Jumbo hut closed till further notice

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20 July 2017: Jumbo Hut closed Jumbo Hut is closed until further notice – an engineer’s report has found the hut is unsafe. Mattresses and gas have been removed. . http://www.doc.govt.nz/parks-and-recreation/places-to-go/wellington-kapiti/places/tararua-forest-park/things-to-do/huts/jumbo-hut/

Popular DOC hut closes because it may be unsafe in extreme winds or snow https://www.stuff.co.nz/environment/94967538/popular-doc-hut-closes-because-it-may-be-unsafe-in-extreme-winds-or-snow

second hut to succumb to the Tararua wind, recently. Luck jumbo didn't end up like old angle knob - down the bank

Which other hut succumbed recently @tararuahunter. Or do you count angle knob as recent?

A few days ago I queried DOC for the Engineers' Report on Jumbo, and have just heard back. (Full names removed just so they don't linger on the internet forever given I know this is an issue that inspires some nasty comments here and there, but contact me if you want the email.) ===== Hi Mike, This is the advice we got from our engineers. Reading between the lines, I think the structure still meets the specs it was built to in 1981, but no longer meets the standards of the latest building code for dwellings in an extreme alpine environment. Cheers [Name 3] Assyst request 34456 lodged by [Name 1] seeks an independent Engineer review of the safety of Powell and Jumbo Huts in the Tararua Ranges. [Name 2] recently asked me to review her work on Jumbo Hut. As discussed with [Name 2] earlier today I have serious concerns about the safety of Jumbo Hut for ongoing visitor use. Having looked at the hut plans, her recent photos and information in AMIS covering maintenance and upgrades since 1999, and carrying out a few very rough order calculations my conclusion is that the building is seriously under capacity to resist probable wind and snow loads. This assessment is based on the following: Wind Load: NIWA have provided design wind loads that take into account the local site conditions, surrounding topography and elevation above sea level. These appear to be a realistic assessment of the actual site conditions in accordance with AS/NZS1170:2011 Structural Design Actions. Assuming a 6 month design life for a building of Importance level 2 requires a design wind return period of 1 in 100 years. The critical wind speed is from the Southerly direction at 65.5 m/sec (235km/hr). There is no sheltering from this direction. A wind speed of this order is virtually the same as for cyclonic regions in Australia. I also note that this hut was originally constructed after Angle Knob Hut, located approximately 2.5km away at a similar altitude, was destroyed by extreme winds in 1981. Snow Load: There is anecdotal evidence of snow accumulating around the building almost up to the eaves at this site. AS/NZS1170 prescribes the open ground snow load (Annual probability of Exceedance of 1/50) at 2.23kPa and the roof snow load at 1.4kPa. This equates to approximate snow depth of 520mm open ground, and 330mm on the roof. Again these loads seem quite realistic. Subfloor Braces: The capacity of Anchor Piles and/or Braced Pile sets is well researched and documented in NZS3604:2011. A braced pile set is two piles with the tops connected together by a joist or bearer, and a diagonal timber brace fitted across the two piles. If detailed according to NZS3604 requirements this “set” has a horizontal capacity of 160 Brace Units or 8kN (20 brace units = 1kN). See Table 5.11 and commentary clause 5.12. Both piles need to have a 12kN fixing between the pile and bearer or joist (fig 6.6). In the N-S there is only one diagonal brace. For a 65.5m/sec wind speed the building requires at least 7 braces in this direction so currently has only 13% of the calculated bracing demand. Wall Bracing: I haven’t done a detailed calculation on this. Bracing may be OK in the N-S direction as there are relatively long lengths of wall with no openings and sheet material 1 side. But I doubt there is adequate bracing capacity in the E-W direction. There are very few brace walls in this direction and they would need to have tie-down straps fitted to the corners to provide effective bracing. Tie Down cables: The tie down capacity is not governed by the cable strength – the most likely failure mode would either be the deadman anchor pulling out of the soil or the bracket attachment bolts pulling through the wall. We don’t have records of exactly how the tie down cables are anchored into the ground or how the brackets attach on the inside of the walls. From [Name 2]’s calculations it appears the existing cables would need to have a capacity of around 12 or 13kN to provide effective tiedown against wind uplift. My experience with this type of anchor is that the capacity is usually less than half this value. Buried galvanised cables can often be subject to accelerated corrosion leading to unexpected tiedown failure. Since these are critical elements in resisting wind uplift and overturning of the whole building we need to have a much greater level of certainty of their capacity. Even adding more tie down cables is not particularly straightforward as these would require careful detailing of the bracket fixing both outside and to the inside face of the wall. Note that there are intersecting walls at some of the most likely bracket locations where it will be problematic to bolt through the top plate. Roof structure: The roof structure is built mostly from 100x50 timber with what appears to be only nailed connections. I believe it would be very difficult to show this has adequate capacity to resist snow loads and wind uplift. Some of it might be OK but the weakest section will always go first and take the rest with it. For example the roof over the bunks has rafters at 600cs spanning around 4.1m up to the ridge board. At least two of these do not have a collar tie. So they have less than half the required capacity in simple bending. The ridge board loads onto the centre of one truss. I would expect these sorts of joints would have loads well in excess of the capacity of two or three nails. Similarly the uplift fixings of the roof onto the top of the wall would need to have a capacity of 5kN or more – well outside the capacity of 2 skew nails. We don’t know if there is some sort of bracket fixing because it is hidden behind the wall lining but given the lack of brackets, straps and tie plates generally in the roof I think it would be unwise to assume they have been used for the rafter/wall plate joints. My impression from looking at [Name 2]’s photos is that the building has been built by staff unaware of just how high the wind and snow loads actually are, and with no Engineer (or Territorial Authority) input. There are hardly any brackets, straps or nailplates used to strengthen joints. The construction just looks too light. The lack of subfloor bracing leads me to doubt that NZS3604 (Light Timber Framing Code) was used to inform the construction (notwithstanding that this building is well outside the scope of NZS3604). We already have anecdotal reports of the hut moving and shaking in the wind and although it has been there in its current form since the early 90s this is no guarantee that it can resist the next storm. My advice is that extensive strengthening work would be required to bring the building to a safe condition. This would include testing and/or replacing tie-down cables brackets and anchors, strengthening rafters and end fixings, removing roof sheeting at the S end and strengthening purlin-rafter fixings and roof sheet fixings, locally removing wall linings and adding anchors at panel corners, adding additional fixings to roof truss joints, and fitting diagonal braces under the building. This is only a minimum scope of work – it is possible more extensive strengthening may be required once hidden joints can be exposed and checked. Very rough order of cost estimated to be $30 to $40k based on a 5 year life until replacement. I think this hut is actually more of a risk than Powell Hut. Leaving it open with a view to replacement in a “few” year’s time is not an option I would be happy with. I believe Jumbo Hut is potentially unsafe in its current condition and should be closed. On this basis I have added a Critical notification into AMIS for this equipment. regards [Name 4] ENGINEER - Structures

@madpom, Powell hut is going to be replaced (next year I believe). An engineer's report found Powell hut shouldn't be used in winds exceeding 180km/h. DOC has been holding some community consultation recently as to how the new Powell hut should be configured.

The test to 235kph wind as a 1 in 100 year event. This has been exceeded twice at Hawkins hill so might not be excessive after all

I remember a night in Tarn Ridge hut in the 80's. We were sitting either side of the table with our elbows on the table top to keep it from rocking, once of us was holding the primus an the other was holding the pot on top of it while it heated up dinner. The whole place was rocking so much you couldn't keep anything on the table without holding it down, let alone a pot on to of the cooker. I remember being very impressed (and relieved) that it didn't disintegrate completely it was moving so much.

builder mate of mine who knows the hut well. he says there has ner been winds ecorded of 235k, kaukau has recorded 205k's. and the hut is already built to a standard to stand up to the winds exp;erienced up there, and it could easily be brought up to a bettter standrd by being braced with cables and deadmen burried in the ground

@madpom was referring to Powell. Makes 2 recent, and one in 1981 or so

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Forum Tracks, routes, and huts
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On 21 July 2017
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