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Question DetailsAsked on 1/5/2018

Can girder style roof trusses be supported underneath by steel columns? Any limits on where supports can be placed?

My garage has a girder roof truss (2 trusses nailed and glued). They support one side of an intersecting roofline whose trusses are connected via steel holders every 16inches to the bottom cord. Each bottom cord is 2x6. The ~35ft span girder truss is deflecting in the middle. I would like to put two steel posts in at roughly 12ft from each wall where the truss is supported to remove deflection while allowing optimal spacing for parking cars and fitting equipment through the garage. The main question that I have is that this is not directly under any webbing, is this OK? I figured that it should be since the walls would still be involved in supporting the girder truss at the ends. Thanks in advance for your thoughts.

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Not exactly, as I will explain in more detail in 4) below:


1) It would take a site inspection by a Structural Engineer (your Search the List category to find well-rated and reviewed ones) for probably about $150-300 to definitively answer this question - because unless the sagging is just maybe an inch or two at center (which would be normal in end-supported wood trusses), this could be an indication of under-design, over-loading due to snow drifting in the valley in excess of design parameters, wood failure (due to rot, insect attack, defective lumber, etc), or failing truss plates.


2) I have seen too many of failing truss plates - the pierced point plates (aka pronged truss plate or truss connector plate) used in most modern trusses are commonly put in using an assembly press to press the plate prongs into the wood, and due to what is in my opinion a major flaw in the building code do not normally have any screw or nail fasteners to hold them in close contact. When new-built (and tested) this may work fine, but after a number of annual temperature and moisture changes, especially in attics or garages or out-buildings outside the "conditioned space" - and also if they start rusting, the prongs on these plates can work their way out of the wood, with the plates coming loose and providing slack in the truss, resulting in swaybacking and in rare cases even structural collapse as the now partly backed-out prongs eventually tear through the wood fibers and fail. I have seen a number of these plates in older attics just lying on the insulation - leaving a portion of the truss with zero structural strength at that connection.


[Normal allowable sag for end-supported wood trusses 35' long would normally be about 1-1/8 to 3.5 inches at center, depending on design and code in effect, so it is quite likely the sag you are seeing, especially if older garage and more particularly if unheated, is NOT indicative of a structural problem - just somewhat disturbing to see. So an evaluation that the sagging is not structurally detrimental might void the entire project unless you are having overhead vehicle clerance problems - say with a tall SUV or pickup.]


3) You are basically right, assuming this is just normal wood sagging due to creep, that putting in additional intermediate supports should not "harm" things because the truss will still be supported at the ends as originally designed. However, putting in supports at the wrong place could cause local failure as the posts pick up load due to the sagging. As sagging progresses, the posts will pick up more and more load which was opiginally carried to the end supports by the truss, especially during short-term snow loading where the truss would normally sag down a bit more during the loading but then relax back to essentially its initial position when the snow melts off.


4) Putting the post under a mid-span of a bottom chord 2x6 chord could cause it to fail in bending (because it is designed to act only in tension in most truss designs, with the cross-bracing carrying the loads to the end of the truss). Also, putting a load bearing post under the truss brings in the issue of column stability - as it picks up load, it will have a tendency to "kick out" - meaning it could put some lateral load on the bottom chord of the truss which should then have lateral bracing put in to stabilize the truss from twisting or kicking "out of plane". In MOST cases putting the posts under truss plated joints, with a bearing cap (like a flat 2x4 a couple of feet long) on top of the post to spread the load out over the entire joint, would do no harm - provided it is put under a joint where there is a vertical and/or matched (one to each side) upward angled bracing coming into the joint from above - like at the center of this truss for instance:


http://www.vermonttimberworks.com/wp-...


But there are some odd designs out there where truss plants use computerized design to absolutely minimize the cost of the truss, and because you say this supports an intersecting roofline the loading on the truss may also not be symmetric, so you would be safer (and in compliance with code) to have a Structural Engineer check it out and give you a letter design sketch for a suitable fix.


5) One other thing - because these are intended to basically just limit swaybacking of the truss, and you are planning on using 2 posts over the 35' span, I would suspect just good quality (with no significant splits) 4x4's would prove to be plenty, using standard Simpson type metal brackets top (Teco nailed to the truss) and bottom (bolted to the slab) to hold it upright and prevent kicking out. Steel would likely be an unnecessary cost, and because of the 7-8 foot unbraced length would likely not be much smaller than wood because of the buckling issue. 3 or 4 inch pipe might also be suitable, though likely more expensive than wood unless you can thread the ends for bearings plates yourself.


6) One other consideration - come resale time these may be red-flagged by the buyer's home inspector, suspecting (correctly) that they are remedial and not part of the original design - so having a sealed (officially stamped with his professional engineer seal) Engineer's report and design for the supports, hopefully (if trusses are structurally OK as they are) stating that the intermediate columns are NOT structurally required and are being installed only to limit deflection and maintain overhead clearance, might help avoid a demand for remedial work on the roof framing.


Also, because they are going to obstruct parking clearance in the garage, if they are NOT needed structurally (and you have the documentation to that effect) I would make them readily removeable so you can take them out and patch the bolt holes in the floor (over expansion shell anchors say) if the buyer demands that as part of the sale contingency items. If planning for that, framing screws should be used to connect the top to the truss so they can be removed easily - Teco nails do not pull very easily.

Answered 10 months ago by LCD




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