huangyhg

equivalent uniformly distributed loads
does anyone know if there is a reference that exists on how to approximate the equivalent uniform distributed loading on beams when there are concentrated loads on them. the uniform distributed load would provide the same or a bit more stress and deflection in the beams. i know there are many variables involved and there is no "one method fits all" ,but, i am looking for a approximate or empirical method to calculate the equivalent uniform load for any concentrated load combination a beam.
any help would be appreciated.
the aisc steel manual has some of this stuff in the beams part starting on page 3-208. to be honest, these never seemed very useful to me, other than to check the output from a program (very important).
krus 1972, why do you want to go to the bother to create an artificially modified solution, when you could just simply solve the shears, moments and displacements by hand very quickly and simply for most problems??
seems pointless.
for a stater, i would base it on the deflection curve, use the maximum deflection and find the uniform load using the deflection equation for uniform loading.
i wold also back check the max shear and moment to make sure there were no problems. with all the simple to use beam programs, this should be a very easy process.
cheers...
mike mccann
mccann engineering
we have an engineer in our department who always uses a large uniform live load in lieu of concentrated loads from equipment and piping supports. his rational is twofold.
first it is his contention that the final locations of concentrated loads will not be known until so late in the design development process that it will be impossible to do a precise analysis within the constraints of the project schedule.
second he claims that a large uniform load provides an additional factor of safety to account for forces generated from operational loading.
the projects that we work on are typically cogeneration projects.
i am currently reviewing his design of a one storey turbine building for which his roof design he has used a live load of 100 psf and a piping load of 25 psf in addition to a roof snow load. he has no basis for these values other than "engineering judgement".
while i agree that these loads are more than adequate for the roof i disagree with this approach for numerous reasons. my biggest concern is that we will never know the actual state of stress in any
steve1-
wow! those are huge loads for mechanical equipment. at least if what we're talking about are typical hvac units.
i have to agree with you on this one. it almost seems like cheating. does this engineer have a concept of how much 100 psf is?!
it's tremendously more economical to obtain a reasonable estimate of the actual mechanical loads. for your "engineering judgment" you could even bump those up by 50% or so. for worst case moment, place it at midspan and for worst case shear place it at the support.
if the building use changes at some point in the future, then it would fall to the new tenants or owner to assess the capacity of the roof and take appropriate measures. it seems unfair to make the current owner pay for what someone may do in the future. (unless the owner specifically requests that the roof have adequate capacity for such a scenario.)
work out bm from point loads and work back converting into a udl.can make a conservative deflection calculation easier than using a number of point loads.
steve1,
you stated that your project is a turbine building in a cogeneration project. so it is an industrial structure, not a commercial or residential one. things are different with industrial structures. 100 psf live load for a roof does sound excessive, but perhaps the other engineer has his reasons which have not been adequately explained to him. who makes the ultimate decisions on this type of thing in your organisation? as the reviewer, and if you can not agree with the designer, you need to talk to the managing engineer.
66,
the purpose of my post was to provide an example of what some of the problems are that can arise when uniform loads are used in lieu of actual concentrated loads.
for my particular case i am well aware, as department manager, of the types of facilities that we engineer. i have been eor on numerous power projects with price tags in the tens of millions of $.
the firm that currently employs me has expanded in the past few years from 15 to 40 employees. unfortunately because our roots are in the mechanical and electrical disciplines the civil/structural department has no representative in the senior management staff. the person who is director of engineering is also the pm for the project that i mentioned above. yes i will have a discussion with him concerning the applicable design criteria.
in terms of bending stress, the equivalent uniformly distributed load for a simple beam is p = 8m/l (kips), where m is the maximum moment (ft-kips), amd l is the span (ft).
steve1,
i was unaware of your experience and position when i posted. as the structural department manager, you are the person who must make the ultimate decisions about these issues. sounds as if you have a training session ahead of you. good luck. old hands can be hard to change.

__________________
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]