Building our retirement home

[CurlyDave]

Or, if you are planning a grade-level patio, design in a railing between the two posts and hide the tension member in the top of the railing.

 

[GPintheMitten]

A tension member at the top of the post would be more efficient, I think. But it would be visible.

 

[CurlyDave]

A tension member at the top of the post would be more efficient, I think. But it would be visible.

I totally agree it would be much more efficient, but I am trying to show a way to do this without any visible changes.

And without creating the possibility of a persistent line of bird poop at the edge of the deck.

 

[TomSeller]

Personally, I think a slack cable is going to destroy the aesthetics of the structure even more than a proper collar tie, but a solid rod would certainly sag less than a cable.

True, but the idea behind the slack cable is that if it stays slack through snow and wind loads, you would REMOVE it.

 

[CurlyDave]

...the idea behind the slack cable is that if it stays slack through snow and wind loads, you would REMOVE it.

I am not certain that is really the case. I think the weak point in the as-build structure is the long-term stability of the post footers. They will want to move through the earth in the direction of the forces on them. The problem I see is that most of this motion is going to be during unusual heavy rain/wind events. Possibly snow load, but again most of the motion will be during unusual events maybe once every few years. Think about a wet, heavy spring snow with the ground thawed, but a couple feet of heavy snow on the roof.

The cables can stay slack for quite a long time but there will be no assurance of indefinite stability.

The other potential issue is that fixing the problem is not going to be easy if it ever does happen. I think part of the failure mode, if it happens, will be the posts tilting outward and the footers shifting underground. Getting everything back to plumb & true is going be be very costly. Sure, if you add a tension rod or cable at the top of the posts, and cut the posts from the trusses, you can probably pull the trusses back into shape. But the posts are now in the wrong position and the footers have shifted, how is that going to be fixed?

 

[GPintheMitten]

The more I think about it I like Curly Dave'so idea of a tension rod at the deck level. Seems like that would interrupt the forces trying to splay the posts out and at least not allow the footings to move.

If any movement was only above the torsion rod, it would be easier to fix.

If you did this, the connection method of the rod at the steel posts would need assure that the wall of the tube could withstand it. So maybe beef that up considerably at that junction.

 

[sld]

When certifying new aircraft one of the ways to validate stresses and loads is thru strain gauges during flight testing.

Can you rig up some very light wire as a way to monitor any movement? You don't need to know the forces involved just if there is any movement. I would leave them slightly slack and check periodically to see if there is any change.

Might not be a bad idea regardless of what you do.

 

[EddieWalker]

Or, if you are planning a grade-level patio, design in a railing between the two posts and hide the tension member in the top of the railing.

I like this idea if the OP is dead set against using a cable or your idea of a rod that is threaded at the end. I like that idea too!!! I guess that would have to be something that would be custom built at a machine shop. Considering that the posts are metal, probably just welding the rod on by a certified welder would be a good solution too.

 

[SirReal63]

The design of the collar ties is a concern to me. The are using tie plates under tension and that is not correct. Those plates are designed to hold wood members in place under compression, not tension as they are not designed to be used the way they are being used. I cannot imagine the truss designer being ok with the plates on the porch section, although it would be ok in the main house section. If it were me, I would add properly bolted collar ties over the existing ones, running across both the upper and lower cord of the truss. The only issue I see with the rod or cable at the bottom of the steel is a 1" movement at the bottom where the cable is would mean several inches of movement at the top, which may be enough to rip those tie plates on the collar tie out. As it is, it may be fine, those steel beams are strong but it is not the correct way to prevent the hinge at the truss/beam location and the existing collar ties do very little to prevent the outward force at the top of the trusses, a properly done collar will reduce those loads.

 

[KiotiKowboy]

The design of the collar ties is a concern to me.

The "ties" were not included for support - per se. They will add "some" strength, but that is just a side benefit.

The intent of the cross-members was solely for the look of the interior ceilings. We did not want the typical point at the top of our vault. You will notice the top of the great room window is cut flat to match the contour.

The trusses we have used were designed to ride on the support walls of the great room - and - with the expectation of a crossbeam on the front/rear porches. The engineers, and the design software, were completely satisfied at that point. Originally, the roof was to be constructed with scissor-trusses but that was going to lower our interior ceiling.

I like the idea of adding support concealed in the deck. And, I've not completely ruled out the cable - but the wife won't like it - lol.