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[e-conf] Crane load combinations in draft IS:800

 
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er_spsvasan
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Joined: 26 Jan 2003
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PostPosted: Sun Oct 26, 2003 2:40 pm    Post subject: [e-conf] Crane load combinations in draft IS:800 Reply with quote

Sub: Crane load combinations in draft IS:800

Hello everybody,

IS 875 (Part 2) indicates four classes of cranes and points to IS:807 for definition of the four classes of cranes. The probability of operating to full capacity is highest for class 4 cranes and lowest for class 1 crane. As per IS:807, a crane is classified as class 4 crane if (1) it is operated for atleast 500 hours per year; (2) lifted load is more than 2/3 rd of the safe load atleast 1000 times in a year and (3) the speed of operation of crab and crane is more than 100 metres per minute.

The mean return period for windload as per IS:875 is 50 years.

The return period for our Himalyan earthquakes is reported to be about 100 years; and they last for less than a minute.

The chances of simultaneous occurrence of full crane load along with either max wind load or max earthquake load can very well be imagined.

Now, in section 1.2, of the draft IS:800, the "Accompanying load" is defined as the live load of lower magnitude which may act together with a leading imposed load. In an industrial structure, invariably, the crane load will be considered the main imposed load and the roof live load will be considered as accompanying imposed load. Referring to Table 5.1, it is seen that full crane load may have to be combined with either full windload or with full earthquake load. This is not correct.

The load combinations involving crane loads has been dealt with in detail in IS:4247 (Part 2) "Code of Practice for Structural Design of Surface Hydroelectric Power Station: Part 2: Superstructure". Crane loads have been considered as distinct from Live loads in this code.

Table 1 of above code gives increase in permissible stresses for 7 load combinations as follows:

(1) DL + LL + Moving crane, loaded to half capacity + Normal Tail Water Level: 0%

(2) DL + LL + Standing crane, loaded to full capacity + Normal TWL: 0%

(3) DL + LL + Moving crane, loaded to half capacity + Normal TWL + WL + Temp: 25%

(4) DL + LL + Moving crane, loaded to full capacity + Normal TWL + Temp: 25%

(5) DL + LL + Standing crane, unloaded + Normal TWL + EL: 33 1/3 %

(6) DL + LL + Moving crane, loaded to half capacity + Maximum TWL + Temp: 33 1/3%

(7) DL + Temporary or Construction load: 25%

Though IS 875 considers crane load and live load as part of imposed load, it will be more reasonable to separate out crane load from live load and other imposed loads. I suggest the following load combinations for limit state of strength:

(1) 1.5 DL + 1.5 LL + 1.5 CL (Moving crane, 50% lifted load)

(2) 1.5 DL + 0.75 LL + 1.5 CL (Moving crane, loaded to full capacity)

(3) 1.5 DL + 1.5 CL (Moving crane, 50% lifted load) + 1.5 WL

(4) 1.5 DL + 1.5 CL (Standing crane, unloaded) + 1.5 EL

(5) 0.9 DL + 0.9 CL (Moving crane, 50% lifted load) + 1.5 WL

(6) 0.9 DL + 0.9 CL (Standing crane, unloaded) + 1.5 EL

(7) 1.2 DL + 1.2 LL + 1.2 CL (Moving crane 50% capacity) + 1.2 WL

(Cool 1.2 DL + 1.2 LL + 1.2 CL (Standing crane, unloaded) + 1.2 EL

(9) 1.2 DL + 0.6 LL + 1.2 CL (Moving crane, full capacity) + 0.6 WL

Load combinations containing erection loads and accidental loads can be as per the present draft.

Regards

S.P.Srinivasan

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