Automatic Inclusion of Member Dead Loads

Question

Automatic Inclusion of Member Dead Loads

msquared48 (Structural)

(OP)

30 Sep 12 16:13

I am designing an odd shaped space frame that has a large cantilever, and I want to spec within the Dead Load BLC for the program to automatically calculate the member weights and have them included in the dead load of the structure - all steel tube sections.

Currently the reactions do not add up, and in reading the RISA manual, the self-weight gravity load paragraph on page 205 of my manual says to do this in the BLC section by applyiong a negative component (-.9 they suggest, but I would use -1.0) in the direction of gravity, the Z direction in my model. In my first run, I did not include the -1.0 and the results did not seem reasonable, but in the second run with the -1.0 included, they seemed more reasonable, but I still have a question. (I will take the .9 or ,6 factor in the load combination section so as not to double dip here as it were...)

The question is, when you enter the -1.0 for the z direction here for the member gravity loads, does the program calculate the dead loads for ALL the members of the space frame, or just the ones you heave entered in the other columns of the BLC for such items as point or uniform loads? Not all the members of the frame see dead loads in additon to their own self weight.

Thanks for any help. I just want to be sure here.

Mike McCann
MMC Engineering
http://mmcengineering.tripod.com

sbouvia (Structural) · Answer 1 · 2012-10-01 09:01:42

If your question is asking what I think it is, once you have entered -1 in the Z-direction (Up Direction) as stated above it will calculate the member weights regardless of other loads on the structure. Usually I make a self-weight case and just tag its category as DL and then under the load combinations I can factor the DL and not have to type in the specific basic load case. I like to keep the self-weight separate most of the time, so I can compare that to some hand calculations of how heavy I envision the structure being.

Hope that helps.

Scott

JAE (Structural) · Answer 2 · 2012-10-01 14:17:14

The -1 factor applies to all members. If you want to omit members from this you need to assign those members a material type that has zero density.

JoshPlumSE (Structural) · Answer 3 · 2012-10-01 19:23:10

JAE is right.

Also, I will have to take a look at the help file / manual to get rid of that -0.9 reference. Any load factors should be handled on the load combinations spreadsheet. In the Basic Load Cases, it should almost always be a -1.0 factor for gravity.

msquared48 (Structural) · Answer 4 · 2012-10-02 11:25:01

Thanks Josh:

As a side comment though, it seems to me that in Risa2D, there used to be a directl question in tone of the screens that said something like "include member self-weight?"

I no longer see that, The comment was very direct and you knew what would be done. The -1.0 is hidden as it were and easy to forget. I would like to go back to the direct comment approach, personally speaking.

Mike McCann
MMC Engineering
http://mmcengineering.tripod.com

JoshPlumSE (Structural) · Answer 5 · 2012-10-03 11:40:14

Mike -

I don't remember a check box like that. You might be thinking of another analysis program. surprise

I believe RISAFoot had something like that, but I'm almost certain that RISA-2D and RISA-3D have always used the Y gravity factor.

JAE (Structural) · Answer 6 · 2012-10-03 12:02:57

That -1 aspect of RISA has been there as long as I can remember using RISA (1988 or so).

msquared48 (Structural) · Answer 7 · 2012-10-03 13:47:41

You guys are probably right - I have used other programs in the past prior to RISA.

Nevertheless, I did like the direct approach...

Mike McCann
MMC Engineering
http://mmcengineering.tripod.com

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