - is there an obligation to speak up?

Some years ago I attended an all day structural seminar in Austin, TX, and one of the professor speakers noted that a three story steel framed office building was being constructed next to their test facility and they watched it frame up each morning as they drove in to work. They noticed a flaw in the design from a distance (lack of vertical stiffeners in the beams running over columns) and finally decided they needed to act and contacted the engineer of record to inquire. Stiffeners were then added.

At the minimum at least send a message to the architect saying 'Hey Mr. big fancy architect, some group of people on-line claiming to be structural engineers are bit concerned about the deck you built. It may be in your best interest to have it checked by a local engineer as the concerns by these people online are along the lines of life and safety of the people using the house/deck"

 

[Ron247]

I left everything fixed.

The big reaction is about 25 kip-ft (ASD). Thrust force of about 6 kips.

Regarding the angle, I don't know that they make a 7x4 that's less than 3/8" thick.

Good point on the angle size, I never looked at my manual just noted the thickness did not appear to be 3/8". Answers that question.

Is the 25 ft-k at one post?

What did you use for your outside edge joist where he has a 3/8" tapered plate?