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Load combinations involving Crane load

 
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JMSSNL at ltecd.ltindi...
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PostPosted: Tue Oct 14, 2003 2:25 pm    Post subject: Load combinations involving Crane load Reply with quote

Dear Sir,

For Industrial structures design ,Crane load  is considered as
Occasional load since crane may not be operating with full capacity all
the time. Hence while combining with Wind/Seismic loads , building
structure is designed for either ,

Reduced Wind/EQ load or
Zero/reduced crane lifted load   (Not wheel load) with full
wind/Seismic load.

This provisions are probabilistic in nature and varies across codes.  


For example Article 1612.3.2 of Section 1612 of UBC-1997 states "Crane
hook loads need not be combined with roof live load or with more than
three fourth of snow load or one half of the wind load".
Load combination specified in Table 5.1 of IS:800 Draft  (Line 2 )
matches with above criteria where WL/EL factor is specified as 0.6 ( 1.2
x 50 % WL ) with full crane load.



Alternate to this combination is another case where  full wind/Seismic
load is combined with zero/reduced crane lifted capacity.

IS:800:1984 (Article 3.4.2.5) as well as IS:800 draft (Article 3.5.5)
makes statement " While investigating the effect of earthquake forces,
the resulting effect from dead loads of all cranes parked in each bay
positioned to cause maximum effect shall be considered."

This means that along with seismic only  (emphasis mine) dead loads of
cranes shall be considered ie. lifted load need not be considered.

Going to Table 5.1 of IS:800 Draft  crane  load factor specified is
0.53 with full seismic load.

To my understanding factor 0.53 is derived by 1.05 x 50 % crane wheel
load. As we know wheel load is factor of (crab weight + bridge weight +
lifted load )and only (crab weight + bridge weight)  will not total to
50 % of maximum wheel load derived with full lifted capacity. This means
part of lifted load is inbuilt in combination involving seismic load.

This shall not be a problem except that article 3.5.5 from draft code
shall be removed to avoid confusion.

Also , since percentage of lifted load that get inbuilt is unknown ,
determining transverse surge loads under this condition will involve
some back calculation.. Surge load is dependent only on crab weight and
lifted load, which needs to be extracted from wheel load calculated
above.

In order to avoid this problem, a better solution will be to specify
percentage of lifted load to be used with Wind/Seismic. I have come
across some international norms which specifies 50 % lifted load with
wind and 20 % lifted load with seismic load.

Other members of SEFI can opine on my suggestion

Thanks,


Jignesh shah
L&T-S&L

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