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Wood Trussed Roof with Cable instead of Bottom Chord?

Wood Trussed Roof with Cable instead of Bottom Chord?

Wood Trussed Roof with Cable instead of Bottom Chord?

(OP)
I am currently engineering a single family residence in California where the architect is specifying a tension cable instead of a bottom chord. Also, the top chords are going to be visible from below, so all connections have to be visually "pleasing".

The span from bearing wall to bearing wall is about 22', and the loads on the top chords are: D=120plf, L=120plf, & S=504plf.  Also this is in a high seismic area (UBC Zone 4), and the UBC wind speed is 70 mph (yes, the IBC has not been adopted here yet).  The pitch of the roof is only 3:12, so there is not a whole lot of room to play with.

Are there any good references showing how to specify sensible connections at the ridge (between top chords) and at the bearing walls (between top chord and cable), or even better: manufacturers who have approved engineered systems?  

I know how to perform the static analysis of the problem, that is not the issue...

Thanks,
CSH
 

RE: Wood Trussed Roof with Cable instead of Bottom Chord?

The connections are the least of your problems.  How do you handle load reversal (uplift due to wind) in a cable?  

RE: Wood Trussed Roof with Cable instead of Bottom Chord?

The only time that I ever saw this done they went to a cable mfg (Crosby) and had cable assemblies made that used pins to anchor the ends.
But Hokie has the real question.  The building that I recall was built to create significant tension in the cables at all times.  In fact if you cut the cables the walls would fall outward.
I have trouble picturing this working with seismic loading rules.

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RE: Wood Trussed Roof with Cable instead of Bottom Chord?

(OP)
hokie66,

Well, that is a connection issue too!  With rafters (similar to a truss top chord), I would typically specify some sort of seismic/hurricane tie connecting the rafter/truss to the top plates.  However, with a rafter, you have the ridge beam to take care of the connection at the ridge.  What do you do when you don't have a "standard" truss with a king post or some other webs connecting the ridge to the bottom chord?  The cable will be in tension, just like a regular bottom chord, but you cannot have any point loads on it.

CSH

RE: Wood Trussed Roof with Cable instead of Bottom Chord?

They won't be point loads; they will just be tension or compression members.  The bottom chord of a truss made with traditional materials would still be in tension, the same as it would be with a cable.  The cable just has to deflect until it develops the internal force necessary to reach equilibrium with the external forces.

As hokie said, load reversal will be very important.  Perhaps a double top chord with web members between the two chords would help with that, that would span like a little truss for uplift purposes.  

RE: Wood Trussed Roof with Cable instead of Bottom Chord?

Can you use a load bearing ridge beam to take out the uplift?

RE: Wood Trussed Roof with Cable instead of Bottom Chord?

Here's a "rod" truss we did some time ago:



RE: Wood Trussed Roof with Cable instead of Bottom Chord?

That rod truss is really neat!

RE: Wood Trussed Roof with Cable instead of Bottom Chord?

StructuresDesign-
I think the point Hokie66 was trying to make is that in a load reversal (uplift) condition, the bottom chord (your cables) will be in compression.  Obviously a cable can't support any compression load.

RE: Wood Trussed Roof with Cable instead of Bottom Chord?

(OP)
JAE:  yeah,  I was also thinking that a rod system would be much better.  That way there can be webs, etc., just like with a "standard" wooden truss.  BUT, the architect REALLY wants cables...  And I am just trying to figure out a way to give him what he wants.  Also, with a 3:12 pitch, there is only about 2.5 feet of height to fit in all the hardware, etc.  To make things even more complicated:  the roof ridge runs at a diagonal, so every single truss is different.  It would therefore be ideal to keep the number of pieces in each truss to a minimum.

StructuralEIT:  clearly, other portions of the roof structure must be capable of handling the seismic & wind loads.  The only "job" of the tension cables should be to prevent the top chords from "slipping" due to gravity loads - and given the seasonal variability of the gravity loads, this must be done carefully so that the cables don't sag in the summer (no snow load).  I understand that this means very large top chords and strong connections at the ridge and walls, ensuring that the two top chords essentially work as one unit and that the roof is appropriately connected to the walls.  

Anyway, does anyone know of a project where this has been done?

CSH

RE: Wood Trussed Roof with Cable instead of Bottom Chord?

StructuresDesign-
I am not talking about lateral loads.  I am talking about simple uplift from wind on the roof.  If the angle of your roof is less than 45 degrees, you will have uplift on your roof due to wind (completely separate from wind/seismic lateral loads).  Unless your dead load is enough to counteract this wind uplift, the bottom chord will go into compression.

RE: Wood Trussed Roof with Cable instead of Bottom Chord?

(OP)
StructuralEIT: yes that is why I would specify hurricane/seismic ties (having both uplift & lateral capacity) for every rafter/top chord.  In this area, the seismic loads generally control, and the hardware I specify to handle the seismic loads will typically be able to handle both lateral and vertical wind loads.

CSH

RE: Wood Trussed Roof with Cable instead of Bottom Chord?

StructuresDesign, how do hurricane ties help with a bottom cable in compression?

RE: Wood Trussed Roof with Cable instead of Bottom Chord?

Structuresdesign,

Just because the architect wants it to look that way, doesnt necesarily mean that you have to design it that way.

Do you have any options for steel/timber framing above that can be hidden in the ceiling space? For example, a ridge beam with timber rafters down each side.

The truss could then be architectural and wouldnt need to do anything.

Alternatively, a hybrid system where the higher dead loads are taken out by the cabled truss, but the ridge beam is capable of resisting the uplift.

or make the ridge beam a truss with timber bottom chord and verticals and cable diagonals.

hope this helps you think of an innovative option.

regards
csd

RE: Wood Trussed Roof with Cable instead of Bottom Chord?

I think the best option is simply to ensure enough dead load on the roof to negate the uplift.  Even the rods in our trusses (see picture above) never went into compression (with 0.6D + W).

RE: Wood Trussed Roof with Cable instead of Bottom Chord?

JAE is correct in that one way of solving your problem is to ballast the roof so that the bottom chord is always in tension.  Depends on your wind loading as to how much concrete or something on the roof that would take.

I would lean rather to csd72's approach.  Maybe sandwich a steel plate between two timber members for the rafters, then let the architect have his cable for decoration.  Not structurally honest, but then it is the architect who is being silly.

RE: Wood Trussed Roof with Cable instead of Bottom Chord?

StructuresDesign-
I am completely failing to see your reasoning on this.  What I am talking about has nothing to do with LATERAL wind/seismic forces.
That is fine if your hurricane ties handle the uplift at the ends of the truss, but if your truss has a net uplift on it, the bottom chords WILL go into compression and the cable can't support that.  
I am clearly not following your line of thought to neglect this condition.

You say, "clearly, other portions of the roof structure must be capable of handling the seismic & wind loads.  The only "job" of the tension cables should be to prevent the top chords from "slipping" due to gravity loads - and given the seasonal variability of the gravity loads, this must be done carefully so that the cables don't sag in the summer (no snow load).  I understand that this means very large top chords and strong connections at the ridge and walls, ensuring that the two top chords essentially work as one unit and that the roof is appropriately connected to the walls."  -  Based on this it sounds like you may be attempting to make the truss not really a truss and have sloped beams (continuous top chords) with a moment connection at the ridge.  If that is the case, then I don't see the need for the OP as the bottom chord would be only architectural anyway and wouldn't be required to do any work.

RE: Wood Trussed Roof with Cable instead of Bottom Chord?

Have you thought about an A-frame truss to take out the wind load, cable would then be useful to prevent gravity load spread (would need to check inward spread for wind loads though).

StructuralEIT makes a very good point about the cable going into 'compression' under net uplift. As you know cables dont do compression.

RE: Wood Trussed Roof with Cable instead of Bottom Chord?

If you are going to have the cable go into compression, just determine an alternate means to support the roof for uplift.  The truss does not have to do both.

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