huangyhg

security safes
i'm often called on to determine if a building owner can put a 4-drawer security safe on an upper story floor. these safes are usually around 900 pounds in weight and are about 22"x39" at the base. i have been determining the load on the floor by dividing the weight by the area of the base of the cabinet (900#/5.96sf = 112.5 psf). since this is greater than the design live load of 50 psf for an office, i have advised against the use of the safe. is this an overly conservative approach to this problem?
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the uniform building code (which i am familiar with) calls for 50 psf uniform loading or a concentrated load of 2,000 pounds applied to an area 2'-6" square. check the applicable building code for the location. if the building were designed for similar criteria, it would appear that the safe weighs less than the 2,000 pound concentrated load (you didn't say whether the 900 pounds includes contents). that is assuming the floor were designed to code. if you have information on the floor, why not analyze the existing floor?
my understanding of the ubc is that the 2,000 pound concentrated load is used by itself without the uniform load present in order to check which is more critical - they cannot be combined. since this is an office, one must assume that the 50 psf uniform load is always present, which negates the use of the 2,000# load as a check.
the code specified uniform live loads and concentrated loads are not meant to be applied concurrently. they are simply an attempt to envelope the maximum load effects that will be experienced by the structure because the real load distribution is impossible to predict. in reality, a live load of 50 psf will probably never occur over a significant area of the floor. aisc's design guide 11 suggests that a realistic typical live load for office areas is 11 psf.
i agree with kramer. analyze the existing structure with the actual anticipated loads to determine the acceptability of the proposed safe. 900 pounds is not a huge load. i would be surprised if most typical office buildings that have been designed per ubc criteria could not handle this.
vmirat---don't forget to extend your load calcs to include the area, floors, hallway etc. that the installer must dolly the safe to get it to it's resting place. quite often, in fact everytime, the unit loading on the dolly's or 'mice' and rollers will be substantially higher than the weight of the safe alone.
rod
a 150 pound woman wearing spiked high heels, 3/8" x 3/8" footprint, and standing on one foot would apply a live load of 153,600 psf. is there any office floor in the world that can support her?
it's all in the analysis, n'est ce pas?
taro -- i agree that the actual loading will probably never reach the 50 psf requirement. in fact, the commentary in asce 7 has a table showing the mean and standard deviation of office loads over a 50-year time period which is around the figure you mentioned. however, i feel compelled from a legal standpoint to use the code required loading.
redhead -- interesting analogy. this is becoming a study in abstracts . if i use the safe as a real concentrated load, then i must be consistent with the other objects in the room (plants, furniture, people!, etc.). this becomes a nightmare! i guess the question is, can you reconcile the difference between code requirements and real world without creating a liabity issue for yourself? the owner is responsible for not exceeding the posted allowable floor load but we don't tell them how to figure it out - that's why they come to us.
what floor area in front of the safe is devoted to operation of the safe door, filling the safe, etc.? if the area in front of the safe is three feet square, then adding the weight of one person (200#) to the safe (900#) and dividing the sum by 15 square feet, the load is 73 psf. that's not the 50 psf you are looking for, but if you consider taro's comments regarding the actual typical office floor loading that will be on the remainder of the span, you might be able to rationalize the loading better. do you have any information on the floor framing? the larger the area you are looking at, the less the effect of the concentrated loading. in other words, if the floor is framed with short span joists, the loading will have more of an effect on the joists directly below the load. if the floor is a flat concrete slab with columns at 24 feet on center, the loading will have minimal effect.
this is a four-drawer file cabinet for storing classified documents. the floor is composite concrete/steel deck supported by w16x26 at 8'-8" c.c spanning 30 feet, simply supported. the safe will be located against a corridor wall within a secure room which just happens to fall directly above one of the wide flanges. so, the beam was designed to carry 50 psf for the office and 100 psf for the corridor. the safe will be located approximately at mid-span of the wide flange.

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