huangyhg

rafter joint
we are constructing a steel framed building with rafter spans of 24m. the steel has been fabricated in the factory by full strength butt welding of 2 x 12m long beams. this fabricated beam has then been delivered to site.
our client now claims the beams should not be welded in the middle. i am aware that for rc construction joints should be at 1/3 span or an area of low stress. however for steel construction i am not aware of any such requirement in fact it would seem unusual to me to contemplate cutting beams to introduce additional joints at a location other than midspan.
am i wrong, is there any code requirement to locate joints away from midspan? alternatively is there any statement anywhere that this is ok...
check out our whitepaper library.
did you ask the client why, specifically?
as far as i am aware, if a full penetration butt weld is used in a welded splice, then you can assume that the weld achieves the full moment capacity of the section.
incidentally, what type of construction is this? is the rafter simply supported, in which case the maximum bending stress will be at mid span as you state, or a portal or moment frame, when the moment at mid-span will be less under vertical loads?
vb
i would think that this is a case of the client needing his consultant's specialist training. ie: you need to (very politely, and quite gently) point out that there is no reason for needing these joints, and explain the actual mechanics of the system.
i am certainly not the most experienced poster here, and i will be quite curious to see what some of these guys have to say, however i cannot see a reason for a joint. even if you calculate the thrust due to thermal expansion and use it as a load case on your wall (never a bad idea when someone brings something like this up), i would hazard a guess that it's going to be minimal and, therefore, inconsequential.
keep us up to date!
regards,
ys
b.eng (carleton)
working in new zealand, thinking of my snow covered home...
young.. i think they joint it because they only have shorter
i believe since you are designing the building that coengineeer may be right. the contractor may prefer to handle two shorter members instead of one long
i wonder how they transfer 24 m beam (shop welded) to the site? that is pretty darn long. are you sure its not field bolted? average flat bed truck in the us is 48 ft long. 24 m is way longer than that.
this is obviously not a us thread as it is given in metric (oh how i miss the simplicity of metric!)anyhow, in australia, for example, you cannot get stock lengths of steel over about 18 to 21m so most portal frames require a splice.
the splice is normally done as a bolted connection for transport and to avoid site welding. it is even better structurally if it is fully welded (butt weld in the flanges, fillet weld in the web).
as the structural engineer, you need to point out to the client that this is a perfectly acceptable way of joining two pieces of steel together. point out that some welded beams are actually formed by welding plates together into an i shape.
regards
csd
you can get a permit for longer flat beds.
thank you all for your responses. i am in thailand and a 24m long trailer can go on the road with some assistance from the police.
the beam is fabricated from 2 x 12m beams. they are welded with a full penetration butt weld at flanges and web. an additional splice plate is added with fillet welds to each side of the web splice (i don't think this is needed).
the rafters (beams) are bolted to the columns (which to clarify a couple of points mentioned in the responses are 24m apart). these beams are the roof rafters so the loads are deadload, service load plus a small allowance for people working on the roof.
the client thinks that the beams should not be welded at midspan and that they should be welded at a point of lower moment. my point is that as the joints are equivalent to the strength of the parent material it would be a bit ridiculous to cut the 12m means to weld them together again to make the 24m beams from 3 pieces.

i see no reason why a splice that develops the full capacity of the section could not be made as you describe. yes one tries to avoid them at midspan but they are often made for practical reasons. i imagine the clients concern would be more on quality control of welding rather than the splice location but presumably one could demonstrate that its properly done.

__________________
借用达朗贝尔的名言：前进吧，你会得到信心!
[url="http://www.dimcax.com"]几何尺寸与公差标准[/url]