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|Framing Repairs, Wood|
Damage to frames can be repaired by the use of sister frames though it is preferred that the frame be replaced if practicable.
The preferred type of sister frame is one of equal size to the damaged one and as long as possible. They should extend at least 18" or approximately four plank widths beyond the damaged area. This frame should be fastened to the planking and other structure with fastenings at least equal in size and number to those of the damaged member.
Care should be taken when recommending that sister frames be of greater size than the damaged frame they reinforce. The weakening effect of bending is inversely proportional to the square of the bend ratio. This means that using a sister frame which is deeper (larger in molded dimension) than the original frame will produce a more severe bend ratio in the sister frame, and may actually result in the sister frames being weaker than the original frames, despite being larger. Often the original frames broke because their bend ratio was too severe in the first place. Successful sister frames may be kerfed if necessary to ease the severity of the bend when that was the problem with the original frames. This greatly increases the effective tensile strength of the sisters without any necessity for greater cross-section.
It is important to note that bending sister frames into hard spots in the hull caused by broken frames may cause locally severe bends in the sisters, which will very likely cause them to break in service. If the hard spot cannot be corrected (this usually requires removal of the original frames), it is actually better to let the sister frame bend fair, spanning the hard spot and then to shim it to the planking rather than bending it into the hard spot.
Long sister frames, well tied in to the main structure of the vessel should not normally butt against damaged frames, though this is acceptable where it forms the best method of tying in the new frame. If the frames abutt, a good bedding compound or adhesive is required to exclude moisture from between the pieces.
Where structural or machinery interference or other reasons prevent fitting a long sister frame well tied into the other structure, a shorter "partial sister" may be fitted as a temporary repair. This should extend as far as practical on both sides of the damage and should be securely fastened to the damaged frame by bolting or equivalent means as well as to the planking and other structure. Provisions should be made to exclude moisture from between the pieces. Temporary repairs of this nature should be monitored closely, followed by evaluation for consideration of further repairs or acceptance as permanent repair. Unusual or nonstandard repairs accepted as permanent should be properly documented in the vessel's permanent file.
A good wood preservative is recommended for use on all faying surfaces. Ensure that precautions are taken that water cannot accumulate at the top of the partial frame and initiate decay. A sister frame should not be used as a repair for decayed frames. The decayed wood will eventually "seed" the sound wood with decay spores in spite of any attempts to prevent it by the use of wood preservatives or to isolate the new wood with sealing compounds. When extensive decay is present in a frame the only permanent repair is to replace it and any adjacent wood affected. If the decay is localized, or such that frame replacement is not practical, the decayed section of frame may be cropped out, and replaced with a new section, using a maximum scarf angle, suitable adhesive, and by mechanically fastening the new scarf joint. A sister frame of the appropriate dimensions may then be placed next to, and centered around the new scarf joint in the original frame. This repair may be considered permanent after proper monitoring and evaluation as previously described.
Where frame damage is evident but sister framing is not practical, consideration can be given to installing interframes between the affected frames or to strengthening damaged or weakened frame areas with fitted metal frames. Such repairs require excellent design considerations and workmanship and should be undertaken with caution.
Decayed Frame Heads
Heads of frames under covering boards often become decayed due to lack of ventilation and accumulation of fresh water leakage. With sawn frames, this can be corrected by replacing the upper futtock. If the futtock is long or the frame is in one piece, it can often be cropped off well below the rot (at least 2 feet is a good rule) and a piece spliced in using a glued and screwed scarf joint of proper dimensions. As an alternate measure a lap joint of sufficient length may replace the scarf. Repairs to more than two adjacent damaged frame heads should not be made by short cropping but should be made by renewing the frames or replacing the damaged sections by scarfing and then sistering the frame.
One of the principal causes of frame head decay is entry of water from deck leakage or condensation into the exposed end-grain at the head of the frame. This problem can be reduced greatly by angle cutting the frame tops slightly short of the underside of the deck, leaving a 1/8" to 1/4" space for ventilation, and, most importantly, by painting the end grain of the frame heads to prevent entry of moisture. The slight gap between the frame heads and the deck also ensures that if the sheer strakes should shrink slightly, the covering boards (margin planks) will not be lifted off the shear strakes by the frame heads.
Excerpted from Wood Hull Inspection Guidance (NVIC 7-95)