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Author Topic: sanity check for my project about to start  (Read 565 times)

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Offline alo53

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sanity check for my project about to start
« on: November 12, 2021, 02:14:00 PM »
Hey everyone! What a cool site to stumble on. So much good info. Weve got a patio cover project designed with some very large beams/spans, and I had a nightmare the other night that the structural engineer missed something and the thing collapsed on my head lol. I am an aerospace engineer and am now trying to learn structural engineering so I can get over my new irrational last minute hesitation

Anyway, the design is 4 posts, 5x5 steel columns (will be wrapped) supporting 8x18 DF #1 beams. There are two king posts with an arched beam (cut from a 6x14) to support the 6x14 ridge beam. Obviously, a lot of big timber on this project. The steel columns are gonna be attached to a 3ft x 3 ft pad that is 3 ft below grade (caged in concrete/rebar).

I have modeled up most of these members on my own and I think it does check out. I am not sure how to model the arched collar tie beam supporting the ridge. At its narrowest in the center,  it is cut to 5x6, but it is thicker near the ends where the moment load is highest. The concealed hardware also scares me but I did look into that and the Simpson HCJTZ carries up to 8000 lbs.

Anyway, is there anything here that anyone thinks requires further sanity check and for me to ask any additional questions from our engineer? We are in San Diego so no snow, but definitely potential of earthquakes.


 
 

 

 

 


 



Online Jim_Rogers

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Re: sanity check for my project about to start
« Reply #1 on: November 12, 2021, 03:19:13 PM »
Interesting project.
First off King post are in the center of the span/roof.
Your label should say "Queen" post, as queens are to the side of the center.
These types of frames need to be reviewed by an experienced timber frame structural engineer to be sure your numbers and sizes are correct.

Good luck with your project.

Jim Rogers
Whatever you do, have fun doing it!
Woodmizer 1994 LT30HDG24 with 6' Bed Extension

Offline Don P

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Re: sanity check for my project about to start
« Reply #2 on: November 12, 2021, 05:03:36 PM »
Let's look,
The rafters are supported from top down by the ridge, a pair of purlins atop the queen posts and the plate.
It is symmetrical so I just drew the tributary areas on the one side of the ridge (red), the purlin (blue) and the plate (black).

The ridge is supported at each end so one half of the ridge's tributary area (pink) is supported by the collar tie in question. Area x (Live load +Dead load) = total load. That load is a concentrated midspan load at the point of maximum moment. You've got the total load P, span L... max moment is PL/4.
The moment diagram looks like this ^, I'm conservative check it as a 5x6.

There is nothing wrong, aesthetically there is nothing wrong at all, with shallowing the curve and making that section beefier, functionally and visually. Right now it draws your eye because whether it is or isn't, it looks undersized to the scale of the other members. Know what the minimum section is and then do the visual as it unfolds.





Edit;  the load distribution of ridge and purlins is largely dependent on how well fitted everything is, but also it could depend on how flexible the arch is or how you build it. If the rafters to ridge joint is sloppy, the purlins take the load. If the arch supporting the ridge is flexible under load then again the purlins take the load. Just for fun you could imagine it without the ridge. Are the purlins adequate then. If so it is your choice whether to load the arch, you could in that scenario suspend the ridge by the rafters above the arch. The devil is in the details. That is all gravity, I'm not seeing what is keeping this upright when the world shakes, hopefully those post/beam connections are moment resisting.
The future is a foreign country, they will do things differently there - Simon Winchester

Offline alo53

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Re: sanity check for my project about to start
« Reply #3 on: November 12, 2021, 06:19:06 PM »
thanks for the replies guys! 

The side 8x18 beams are being held bolted to the column via knife plate. The front and back 8x18 are hanging on this bracket above the column: https://www.strongtie.com/specialtycollection_architecturalproductsgroup/hcjt_tie/p/hcjtz#LoadTables

your second comment: re earthquake is what scares me. 

Lastly, what is the span I should assume for the collar tie / ridge support? From where it touches the rafters, or from queen post to queen post.

Offline Don P

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Re: sanity check for my project about to start
« Reply #4 on: November 12, 2021, 07:11:15 PM »
I'm a carpenter so I go brutally basic, the span is between the posts... and it doesn't hurt to round that upwards an inch or two, technically it is the distance between supports plus half the required bearing at each end. That is the tip of your mental simple support triangle.

I'm just seeing vertical loads on that Simpson, something else needs to keep you from rocking. That steel column bolted down may be adequate.

If you haven't found it scroll down the column on the left of the page to the red toolbox, click it and go to the bottom, there are wood beam and column calcs there.
The future is a foreign country, they will do things differently there - Simon Winchester

Offline alo53

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Re: sanity check for my project about to start
« Reply #5 on: November 12, 2021, 08:05:03 PM »
for the arched beam I calculated 1210 dead and 1405 live load and applied as a point load at mid span. the total span is 11'10''. the beam is failing by quite a bit.  i must be completely off on the loads. 

Offline Don P

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Re: sanity check for my project about to start
« Reply #6 on: November 12, 2021, 09:58:08 PM »
Lets see,
~12' between queen posts so trib width bearing on the ridge is 6'. 21' ridge span? so half that 10.5', looks like a 2' gable overhang so 12.5' x 6'=75 sf. LL no snow, 20 psf wind + 15psf DL...75sf x 35psf=2625 lbs total ... doesn't get much closer to agreement than that.
The future is a foreign country, they will do things differently there - Simon Winchester

Offline alo53

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Re: sanity check for my project about to start
« Reply #7 on: November 12, 2021, 11:20:26 PM »
haha well i can't say that i am happy that you agree with me.... that load on the 11' foot span of a 5x6 is way past failure. it takes a 6x12 for that beam to not fail. i am not sure how i address this with my architect and engineer who i paid to design this. the city approved the plans and has issued the permit already. i am just glad i have an engineering background and had some basic intuition that it didn't feel right. 

are there any other concerns you see with the design i should bring up with the engineer? i do not want to spend money putting a safety hazard on my property. i am thankful for your input Don!

Offline Don P

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Re: sanity check for my project about to start
« Reply #8 on: November 13, 2021, 08:50:23 AM »
Check your ridge, purlin and plate sizes. I haven't but the size distribution, well, I'd check it.

Read the note on the Simpson connector and green timber splitting here;
T-C-CJTZ21.pdf (widen.net)

Look at the details, if you can't find a lateral load path, you need one pointed out with the connections. 

All depending on the department a running joke among carpenters is that "plan review will happen at the framing inspection".
The future is a foreign country, they will do things differently there - Simon Winchester

Offline alo53

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Re: sanity check for my project about to start
« Reply #9 on: November 13, 2021, 08:53:36 PM »
i think the perlins and ridge look okay

perlin: 6x12 DF1,  20'6' span w 3'6'' overhangs.trib width = 6'6'' applied 14 DL, 20 LL.
ridge: 6x14 DF2, 20'6' span w 3'6'' overhangs. trib width = 5'6'', applied 14 DL, 20 LL.

maybe the suggestion i bring to the engineer is to just take the sportiness out of the design. put 8x8 posts under the queens and hook them up w T bracket to the horziontal beam.
that defeats the goal i gave them of open up our view as much as possible, but i would rather err on side of safe.

still doesn't fix the ridge support arch... which needs more beef i guess.

Offline Don P

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Re: sanity check for my project about to start
« Reply #10 on: November 16, 2021, 08:17:06 AM »
From those tributary areas it looks like it might be better to use the ridge dimension at the purlin locations and the 6x12 at the ridge. If you look at the beam equation for a continuous beam across 3 supports, the center support takes 5/8 of the load, each end 3/16... the ridge taking that from a rafter on each side has a 3/8 load.

Wood engineering stuff;
There is a good publication of beam equations used in the code referenced NDS for wood construction at awc.org under the publications tab, it is called DA6- Beam Equations
The NDS is there as well which is the engineering code ref for wood

You have an engineer, I'm not seeing all they did, never forget that we are 2 amateurs on the internet.
For bracing if there is no load path, you have a steel post. I keep wanting to weld a steel blade extending out along each 90 degree beam axis for a few feet, weld a support plate flatways under that knife plate. Split the beam and set it over the plate, pin low and securely at each end of the plate to the wood beams, a long rigidly connected lever arm. It's the same as the simpson connector only heavy, long and welded and doing so out of both 90 sides of the post. In other words I'm seeing some awesome steel posts easily fabricated to something rigid and then these wimpy bent tin connectors.( apologies to Mr Simpson, but theres a honkin heavy post there begging for some hot arc connections)

The light ridge support arch could be somewhat similar, make a flitch plate composite beam, a sammich of a center steel plate shaped and sized for the load with wood side plates checked for buckling. That can probably get you back in that open direction. In all of this beam work if lightness is a factor glulam has the appeal of heavy timber but carries around triple the allowable design loads. Shrinkage allowances around connections are much lower.
The future is a foreign country, they will do things differently there - Simon Winchester

Offline alo53

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Re: sanity check for my project about to start
« Reply #11 on: November 17, 2021, 02:49:32 PM »
From those tributary areas it looks like it might be better to use the ridge dimension at the purlin locations and the 6x12 at the ridge. If you look at the beam equation for a continuous beam across 3 supports, the center support takes 5/8 of the load, each end 3/16... the ridge taking that from a rafter on each side has a 3/8 load.

Wood engineering stuff;
There is a good publication of beam equations used in the code referenced NDS for wood construction at awc.org under the publications tab, it is called DA6- Beam Equations
The NDS is there as well which is the engineering code ref for wood

You have an engineer, I'm not seeing all they did, never forget that we are 2 amateurs on the internet.
For bracing if there is no load path, you have a steel post. I keep wanting to weld a steel blade extending out along each 90 degree beam axis for a few feet, weld a support plate flatways under that knife plate. Split the beam and set it over the plate, pin low and securely at each end of the plate to the wood beams, a long rigidly connected lever arm. It's the same as the simpson connector only heavy, long and welded and doing so out of both 90 sides of the post. In other words I'm seeing some awesome steel posts easily fabricated to something rigid and then these wimpy bent tin connectors.( apologies to Mr Simpson, but theres a honkin heavy post there begging for some hot arc connections)

The light ridge support arch could be somewhat similar, make a flitch plate composite beam, a sammich of a center steel plate shaped and sized for the load with wood side plates checked for buckling. That can probably get you back in that open direction. In all of this beam work if lightness is a factor glulam has the appeal of heavy timber but carries around triple the allowable design loads. Shrinkage allowances around connections are much lower.
i love this idea. of increasing the size of the steel plate to also go under the horizontal, and potentially  knife plating both of the beams. is that what you're suggesting?  
 
i don't think that would take anything away from the look of the design. 

agreed we are both amateurs :) , especially me. but ya im sure the simpson joist would hold it fine, but i don't mind a little extra security. 

Offline kevin5055

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Re: sanity check for my project about to start
« Reply #12 on: November 17, 2021, 03:19:37 PM »
Maybe I missed it, but how is the collar attached to the rafter. Worst case is if the ends can rotate, it would then be a simply supported beam and have a moment of PL/4. If it is sufficiently connected to prevent rotation of the beam ends, it could potentially have been viewed as a fixed-fixed beam. This drops the moment to PL/8.  

Even with PL/8, I am coming up more stress than allowable for DF No. 1.  You didn't specifically state the grade of the collar tie, so I am assuming it is also DF No. 1?

Offline kevin5055

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Re: sanity check for my project about to start
« Reply #13 on: November 17, 2021, 03:33:00 PM »
If you look at the beam equation for a continuous beam across 3 supports, the center support takes 5/8 of the load, each end 3/16... the ridge taking that from a rafter on each side has a 3/8 load.
I agree with this observation, but I think the center takes 5/8 of each span (total 10/8 and the end supports take 3/8; the total should be 16/8 for all supports. This would make the ridge 6/8. EDIT: We are saying the same thing. I was matching the diagram from the AISC manual you posted the other day where everything is in terms of wl (where l is for one span); you were looking at in terms of total load over two spans so it works out the to be the same thing. 

It is also worth noting this is for two equal spans. Since the center support (queen post) isn't at the center and closer to the ridge, it should further reduce the reaction at the ridge beam.

If you paid for the design, I would think you would be able to get your hands on the calculations?  If I had to guess, the rafters were designed as continuous beams with the tributary width of roof being applied. Then the purlins and ridge designed as simple spans with point loads where the rafters rest on the purlins and ridge (using the reactions from the continuous beam analysis). The end reaction of the ridge beam would then be used to size the collar. Lastly, the 8x18's designed for a purlin reactions (with 1/2 the ridge reaction added to each queen post location).

Offline alo53

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Re: sanity check for my project about to start
« Reply #14 on: November 17, 2021, 08:45:23 PM »
here's what i was able to download from the permit website. i haven't spoken to anyone yet.

what i find odd is he has the ridge support modeled as a 5'10'' span in his calcs.  i don't get it. attached the detail of that beam below. 5'10'' is the span from where it touches the rafters.





 

 

 

 

 

 

 

 

 

Offline Don P

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Re: sanity check for my project about to start
« Reply #15 on: November 18, 2021, 06:57:40 AM »
Yup, "Lucy!, you got some 'splainin' to do". I'd ask about that ridge support.

The plus side is the lateral load path is explained in that, fixed end steel posts embedded and constrained... deeply embedded heavy flagpoles surrounded by a slab. I assume there is an attachment detail elsewhere in the plans.
The future is a foreign country, they will do things differently there - Simon Winchester

Offline alo53

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Re: sanity check for my project about to start
« Reply #16 on: November 18, 2021, 12:58:39 PM »
yup indeed. i will let kevin respond too since he was interested in the calcs to see if he sees anything else worth questioning. 

i am thinking about what to suggest - whether i want to switch material like you suggested (even consider just steel?), or just add a king post below the ridge. that for sure changes the aesthetics. i still think the 2 suggestions i make are figure out this ridge support issue, and replace the simpson joist with a stronger connect to the steel column. 

here are the structural details, including how the steel column is attached to the pad and caged in pole footing.



 

Offline kevin5055

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Re: sanity check for my project about to start
« Reply #17 on: November 23, 2021, 04:05:24 PM »
yup indeed. i will let kevin respond too since he was interested in the calcs to see if he sees anything else worth questioning.

i am thinking about what to suggest - whether i want to switch material like you suggested (even consider just steel?), or just add a king post below the ridge. that for sure changes the aesthetics. i still think the 2 suggestions i make are figure out this ridge support issue, and replace the simpson joist with a stronger connect to the steel column.

here are the structural details, including how the steel column is attached to the pad and caged in pole footing.


(Image hidden from quote, click to view.)

Sorry to keep you waiting; I agree to question it.  I could maybe see the 4'-10" span if it was was rigidly attached to the rafters, but I don't see any attachment details for how it connects to the rafters and the rest of the design doesn't jive with that load path (that is, the load path would be back through the rafter to the post).  The corbels look a little small too to me with only a 2" thick "ledge".

The good news is that it looks like the 8x18 DF beam is designed as if there was a post under the ridge beam so if you added a post, I don't think it would change the beam size.  Applying the ridge beam load at mid-span of the 8x18 will produce a higher moment than if it was split between the two posts under the purlin beams.


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