Upgrading Deck to 3 Season Porch - Resolving Lateral Windload

[jacktbg]

Hello,

I am a former Structural Engineer, was specialized in residential design in high wind areas, but the vast majority of the homes I have worked on have been able to be designed either using the MWFRS method to develop loads and then design walls/windows for in plane loading by hand, or some designs complied with Prescriptive wind design. Was mostly wood framed homes, used sheathing to resist in plane loading, etc. I have since then moved on and into my original degree, Mechanical Engineering. But I feel I have enough experience to design the following, at least a preliminary version.

I now am looking to design a 3-Season Porch for my parents, upgrading their existing deck footprint. But, came across a roadblock that I feel a little out of my element about. I haven't actually designed a 3 season porch addition before, I have designed houses, regular or screened porches, and decks, but never a 3 season porch, which to my knowledge requires wind load analysis (because we are using solid windows), and I have zero wall to work with, sheathing to assign, etc. I'll be submitting my preliminary thoughts, calculations, and drawings to an engineering company (which I used to work for) to get a PE to look over it and tweak what needs to be tweaked. I know they would design it from scratch for me, but frankly I should be able to do this and I don't want to embarrass myself by either vastly over-engineering it, or by making any critical mistakes. So, I'm turning here for ides, this community has helped me out before and I highly value the advice I received last time.

So let me give a quick description of the project, then tell you where I'm stuck.

This is in Minnesota, in a 109 mph zone, risk category 2, and on the edge of a lake, so I'm saying exposure category D. We're upgrading what is now a deck (3 feet off the ground) into a 3 season porch with a gable roof. Right now, it's 16' wide by 14' away from the house. We wish to use existing footings, and send the joists a further 2 feet away from the house, cantilevering them over a drop beam, etc. The gravity, and the uplift, I can handle. I ran the joists, they can handle the cantilever with the gable on the end and the weight of the floor truss and live load. I have no problem generating the uplift for the structure, and designing all the connectors (going to use some standard simpson connectors, H10A etc.), and sizing all the beams and columns. But, if this were a house I were designing, I would be able to simply design the space below the floor to resist the lateral loads and then develop the necessary wall length, portal frame what needs to be portal framed etc., for the first floor. I haven't really done any rigid frame design before, so I have a few questions...

My initial thoughts were that A.) At the ledger from deck to house, I was planning on installing to existing floor trusses with DTT2Zs, at both the floor level, and to tie the roof beams to the house similar to the method described in DCA6... I can assume the house can handle any wind loads which feed into the house as there is more than enough wall along that line on both the first and second stories of the house. And, B.) That on the other side of the structure, I might be able to use a post-installed Column Base which is strong enough to resist some amount of moment about the ground, alongside some simple column bracing at the outer most columns, as you would see recommended ijn DCA6 for deck design. I believe I can just use traditional statics and some software like Skyciv to resolve the portion below the deck, which is going to resist the lateral loading. Finally, assumption C. was that any in-plane loading on the "walls" of the porch parallel to the joists, would feed into the joists through the ledger/DTT2Z, on top and bottom, resolving the lateral loads coming from the lake towards the house. But, here is the crux of my question:

What do I do then at the cantilevered side of the deck, to send posts up to support the gable end, that could be a rigid enough moment frame, without using any wall to slam a portal frame in? I see pictures all over the place online of wood framed 3-Season-Porches with virtually no wall along the length of the windows - just windows, doors, and posts, and no cross bracing. How does the wind load actually develop and translate to the ground in those cases!? I get that I could use some kind of steel moment frame from posts, or potentially a really beefy post cap into the gable beam that could lock the structure into place/resist the necessary moment. But do the engineers who design those assume that, via a Rigid Diaphragm Analysis or the likes, most of the lateral load gets transferred through the "deck"-ledger/DTT2Z and into the house system?? I get that that is the idea with a deck, which does not receive the type of lateral load a 3 season porch does, hence the ability to omit cross bracing below the decks per DCA6. How do Engineers actually ensure, from a wind design standpoint, that the columns, windows, and beams are able to resist that in plane loading, and keep the system from racking? I certainly can't get a rigid enough connection at the bottom of the posts above the cantilevered portion to fully resist the moment/racking, and to my knowledge the windows offer zero rigidity, unless I'm missing something. And, I've scoured the web for a top of post connector that would offer a rigid enough connection to another wood beam to solve my moment crisis.

My suspicion is that I might need to A.) get rid of and repour the existing footings, which my parents wanted to avoid at all costs, in order to install a more moment-resistive post base that can be embedded, or B.) get rid of the cantilever, simplify the design, to avoid having to resolve the moment at the top of the cantilever part, or C.) Both. Which would be a shame, because until I got to the lateral loading this all seemed very feasible.

Again, my old boss will check over my final design, no worries. But, would like to save the headache of looking like I have no clue what I'm doing. This is something that SHOULD be super easy, I simply don't understand how all these other designs I'm seeing can have fairly basic post, column, and beam connections, and still get away with having the deck be basically 100 percent windows with a few columns.

ANY input at all would be so greatly appreciated.

For reference: I've included a quick Skyciv sketch, from a couple angles, of the system I'm trying to create.

Thanks SO much in advance!!!

 

[jacktbg]

Follow up - I suppose since I'm not using sheathing for those in plane loads, I should be using Components and Cladding loads then? Want to make sure I'm developing my loads in the right way.

Thanks!

 

[phamENG]

To resist racking of the entire structure MWFRS would still be appropriate.

I've done something similar to your tension ties. I used them at the floor level to tie into house floor diaphragm and at the roof level to tie into the house's roof diaphragm. You have to be careful doing that, though, since the point load on the diaphragm may need reinforcement regardless of the amount of wall you have. You're also inducing torsion on the main house's LFRS which may cause problems.

Your porch is a bit long, though, so you will need something to stiffen things up out at the other end. Could you try using a truss in the gable end and lowering the bottom chord a bit? That'll give you something of a truss moment frame to stiffen it up. A Simpson Moment Base is also a good idea to try. They don't give you a lot, but if you combine the methods they may contribute enough stiffness to get you there.

I don't like the cantilever. I'm sure it could be done, but I think it's going to over complicate things since I'm guessing you won't have a fully competent diaphragm on the porch.

 

[1503-44]

Yes please do get rid of that cantilever. At the very least brace it back to the nearest footing. AFter a few years, it will start creeping lower and lower.

“What I told you was true ... from a certain point of view.” - Obi-Wan Kenobi, "Return of the Jedi"

 

[jacktbg]

Thanks for the replies!! We might get rid of the cantilever - bracing it back to the existing footings isn’t a bad idea, but resolving that windload parallel to the house still seems problematic for the gable end in-plane loading. I wanted so badly to make it work, but honestly by the time I engineer in a rigid enough set of connectors, and tighten it all up, it might be cheaper to just repour new footings 2 feet out if we want the space.

We are still verifying if the existing footings are large enough to hold up the structure (the deck was also an addition and is in city records so we will know what size, it will just be a matter of checking soil bearing pressure etc.) I think our plan has become to, if footings are big enough, use the exact existing footprint (14x16) of the deck and keep the same size, or if we find out they are 18”x18“ or something and don’t work, we will repour footings 2 feet out if the city allows. Advantage there would be, as you pointed out, I could use an embedded post base that has some level of moment resistance (I understand not much but seems like anything helps.) my new plan is therefore to install bracing, maybe even beyond the 2 ft diagonals called out in DCA6 and just going with full cross braces, from the bottom of the center column to the top of the outer columns, and going continuous to the gable truss. We are using 6x6 posts - I haven’t crunched the numbers yet but I’m thinking I might be able to use those posts as the primary wind resistant member on the gable side then, and design them as partly cantilevered beams supported 3ft up, provided the columns are beefy enough, and the (embedded or post installed, depending what we hear from city) post bases have enough lateral capacity. Does that sound about right? I would probably just hanger the beams in at the floor and ceiling level, unless you think putting a beam on top of the post and installing a more rigid post cap would be a better design?

I’ve got a good idea of my options now that I’m throwing the cantilever out the window. It’s an interesting idea and would have been a cool design, potentially saving us from needing new footings poured. But, why not simplify the design? Without walls to brace the upper half, even enough for a portal frame with a few feet on each side, doesn’t seem feasible. I think we will appreciate being able to take the windows all the way to the corners more than the 2 feet of extra space.

If any other thoughts please let me know! I haven’t fully ruled anything out, just thinking the cantilever might be out of my league with wind loads / no walls. If any creative solutions though, would love to entertain them.

 

[ChiefInspectorJ]

I would think that using wood to solve this problem is too complex. A cantilevered rigid roof diaphragm
might be able to be dreamed up on paper but don't know if it would work.
Use a steel or Concrete/CMU frame.Although as a structural inspector I have seen thousands of porch open and enclosed conditions where 4x4 post are put in standard post bases on 16"x8" strip footings . I knew this was incorrect but" An Engineer stamped it" was the answer I would get. In a high wind event the top beams usually rip off the post
as the load never gets to the base.
There are thousands of screen rooms with no shear walls that try to use loose cables to resist the loads.
Not to mention loose uplift rods that loosen in the walls.

 

[jacktbg]

ChiefInspectorJ, I believe we are going to scrap the cantilevered floor and roof idea in favor of just pouring new footings 2 feet out from the existing footprint. Without shear walls, I was hesitant to design the post to footing connection as a moment resistant connection, but what would you think about "fixing" the posts via cross braces below the floor and considering the posts above the floor as cantilevered beams, assuming the statics of the cross braces works out to allow the foundation to post connection and the connection from post to floor to be considered "fixed"? (And that I can get enough lateral resistance out of the post base.) Would that be a reasonable way to tackle the "no walls but wind load" dilemma?

Thanks!

 

[ChiefInspectorJ]

jacktbg,

Plywood or other window covers would solve the problem resting on a floor transfer beam on piers.
As you said most of these structures don't follow any engineering methods or codes but have stood the test of time. Most of the times in hurricanes the loads never get to the foundations as there are weaknesses in other places. And I think the window frames function as a type of frame which I think would be a good product to develop.
As you said I think you will need new piers and footings on which you can place a 6x6 column with moment connector on it
You can use a skirting between the post or piers as a shear wall if there is a footing, or as a braced frame.

Or you could order a set of engineering from a company like below.

I
Here is a site with engineered rooms.

https://www.engineeringexpress.com/calculators19/?fwp_calculator_category=aluminum-designs

Preengineered plans:

https://www.engineeringexpress.com/store/?fwp_product_categories=aluminum-structures