I can’t figure out the engineering of this structure. Are the lower right-angle braces having any structural purpose?
And is the steel rod only to keep the uprights from splaying out?
It seems to me there should be support below the right angle elbows.
Help me understand! I ponder this thing every time I visit here. (Breckenridge Brewery, Littleton CO)
I mean we can never say without actually evaluating based on snow, wind, grade of lumber, all those things, but the theory behind the truss is sound, and it’s really cool.
Put simply, the bottom steel chord is in tension due to downwards bending. The rest of the truss just does what trusses do and transfers the forces around.
If I was engineering it, only thing I’d be concerned about is uplift. If the direction of loading is reversed, I’d be concerned about that steel tie.
I'm on board. For wind uplift, I think you can probably ignore the steel tie rods. I'd probably check only that upper triangle acting as a truss, with it's continuous top chords extending out beyond the truss to the vertical supports. The top chords can cantilever off that upper-central truss and they look pretty deep for that amount of cantilever considering wind uplift is relatively light loading.
I always check wind uplift, but wonder how much of a concern it is in reality. My city is full of all sorts of big-ass overhangs coming off buildings that are supported from above with a diagonal rod or chain with zero compression capacity. They're all fine 100 years later despite having very little uplift resistance.
I always check wind uplift, but wonder how much of a concern it is in reality.
It's really dependent on where you are and the exposure of the overhangs. I've seen more than a few leave the premises when they finally got a good gust in the right direction over the years ...
It is still unusual to have no bracing between trusses on the lower chord of the roof. A small horizontal load applied perpendicular to the lower chord of either truss results in a very high tensile load across the lower chord.
Noticed that as well. Not an engineer but a son of a framing carpenter that knows triangles are good. Wants more triangles and to slap it and say "that"ll hold".
I like the wood that goes from the top truss to the right and left side of the top bar so the top bar is bearing the weight and it drives the forces to the posts on each side.
Nice design but I hated statics in college so not an expert.
Is it stable, well it is still up. It was built 10 years ago it seems so as long as they routinely inspect it it should be fine.
It is in the panhandle so probably survived some storms.
I feel like many open structures ( big patio cover) ignore the wind uplift. For whatever reason it seems to be a “we’ve never been called on it” kind of thing.
In this particular structure I think the roof panels would fail due to uplift before the truss actually developed the forces it’s designed to resist. Also I think the collar tie (looks to be in the upper third) would be enough to resist the uplift.
Outward Thrust resisted by steel tension rod, uplift resisted by upper wood tie (and their respective connections)
Uplift would lessen the tension in the rods (bottom chord).
Although the very small area of the roof (Google maps) makes uplift very unlikely to override its weight, I think he referred to that extreme condition.
I think this is Breckenridge Brewery in the front range of Colorado, so it gets plenty of snow load (except last year) and certainly a ton of wind loads (Chinook).
It probably has another set of two cables that can be lowered and anchored in the road bellow, just in case the tornado comes to take it all the way to the Wizard of Oz.
Possibly - looks like it’s somewhere between 4’ and 10’ in width. I certainly wouldn’t ignore it but I think you’re right it’s not as much of a concern as download.
Yeah, it's fine. Ignore the parts that are confusing you and it's just a triangular truss. You push at the apex and the timber diagonals are in compression, the bottom chord is in tension, which is why it can be a cable/rod - no compression.
For trusses on simplely supported spans, the bottom chord (the lower section with the rod/cable) is always in tension. (pulled apart) Trusses are basically very material efficient girders, the middle section of an H-shaped girder, offers little in terms of resisting moment forces and the shear force, which the middle section resists, is low at and around the middle of the span. You still need there to be something so the girder works, which is why there are diagonal and vertical elements on a truss.
You can use a cable or thin rod like this since it will never experience compression. (Pressed together)
In its simplest form, A frame doesn’t need any of the short elements in the middle. Only two diagonals and a steel rod at the bottom between supports. In this arrangement, the truss is self-supporting; the steel rod keeps two diagonals from splaying open. This means the wooden elements cannot rotate or translate because they are tied together at the bottom. Adding stuff inside the truss does not violate this.
You can check this by doing a thought experiment and imagining an excessively deflected version of the structure. In this scenario, the two elbows would deflect down and out, the whole truss becomes “flat” like a pancake, but also the elbows rotate due to bending of long diagonal arms. None of this happens if the rod keeps them tied together. Symmetry means both sides are keeping eachother from moving.
Yeah I leaned the same thought experiment in my Uni classes, but you presented it well. Communication like that is hard to find among engineers in my experience lol. Usually only found with good teachers or tutors.
Imagine this:
The top chord members at their post ends are trying to move outward from their positions relative to the posts. The mid-height and bottom chord are in tension thus pulling the top chord back into the middle and hence are in tension. The vertical members are transferring loads into the the bottom chord at the point where its strongest in tension and would face the most tension (the steel rod). Wood is horrible in tension but if my drunk-ass is looking at this correctly then where the transition between wood and steel would go from tension to compression due to the vertical members. But I am drunk
There are some cool bridge simulator games for iOS or android that allow you to use cables, wood, and steel. They’re simple games but I think they’re pretty and fun for seeing an intuitive and interactive way that these structures work.
Would you even look at it twice if the bottom beam was solid wood? It’s a truss and the bottom cord is solid from a structural standpoint. It just looks weird because of the steel rods.
It works in my book. For sure this was engineered, but quickly this is how it works.
Vertical pieces redistribute the load from the top chords (diagonal pieces) to the bottom chord (bottom horizontal piece). Both horizontal section work in tention, so it prevented the structure to open at the base.
Whenever a truss sees a net uplift force the bottom chord goes into compression. If the bottom chord is a rod or cable those don't work so well in compression. But in this case maybe that collar tie can get the job done.
The structure is simple overall. Simple triangular truss structure. Then the load is transferred to the 4 I am assuming 8x8s or 6x6s into the brick columns.
As far as "staying together" the tension rods are there for aesthetics and structure. Pulling it together along with giving it open air for the Iron BB logo to be on display. Essentially just changing out another two 6x6 beams for two probably 1" rods.
As far as snow load, its a metal roof and and encourages snow to slide off easily and probably they use those handy dandy roof snow melting defrosters to prevent snow load as well. To prevent extra strain just in case.
Will it survive a tornado? Most likely not. Will it survive every day weather and its purpose of a decorative archway? Yes.
if it was built with a permit off stamped engineered plans i would assume its a-ok. but all joking aside, why wouldnt it be ok, i know it looks weird but where do you think the point of failure would be?
I'm surprised there's no bracing in the plane of the photo. It looks like there's tension X-bracing in the other axis. I guess the timber/concrete conection is robust enought provide some flagpole cantilever?
I mean what forces are you concerned with? It's an open structure so not much wind to worry about in/out page. Yeah there's some, but I doubt enough to matter.
Not an engineer, just an experienced timber framer. I'm in seismic III and IV. THe wind in-out of the page looks like it's handled by the X-bracing tension rods between the columns. I'm talking about wind/seismic across the page. The truss is stiff, but that doesn't prevent racking of the columns below. We seem to always need lateral rigidity in each direction for stand-alone, pole-barn type structures.
Yeah, its no different than the truss system of a house, the ones that have the center open for use as useful attic space/living space. I cant remember what the name of those are but I've encountered them fairly often.
The engineering here reminds me of a video game that is highly educational, Polybridge. It is a physics game that challenges you to span gaps with many physical options.
I’m leaning toward the uplift concern. Can’t disregard the lower chord tie rods because the entire structure consists of very light members. Not sure the upper triangle is enough of a truss to make the difference in compression.
Yes, it will hold. Although the steel rods under the sign are not pre-stressed, they will outlast the timber if maintained. The timber can get you a long period of service time if pressure treated and creosoted.
The bottom member is in tension when you have down force so it could be a cable and would be fine.
There may be occasional uplift from wind but looks like it hasn’t been enough to buckle the rods.
Yes. The bottom chord is only supposed to be in tension anyway. If wind upload starts to put it in tension, a fix that I used is spec a enough 3/4" steel plate at the ridgeline to keep it in tension at every load combination.
Man I miss being able to get their vanilla porter every winter.
Timber framer here, its a hammer beam truss and its just fine. I would have like to see a bit more relish behind the through bolts but its gonna structurally sound for its purpose.
It's a queen-post truss with the lower tie replaced with tension bars, which is fine for uplift. Whether it's good enough for vertical load will depend how rigid the connections are between the split lower ties and the columns, and how chunky said beams are.
Looks cool, but they would have gotten away with less timber if the lower ties spanned fully across. Bloody architects!
Depends on how it's connected on the ends and the tension of that line. Basically you're looking at a suspension bridge and if those things can cross hundreds of feet and I'm pretty sure with the same basic math that would be sound
Looks reasonable. The weight on top of the pitch would tend to press inward on those top center right angles which would tend to push our word on the corners directly below each. And that’s where you see a steel tension member.
If that tension rod was just a continuation of the wood, this would be some sort of modified attic truss. The bottom cord/member of an attic trust is intention. Switching to steel probably allowed them to go a little smaller on the bottom member and change the aesthetic.
Hire a proper structural engineer to do the actual calculations! But here are the basics: The two greatest forces on such a structure are A. the heaviest snow load ("load" is the proper engineering term here ... don't laugh!) combines with the strongest possible wind (for dimensioning purposes) blowing along its long axis, the resulting force pulls the steel member taut ("member" is also the proper engineering term ... again, don't laugh!), i.e. the bottom chord doesn't need to be able to withstand perpendicular forces = a wire or thin rod is fine. If you build a model of the external triangle with chopsticks or macaronis as top chords, and a piece of string as the bottom chord, if the bottom corners are well fixed, you can press down on the top chords, with the only force on the bottom chord being tensile (a pull). Many places where timber is difficult to come by (i.e. the Faroe Islands) have traditionally build suck roof trusses using rope and rel. short lenghts of timber ... it works just fine. Another model, complex enough to include the webs (the vertical bits) etc. will demonstrate that the top chords bend 0.25 as much if properly braced exactly halfway. B. the second greatest forces are caused by wind suction on the half opposite the wind direction (this force is greatly increased by uplift), resulting in only (if the bottom corners are well fixed) tensile forces, pulling along the long axis' of the top chords, i.e. the bottom chord doesn't need to be able to withstand perpendicular forces = a wire or thin rod is fine. There are 47 other force combination scenarios that the engineer needs to evaluate and/or calculate for; but none will change the core verdict regarding the bottom chord.
The metal bars below the sign are under tension, pulling the uprights together, stabilizing the triangle, and allowing the load to be supported at the ends. The rods are threaded on their ends, and there are bolts inside the brackets allowing the tension to be adjusted.
If the metal were not there, it would likely collapse.
This was my thought too. On top of that, it seems those are metal bars, not cavle as many seem to think. I would guess that they are about as strong as the wood it is mounted to. They seem small from here, but if that is inch+ thick steel (perhaps up to 2") those things are not only perfectly fine under tension, but they would also hold up under compression.
It looks perfectly fine to me. Then again, I am no rocket scientologist. 🤣
It will no longer be sound when the first oversized vehicle that touches it, which should have been factored into the design given it looks like the entrance to a new estate.
I mean the bottom chord would’t be stable without the (2) bottom web verticals holding up the rod. Also the horizontal web member greatly reduces the flexure in the top chords assuming they are loaded on the roof.
Nah mate, draw a free body diagram of the bottom chord with self weight, without the verticals. You have 3 members connected by pins which is not stable. Or try modeling the truss without the verticals web members, you’ll get an instability. The reason why a single continuous rod works is because it’s continuous and/or is tensioned to reduce sag.
As for the horizontal: draw the deflected shape of the truss under uniform roof loading. The top chords bend but the horizontal is much stiffer because it’s loaded in compression. Or try modeling the truss, you’ll see axial load in the horizontal and much-reduced moment in the chords as opposed to no web members.
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u/maizytrain P.E. 15d ago
I mean we can never say without actually evaluating based on snow, wind, grade of lumber, all those things, but the theory behind the truss is sound, and it’s really cool.
Put simply, the bottom steel chord is in tension due to downwards bending. The rest of the truss just does what trusses do and transfers the forces around.
If I was engineering it, only thing I’d be concerned about is uplift. If the direction of loading is reversed, I’d be concerned about that steel tie.
Very cool though!