Strap Reinforcement of Existing Rafter
Strap Reinforcement of Existing Rafter
(OP)
I need to reinforce existing rafters for the addition of solar panels. The simple solution of sistering another rafter along side the existing one isn't an option due to confinement issues of the attic access (rafter is too long to fit).
My solution:
Attach a Simpson coil strap "CS14" (1/16"x1-1/4" steel) at each end and run it under blocking that would be between the strap and the rafter to create a deeper beam (see detail).
My Process:
•Find required section modulus by solving Mmax = Fb' x Sreq.
•Transform the steel to the equivalent wood section.
•Determine the neutral axis & new moment of inertia for the transformed section.
•Compared the section modulus above & below the NA to the required S
•Used the allowable deflection to determine the required moment of inertia & compared it to the one for the new transformed section.
My Question:
•Does anyone see a problem w/ this solution and is there anything I overlooked?
My solution:
Attach a Simpson coil strap "CS14" (1/16"x1-1/4" steel) at each end and run it under blocking that would be between the strap and the rafter to create a deeper beam (see detail).
My Process:
•Find required section modulus by solving Mmax = Fb' x Sreq.
•Transform the steel to the equivalent wood section.
•Determine the neutral axis & new moment of inertia for the transformed section.
•Compared the section modulus above & below the NA to the required S
•Used the allowable deflection to determine the required moment of inertia & compared it to the one for the new transformed section.
My Question:
•Does anyone see a problem w/ this solution and is there anything I overlooked?






RE: Strap Reinforcement of Existing Rafter
I did a quick and dirty analysis of the geometric properties of a SPF #2, 2x12 with a 14GA(.0747")x1.25" plate attached to the bottom and determined the following:
-"I" increased by 38%
-"S" top for the transformed section increased by 23%.
In the past, I have found the weight of solar panels attached to a pitched roof to be relatively light (less than 5psf).
You should be able to justify the existing rafters by connecting the straps directly to the bottom of them. Be sure your analysis is two parts in that you should analyze the rafters for the existing loads based on the section modulus of the existing beam only. After that, you should analyze the rafters for the weight of the solar panels and live load with the transformed section.
Good Luck!
RE: Strap Reinforcement of Existing Rafter
The only change I would make would be to twist the strap at each end , nailing the strap to the side of the rafter rather than the bottom and clinching the nails. Using this approach, you could use two straps side by side if needed.
You know though, sistering could still be an option here as you can sister on smaller members rather than the same size. The calculations are more involved, but it may be simpler. Similarly, the sistering does not necessarily have to be full length.
Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
RE: Strap Reinforcement of Existing Rafter
BA
RE: Strap Reinforcement of Existing Rafter
Larsacious:
I agree usually the small increase in DL of the solar panel is offset by the decrease in SL from changing the roof surface to a slippery surface. If the panels were to be mounted to every rafter there would be no problem I would absolutely make the "net reduction in load" argument, but the problem arises when the mounts skip one or two rafters and concentrate the load on a single rafter.
msquared48
I know it seams minimal, but are you ever concerned that twisting the strap and attaching it to the side will introduce torsion in the rafter? Granted, it could be resisted by blocking, however I'm trying to make this as efficient as possible & more blocking, means more labor, which could make this option less efficient then another.
BAretired
I had intended use wood screws instead of nails to attach the strap. The first screws would resist the pull out force (via NDS) and the following nails would transfer the tension in the strap to the rafter.
RE: Strap Reinforcement of Existing Rafter
RE: Strap Reinforcement of Existing Rafter
The proper analysis is to apply one sloping force at each end and one vertical force at each block. The compatibility condition is that the deflection of the joist at each block is equal to the vertical deformation in the steel strap under combined loading.
Another point to consider is temperature effects. A change in temperature will affect the steel strap more than the wood joist.
BA
RE: Strap Reinforcement of Existing Rafter
There is a problem with your solution. It is not correct. While it may not make a great deal of difference in the particular situation which you describe, it is important to note that the method is fundamentally wrong.
BA
RE: Strap Reinforcement of Existing Rafter
RE: Strap Reinforcement of Existing Rafter
I agree that technically this in an indeterminate structure and should be analyzed as such. However if I were to try and invoice the client for that I doubt I'd ever get another job. As right or wrong as that may be it's the nature of the beast. I'm just trying to provide my client with the most efficient overall process I can. If the cost for extra analysis will save my client money during construction then that's what I'll recommend. I just can't see that as the case here.
I did look into the thermal expansion, and with a temperature change of 150 degrees for this span the differential change in length it only 0.11"
SpecialEddie
In this region (Massachusetts) roof live load rarely governs it's usually snow load which still needs to be included, although it usually does decrease enough to offset the additional load form the panels. The problem arises when the panels aren't supported by every rafter but every other one, or every third one. This is due to the contractor's preference to reinforce the rafters instead of having additional roof penetrations that may leak over time.
RE: Strap Reinforcement of Existing Rafter
Without running numbers, it feels to me that the strap and screws (not being prestressed) would have to stretch a bit before taking any significant load. What happens to the system if say each end of the strap deflects/stretches even 1/16"? Does that mean the existing rafter would have to deflect (i.e. take a larger share of load) before the composite behavior is engaged?
RE: Strap Reinforcement of Existing Rafter
RE: Strap Reinforcement of Existing Rafter
RE: Strap Reinforcement of Existing Rafter
BA
RE: Strap Reinforcement of Existing Rafter
In a normal "stick" built house, the addition of a strut could be easily accomplished. This would cut down on your span which would greatly increase the strength of your rafter (although you would add more weight to other portions of the structure.
RE: Strap Reinforcement of Existing Rafter
Good point about the pre-stressing of the steel (thanks PMR06). I think that the additional cost of hold downs, threaded rods, & connectors vs. only steel strap might make this option less efficient than another solution unless I can minimize the locations that need reinforcing, but that's something I'll need to run the numbers on. It the rafter is jacked up during the strap installation to relieve the stresses, then when the jack is removed the rafter would deflect and pre-stress the strap. I know it's mathematically the equivalent of shooting from the hip, but what is everyone's thought on that?
SteelPE
That would definitely resolve the rafter issue. However (as you mentioned) the new load would have to be accounted for in an existing house, and it seems people want to avoid that like the plague.
RE: Strap Reinforcement of Existing Rafter
I've been wrong lots of times before, but I thought you only had to sister the middle half.
Mike Halloran
Pembroke Pines, FL, USA
RE: Strap Reinforcement of Existing Rafter
RE: Strap Reinforcement of Existing Rafter
The prestressing concept is the correct way to handle what you showed in your sketch, and I thought that was what you had in mind, so I didn't go through your transformed section verbiage in detail. The difficult part is the end connections of the straps to really allow the strap tension to be induced. There is considerable relaxation at those connections which tend to defeat your tensioning. You've really got to think that out and over design that detail. And, Haydenwse has about the right way to induce the tension on this type of job. Otherwise, it's a bit of seat-of-the-pants engineering: since the length change of the strap, due to the blocks implies the strap tension; but the upward deflection of the rafter and connection relaxation reduce that tension. A system with the blocks (stand-offs) and a turnbuckle allows you to watch the rafter deflect and connection movement during the tensioning process.
You have an interesting (dangerous to yourself) engineering philosophy, where you are getting your tit surprisingly close to the wringer to save the client money in terms of your fees. Nobody is suggesting that you way over engineer the problem and solution, but you've got to get it right. You have to really think it through, and have a real leg to stand on; that is calcs. and concept well thought out, etc. including some notes on the approx. nature of the entire process. Because, even if you save the client some fees, by doing a half assed engineering job, when your solution goes south, you can be sure the client's attorney won't remember that the client wanted a solution on the cheap. They need your help and should be willing to pay a reasonable fee for that help. These jobs are usually interesting, but they are kinda dogs for all of us; gosh it's a simple problem, why does it cost so much; if it's so damn simple why do they need you? One always wonders if a $500 job, that really should have been billed at $1500, is worth the $3000+ it could cost you to defend yourself, all for having saved the client money, no mention of solving his problem practically and economically.
RE: Strap Reinforcement of Existing Rafter
I'm more conservative than I appear to be in this post although I appreciate the concern. I am guilty of trying to think out better ways to accomplish something, however I do know when to jump ship. After reading Haydenwse's post I really don't feel comfortable pre-stressing the strap w/ blocks & wedges. I have previously used turnbuckles and used torque to get some indication of the tension in the threaded rod so I might still recommend that as an option to my client. Unfortunately, due to the additional cost I think we'll be right back to square one and open up the roof to get lumber in the attic and sister the existing rafters.
I love sending invoices like these out.
Thanks for everyone's help.
RE: Strap Reinforcement of Existing Rafter
If that doesn't work, is there anything else that can be done? Have you really dug into the code to see if you can gain any additional capacity or reduction in load? I know Massachusetts had a code change a few years ago. I also know that they have made multiple revisions to Chapter 34 pertaining to the addition of solar panels to an existing roof.
If you have exhausted all possibilities then I guess you truly are left with ripping the roof off.
RE: Strap Reinforcement of Existing Rafter
Determine how much tension (P) you need. Length (L) is the distance between the ends of the anchorage zones. The Moment of inertial (I) is the biggest challenge because you need to get an effective I due to the presence nail holes along the strap but you can make a good approximation with a ratio of (L*net I / L*full I). Once you determine how much elongation you need, some high school trig will tell you how tall to make the blocks to get that elongation.
Not very difficult to calculation and install, yet very accurate.
RE: Strap Reinforcement of Existing Rafter
I don't think it's an issue of ripping the entire roof off, just an opening in the roof or siding (maybe a gable vent) to pass the lumber into the attic. Which, by the way is starting to look like a better and better option.
Haydenwse:
I'll have to check out that section of the commentary.
My concern w/ the using the tension in the strap is that I was designing the strap to be 7-1/4" out from the bottom of the rafter. In order to obtain that distance, the strap would have to be attached w/ a lot of slack in it. My concern is that if I give them;
•the length of strap required between attachment points,
•the distance (along the rafter) between the attachment points,
•the depth of and distance between the blocks with the precision required to determine the tension,
is this going to be do-able from a construction stand point or is it going to be like giving out a foundation plan w/ dimensions that have 32nds of an inch on it?
RE: Strap Reinforcement of Existing Rafter
Just an idea - could you add bridging between the rafters along the fastener lines of the solar panels to share the load among the other rafters?
RE: Strap Reinforcement of Existing Rafter
If you run the strap over a pair of 2x8's I would ease the edges to avoid a hard bend in the straps. You will also want to fix the blocks in place with plywood gussets on each side. I would also suggest running a short section of strap nailed to the sides of the rafter over the in-board end of the anchorage zone to take the vertical component of the tension. I personally would not trust a couple of screws in the last holes for that.
The suggestion from bones206 of distributing the load to adjacent rafters on the inside will also work. I wouldn't use bridging though. I would install a strong-back to the under-side of the rafters at each intersection. Depending on the load, the connection could probably be made with Strong-Tie TS straps.
RE: Strap Reinforcement of Existing Rafter
I don't agree that it is that simple. In a perfect world yes, you could calculate elongation. This isn't a straight item (I didn't look it up, but I assume it is a coil strap). The amount of elongation would probably not rather small, and there will slack from installation and slip at the end anchorages. That would make calcing elongation a bit of a crap shoot.
If using a turnbuckle (and threaded rods), after the slack is taken out of the system you could calculate number of turns of the nut that would be required. That would give a little more accurate measurement of PT. But it seems to me there has to be an easier way to skin this cat (interior partitions, partial length sistering, etc)
RE: Strap Reinforcement of Existing Rafter
You are kinda missing the point, Haydenwse is still right on the money, I generally agree with him. Except to add, reread my first post, the correct thoughts are there, but you have to read btwn. the lines a bit, so let me try it with different words. Your original scheme is O.K., but you are thinking about it from the wrong angle. However you do this, at this stage, it is a bit of a seat-of-the-pants process, but with some good engineering judgement and attention to detail you can arrive at a satisfactory solution, albeit not an exact solution. Applying the strap to the bot. of the rafter by hand, even over your blocks, would buy you nothing. The rafter would have to deflect too much to bring the strap into play and to overcome the relaxation at your end connections. And, the strap would only start picking up added load after that delta. Jack the rafter up enough so can call it a flat arch, apply tension to the strap by mech. means, nail it off, remove the jacks, and you have a tied arch and can make some attempt at calc'ing. its thrust (tension in the strap) and watch the end connections relax and the arch settle, again an approx. which adds some cap'y. to the rafter.
Torque on the turnbuckle is an approx. for the reasons AISC and Haydenwse suggest, friction, etc. But, if you know the pitch of the threads, then three full turns of the turnbuckle after you release the wrench and the rods or strap detwist, means so much elongation of the rod, which suggests a tension. T = E*A*rod elongation/rod length. Except, upward deflection of the rafter, slack in the rod, and end connection relaxation effects must be subtracted from your final tension calc., and these are somewhat unknown, thus your engineering judgement is needed, for a final approx. A string line on the rafter gives you upward delta, tick marks on the strap and rafter at each end gives you their movement, but add a little more because this is a long duration loading at the connection, thus more relaxation over time. After you've done a couple, I'd have a good feel for the rafter delta, but I'd continue to watch the end connection movement. For example, using 6" blocks, 4' on either side of the center of the rafter gives you rod or strap geometry, you make a reasonable estimate of the strap tension, and this imparts two upward forced on the rafter, increasing its moment cap'y, as long as it can take the new axial compression and stresses around the end connections and they don't relax too much.
Haydenwse's wedges and blocks method works too. I would apply some mech. tension to the strap to remove any slack, and nail it off at the ends. Now start inserting wedges 4.5' from the rafter center; watch the rafter delta upward and the end connection movement, and calc. the tension. The new strap geometry will suggest an approx. change in length of the strap once the above unknows are included, and thus the upward forces a 4' to give you upward moment you want. Insert the proper ht. blocks at 4' from the center.
RE: Strap Reinforcement of Existing Rafter
I also agree that the turnbuckle idea will work just fine. You just can't use torque to determine tension. You have to go back to elongation. The biggest deterrent to that system is that it is very expensive compared to other alternatives.
RE: Strap Reinforcement of Existing Rafter
dhengr:
I love the idea of using the pitch of the thread to gage the tension.
I'm pretty sure I see what you mean, but just to verify, let me run through the process.
•Install the strap as tight as possible at approximately the depth away from the bottom of the rafter.
•Snap a chalk line along the rafter to later measure upward deflection.
•Put a couple of tick marks at the ends to later measure the slip at the anchor points.
•Start inserting temporary wedges slightly further from the center then the location of the permanent blocks.
•Continue inserting wedges to stress the strap to the tension required (while subtracting losses due to upward deflection, and slip at the anchor points.
•Once we get the tension required, cut and install the permanent blocks and remove the wedges.
I think I prefer the rods to the straps, but Haydenwse is right I think they are cost prohibitive.
RE: Strap Reinforcement of Existing Rafter
RE: Strap Reinforcement of Existing Rafter
SRO:
You're heading in the right direction. The thread pitch/turn of the turnbuckle works reasonably well, with a reasonable degree of confidence, when you start out with a slightly pre-tensioned rod (any sag already removed), with end connections which do not slip or relax, and where rafter deflection is not too great wrt the tension applied; since all of these are factors which you have some trouble accounting for in an exact fashion, on this problem. On larger jobs, the saddles which harp the tendons and the tendon end bearings are fabed. and much less yielding than what you have in your problem. And, they actually pull the rods and measure their elongation or read the pressure on the pulling jack, then run nuts tight onto the bearing to set the tension. You must pre-tension the strap a bit; whether it's flat to the bot. of the rafter (then you use wedges to move it away and really tension it) or over precut blocks and you move them longitudinally, you don't want to use half of your tensioning movement just to take the slack out of the strap.
All of the calcs. are done before hand so you know how to design your end connections, and they are the most difficult part of the problem. You can't get enough nails in them, and their movement is so critical. We normally think nothing of a 1/32 or 1/64" movement in a wood framing joint, but that's a lot in terms of steel elongation and final tension. What tension change does 1/16" elongation change cause? What elongation or length change is caused by a 1/16" rafter deflection change? Without knowing your rafter length, new loads and exact geometry, you will find that the strap tension gets pretty large to impart 100 or 200# upward on the blocks. Thus, the end connections become difficult quickly. The end tick marks must be sharp and accurately done because you will be looking for fairly small movements, with large effects. Do a calc. with 100 or 200# block loads, what deflection change does this cause? Snap the chalk line, or make a few fine marks, and leave the tight string in place so you can actually watch the deflection as you do the tensioning. As much as anything, this deflection might be the best indicator that you have achieved the result you wanted. But, you have to make adjustments on the fly, during the tensioning process, so you have to make tick mark readings on the fly too. Finally, after factoring these things together, you make the judgement that you have enough tension, and then hope no body asks you exactly what the tension is or will be in a year. But, again, think conservative end connection design, and remember nails in shear parallel to grain are not as good a nails perpendicular to the grain. And, on the first couple rafters I would check these movements and deflection changes for a few days to see that they hadn't changed much.
You could use aircraft cables, one on each side of the rafter. Then you need matching thimbles, cable clamps, and long turnbuckles, but this might still be no more expensive than a good grade of strapping and all the problems associated with its end connections.
RE: Strap Reinforcement of Existing Rafter
RE: Strap Reinforcement of Existing Rafter
The issue of losses of prestress can be a bit overwhelming the first time you look at them. That is why I suggested in an earlier email, that maybe a strong back under a group of rafters to distribute the load out to additional joists might be a simpler solution. There again, the challenge is stiffness. The strong back should be designed for stiffness instead of moment capacity.
RE: Strap Reinforcement of Existing Rafter
As to the issue at hand, I'm adding lumber... .02