huangyhg

to check if beam crack is live or not
there are minor cracks in the midspan of the beam, i want to check if the cracks are "live" and still increasing or not. i usually paste 2mm thick 5" long glass stips exactly below the crack, so if glass breaks it means crack is still expnading ? is this correct or not ? any better suggestions ?
there are a number of ways you can mark it for this.
the simplest and most crude is to mark two spaces a certain distance apart.
another is to use 'tell tales' two tear shaped plastic items which are nail fixed either side of the crack. you then put a felt tip mark across both so that you can tell when they move.
glass is a suprercooled liquid and as such will flow with slow movement over time, thus it may not necessarily indicate movement.
there are crack monitoring devices specifically for your situation and here are two places to look. i've used the avongard monitors in the past and they are economical, easy to install and simple to read.
quote:
glass is a suprercooled liquid and as such will flow with slow movement over time, thus it may not necessarily indicate movement.
glass does not flow at room temperature. this is an oft repeated jewel of "wisdom". i don't want to push this thread completely off tipic, but i do hate to see misinformation spread. do a couple of google searches and you'll find some sites debunking it.
-b
a common method to measure crack propagation in a fatigue test is to use a 'crack gage.'
drill an 1/8" hole on either side and install a concrete nail in each. use dial gauge vernier calipers to measure any movement. holes can be 2" away from crack. can measure at 1 day, 1 week, 1 month, 6 months, 1 year, 2 years, etc. if no movement then not necessary to check further.
dik
zeeshanahmed,
where are these cracks located at midspan and at what orientation? what is this beam functioning as? are there diaphragms present here?
vod
why so sophisticated ? an indelable ink mark at the current tip position, and another 1" away would tell you plenty ... do you need remote monitoring ? add a video head, heck add a laptop and have live monitoring over a webcam ...
prost--dangerously off topic indeed. if you want to see how it was already discussed to death a decade ago (including the "thicker on the bottom" false reasoning), go here:
let me shed some more light on the subject.
this is a building of telenor (a cell phone company) its a msc building which means loads and loads of equipment inside which is so dust sensitive that when you go inside the hall you need to wear shower caps, gloves and shoe covers. on the 2nd floor in this two story building all the longitudinal (major span) beams have typical bending cracks in the center of the beam starting from bottom of the section and moving up on sides -its a good textbook example of bending cracks- appernetly there are heavy aircons unit on the roof which are directly on top of these beams and it seems that these loads were not taken in account while telenor was renting this building. anyways even the thought of some kind of retrofiting creeps the hell out of the management here as it would mean to shut down a complete sector, pack the equipment inside and then do some retrofiting work. so i performed an analysis (taking the aircon loads in account) and failure ratio roughly is 1.2. i have suggested to them that lets put some type of gauges on the crack to observe the cracks, if there is no more propagation in the cracks then lets live with this as it is, but if the cracks are expanding then there is not other option other than going through the retrofiting of building. the building is a simpe 25'x20' regular grid of 120' x140' , two story building. so hows that ? ?
i'm not sure why a crack would 'die' as such unless the loads were transmitted elsewhere in the structure, and as such the crack is live. crack propogation will increase under some paris law until eventually it reaches the critical crack length and then 'ping' - to use a technical term. how close is the crack to the critical length. has the neighbourhood been evacuated?
i'd maybe drill a hole at the crack tip to arrest crack growth and then add some kind of stiffener to it so that the beam meets design requirements for the expected loads.
corus

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