Deck lateral load 4

[DoubleStud]

So, what do you typically do now when you design a deck? How much lateral load do you use? Do you calculate the tension chord force at the end of the deck and use a tension hold down device like IRC Figure R507.2? According to NDS, the allowable shear capacity of a deck with horizontal decking member is only 70 plf. I have a deck with really tall columns (20 ft). The columns and beam will not give that much restraint laterally. I read an article somewhere about a test on the deck where people move left and right simultaneously and it generated about 12 psf of deck area laterally in the middle. That seems crazy high. Luckily the deck has a pretty good width to depth ratio (24 X 11). I just want to make sure I do this correctly since the columns are so tall. Any advice?

 

[phamENG]

I typically go for 1/4 of the live load, so for a deck that's 10psf. I think that report suggested values as low as 4psf might be acceptable for most residential decks, but I go for the upper end. I've been on too many wobbly decks.

As for design, that's my approach. I look at it as either a lumber diaphragm OR I design discreet horizontal braces (most composite decking and anything using hidden fasteners is NOT a diaphragm), and then tie the chords back into the house diaphragm. That, or I make it free standing.

At 20ft, make sure you check lateral drift and secondary effects in those columns.

 

[DoubleStud]

phamENG, if hidden fasterners is not a diaphragm, what value do allowable shear capacity do you use? 0? How do you install this horizontal braces? Flat below the joists? What do you use as a member typically?

 

[Tomfh]

Metal cross bracing between the columns, or cross bracing below the joists, connecting the columns to the house diaphragm, or both.

 

[jerseyshore]

I've never seen a deck explicitly designed for lateral LL forces before, but that's because they are rarely 20 ft tall. At that height checking the columns is much more important.

I always specify extra blocking on decks to create good diaphragm action. 99% of the decks I see are composite decking/ fiberglass so you can never rely on that as pham said.

Knee braces are rare in my area because no one wants to block their water view so I always specify the heaviest Simpson post caps to create rigid connections at the top of the posts.

 

[DoubleStud]

I am going to use steel beam and 20 ft tall 5x5x1/4 HSS columns with simple post cap and 4 bolts. It should give it some fixity but not much. I am going to ask the architect if we can use diagonal 2x wood decking to give it high shear capacity and specify it on plan. I will tie the ends to the floor diaphragm. Let me know what you guys think.

 

[XR250]

I am going to ask the architect if we can use diagonal 2x wood decking to give it high shear capacity and specify it on plan

I would not count on that as it will likely be changed out to composite with hidden fasteners in the future regardless of the plans.
I usually use 20% of the live load for lateral when I actually check it. If they don't want to see bracing, I generally do under deck planar 2x6 diagonal bracing that ties to beefy 6x6 blocks that is through bolted to both bands .
Best bracing would be X-rods between posts though.

 

[EngDM]

Since you're using steel columns the deck will likely last a long time without needing any replacement for reframing (of the columns at least). Have you considered that down the road the owners may replace their rotting 2x wood decking members with composite? When people replace decking I could see them just doing it instead of contacting an engineer for review.

Edit: XR250 posted as I was typing out my reply. I agree with XR250.

 

[zackngineer]

I typically go for 1/4 of the live load, so for a deck that's 10psf.

This would indicate that you're using a live load of 40 PSF for decks. The live load for decks is 1.5 x the area served. Assuming residential, your live load for the home would be 40 and your live load for the deck would be 60.

 

[lexpatrie]

That 1.5x was true at one point, but as I recall, no longer. Or it's for balconies, i.e. no support at the exterior, cantilever, etc., not decks explicitly.

On the lateral load question: Woeste, the answer is always Woeste, or Bender, or Loferski.

Lateral Loads Generated by Occupants on Exterior Decks, Parsons, Bender, Dolan, Woeste, Structure Magazine, January 2014.

The highest load they were able to produce was 12.1 psf equivalent.

Just created a FAQ entry for this article over in the wood design forum.

 

[DoubleStud]

yes lexpatrie, I have read that. If I use 12psf, I will have 146 plf shear on my diaphragm. SDPWS only list 70 plf for horizontal lumber sheathing. But if we install it diagonally, it is 420 plf. But everyone brought up a good point about future decking that people may use composite decking with hidden fasteners.

 

[DoubleStud]

I generally do under deck planar 2x6 diagonal bracing that ties to beefy 6x6 blocks that is through bolted to both bands

I am not able to visualize it. Can you explain further?

 

[phamENG]

DoubleStud - XR250 already covered it. I'm pretty sure I developed my method out of some advice he posted here some time ago.

zack - 2021 IRC indicates the design live load for decks and balconies to be 40psf. For commercial spaces, I use 100psf as they are almost always assembly areas (or have the ability to be quickly converted to assembly areas). For buildings that fall under the IBC (anything bigger than a house), then you are correct. But I find it's pretty rare to have a wood deck attached to such a building, unless we're talking about balconies on a multi-family building, but those are rarely a concern for lateral given their normal proportions.

 

[lexpatrie]

You can't design for every what if, there used to be some strap bracing that you could put under the boards, that could take some of the load, or you could mess a bit with the dimensions of the deck, perhaps, to get the load a bit lower, if needed.

Somebody here once suggested on an attic repair have the sealed design stuck in a plastic baggie and taped to the attic framing, can't quite do that here, but you could put something on the drawing that it's to be provided to the next owner in perpetuity, until it is fully replaced, and you could also specify a design professional inspect the deck at some time in the future, 1 year for a shakedown, then 2 year intervals from there? NADRA may have some framework on how often a deck should be inspected.

To me that 12.1 psf is what they call "attractive nuisance" or "forseable misuse" type of loads, i.e. 12.1 psf is unlikely to happen outside of deliberate sabotage/mischief. Just how far down you adjust that lateral load is engineering judgement and most engineers probably ignore lateral load concerns on decks (if seismic or wind created larger loads, then the lateral load from occupancy isn't that great a concern, unless you're considering some kind of "earthquake watch party" on the deck during a design seismic event). I would think using a higher lateral load on places that could more plausibly see a flash mob, an assembly occupancy, or things like that. There are sway loads for bleachers, but I'm not aware of the magnitude anymore.

When it's a residential deck, beyond the housewarming, there's normally some furniture that obstructs full occupancy. And not a lot of rhythmic dancing...

 

[phamENG]

And not a lot of rhythmic dancing...

Hey, lex: just because your deck is boring doesn't mean the rest of us have to have boring decks.

 

[bones206]

There should be an explicit frat party load combo in the code for deck design.

 

[jerseyshore]

In the NJ IRC balconies are 60 psf and decks 40 psf. I will always use minimum of 60 psf for decks though. In the NJ IBC it's typical 1.5x LL served.

I never mess around with decks (or balconies) because they are by far the most likely to be fully loaded and are constantly abused by the elements.

I went to a 4th of July party in Brooklyn many years ago and everyone was packed like sardines on this balcony that overlooked downtown Manhattan to watch the fireworks. Was a typical Brooklyn high rise residential building and we were probably 10 stories up.

Ever since then I always use that night as justification for higher LL's and extra care in framing and detailing decks & balconies. People will pack a deck for fireworks.

 

[zackngineer]

That 1.5x was true at one point, but as I recall, no longer. Or it's for balconies, i.e. no support at the exterior, cantilever, etc., not decks explicitly.

zack - 2021 IRC indicates the design live load for decks and balconies to be 40psf. For commercial spaces, I use 100psf as they are almost always assembly areas (or have the ability to be quickly converted to assembly areas). For buildings that fall under the IBC (anything bigger than a house), then you are correct. But I find it's pretty rare to have a wood deck attached to such a building, unless we're talking about balconies on a multi-family building, but those are rarely a concern for lateral given their normal proportions.

So I did a little code digging on this and here is what I found:

*IBC-R 2024 Section 301.5 Live Load: Table R301.5 Balconies and decks: 40 psf**e; e = See R507.1 for decks attached to exterior walls... R507.1 just refers back to R301.5. Seems like a circular reference.
*IBC-B 2024 Section 1607 Live Loads: Table 1607.1 Balconies and decks: 1.5 times the area served, NTE 100 psf
*FBC-R 2023 Section 301.5 Live Load: Table R301.5: Same as above.
*FBC-B 2023 Section 1607 Live Loads: Table 1607.1: Same as above.
*IBC-R 2000 Section 301.4: Table R301.4 Decks: 40 psf. This has been the case for a long time it seems.
*ASCE 7-22 Section 4.3 Uniformly Distributed Live Load: Table 4.3-1: Balconies and decks: 1.5 times the area served, NTE 100 psf. There is no reduction or provision in the ASCE 7 as far as I can tell that allows a 40 psf live load on decks.

I always use the ASCE live load chart. I'm not going to stop, but I see where the confusion was on my part.

 

[lexpatrie]

Okay, it's designed for regional/local practices, we don't dance or have fun. We're stoics. We gather and sometimes coffee is involved, think of it as AA without the prior drinking.

I did at one point do an "ornamental" deck that was a three level viewing platform, 100 psf occupancy live load, ramps for ADA, foundations, free standing, exit stairs (might have been "by other"), instructions on coating the ends of any cut PPT lumber, the hangers were all worked out for chemical attack, deicing salts were prohibited (baseball stadium, we aren't that hardcore, this isn't Canada), inspection requirements for it on a scheduled basis on the drawings, and it was beautiful in it's own way.

They wanted as few pieces as possible so I found some 50' - PPT glulams that could come from Washington state, through the mountains, eventually it got into this big "we don't like it" "you made it too stupid" "neener neener" "why are there concrete foundations" and it never got built. It would have been fantastic, and it was designed for 12 psf lateral, as it was a viewing platform for a baseball team. I may still have the drawings as it was a "Piece" in the parlance of the street artists. I expect the calculations are long lost. I have found several times when somebody wants to "max" something out, they usually don't understand that the lack of redundancy creates forces that MUST be handled at the point of concentration, i.e. the lateral system gets a lot more formal because there's so much less of it. There was cross-bracing, everything was detailed to drain and not accumulate water, it was quite a thing.

No argument on that, Jersey, but it's a balcony, not a deck, so I think the 1.5x occupancy went away in the base code, or the language got aligned I can't recall at the moment. You all keep spouting code, but I'm never convinced anybody here cross-checks versus their brute force recall. So I thought it went away except for full on balconies. It would be something worth noting elsewhere, like in the FAQ but I don't know that offhand and it's a busy week and I don't want to get diverted, I'm having enough fun trying to convince CAD to plot the contents of the viewports but not the borders.... and ship the drawings.

Just a minute to clarify - as I get caught up - there's two items at play here -
[ul]
[li]Lateral load on a deck or balcony - functionally I would expect these are not all that different. i.e. 12 psf is the maximum one can get without trying really, really, really hard. That's the real focus of the OP's question.[/li]
[li]Vertical live load on a deck or balcony - which is not the OP's question.[/li]
[/ul]

But it's worth diverting into as we seem to have "handled" the lateral load question. Here's where the difference probably lies. Balcony 1.5x occupancy is probably still there, it's the definition of "deck" versus "balcony" in the IBC/IRC/ASCE that's going to differ, and for a deck, as I recall, it's occupancy served. It used to be 1.5x, but that's not in the code any more. I know I'm speaking off the cuff and I just complained that people don't site code and check, I admit and concede that point, I just don't have time for it ATM.

I'm not a giant fan of "occupancy served" but that's what is in the code. If you choose to exceed that, I have no argument against that.

 

[zackngineer]

There should be an explicit frat party load combo in the code for deck design.

There is! In the ASCE 7: Balconies and decks are recognized as often having distinctly different loading patterns than most interior rooms. They are often subjected to concentrated live loads from people congregating along the edge of the structure (e.g., for viewing vantage points). This loading condition is acknowledged in Table 4.3-1 as an increase of the live load for the area served, up to the point of satisfying the loading requirement for most assembly occupancies. As always, the designer should be aware of potential unusual loading patterns in the structure that are not covered by these minimum standards.