How to get STD Steel JOISTS chord areas 1

[sponton]

I'm doing a vibration analysis with the AISC design guide 11 on a set of steel joists, the problem is that the manufacturer tables do not really give me the chord areas that i need to perform the analysis, are there any existing tables that have these properties or a way to calculate this as accurately as possible?

 

[KootK]

New construction? I'll assume so. Most joist catalogues will give a formula for estimating stiffness without knowing the exact chord size. That will allow you to complete your vibration checks. I typically assume a reasonable joist stiffness value and then express that on my drawings as design criteria for the joist supplier.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[sponton]

Yes indeed, I don't have any issues for the stiffness part, because as you've mentioned a formula is provided. The thing is that if you follow the DG11 procedure and assume the steel deck and joist behave as a composite beam I need to get the joist area to get the composite moment of inertia. I just decided i'll take it as
A= 2*(12*(550 lb/ft)()span ft)^2/8)/((50000psi)*depth of joist)

it won't be the most exact thing but at least it will give a nice enough estimate of the composite moment of inertia, in any case I don't really fancy the way the AISC deals with vibrations, it's lucky it does so at all.

 

[KootK]

Using the 550 number seems optimistic to me. That would likely only be applicable for relatively short joists for which vibration isn't likely to be an issue. I think that you'd be better off with something like: 2 x I_eff x 1.15 / d^2. 1.15 = shear flexibility correction; d = distance between chord centroids.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[sponton]

I guess you're right, I'd rather just stay in the safe side.


There's something that bothers me terribly about the design guide, the example is wrong, they use a joist that's not standard for the span analyzed. -_-'

 

[KootK]

Be careful what you assume to be the "safe side". Vibration design is mostly about making sure that the natural frequency of the structure is safely outside the anticipated frequency band of the exciting function (occupancy). Using your scheme, you're likely of overestimate the composite stiffness of your floor system considerably.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[Hokie93]

Many years ago I addressed this very same question to Dr. Murray at Virginia Tech (author of AISC Design Guide 11 and the FloorVibe software). His response was to use I=Ad² and assume the top chord area = 1.25 times the bottom chord area. This assumption is apparently incorporated into the FloorVibe software.

 

[KootK]

That's great to know -- thanks Hokie.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[sponton]

Actually thanks very much that's really valuable information, I had e-mail Dr. Murray about the way the floorvibe software handled numerically the problems and even though his reply was very fast it was kind of reluctant, he told me they were re-writing the design guide and some of the issues I had inquired about would be addressed in the new version, that's good but not exactly in compliance with the schedule I have now.