Assigning Loads To Frames

[nikgr82]

I want to run a space frame with and i dont know how to apply uniform loads on the beams.i used rectangular shell elements with very very small bending thickness (0,000000001.Are the uniform loads assigned correctly to the beams this way

 

[popabf]

There are 2 major possibilities to assign loads on bar elements (beams): according to the Local Coordinates of the element (1, 2 or 3) or accord. to the Global Coordinates (X, Y or Z). The help files contain all the informaions necessary.

 

[Diquan]

"I want to run a space frame with and i dont know how to apply uniform loads on the beams." -> Select the element/frame then go to "assign->frame/cable/tendon loads-> Distributed" there select the case (dead, live, quake, etc...) the units of the load (kip-ft or ton-m, etc...) in the coord system select the desire condition (the load is applied perpendicular to the global axes or another situacion, etc....) at the bottom says "uniform load" the load is applied in force/lenght units (kip/ft, lb/in, ton/m, kgf/m, etc....)

Diquan

 

[nikgr82]

Thanks both of you for the advice but i did not write the thread correctly so you did not understand what i want.i am working on a steel space frame.my problem is how to transform a uniform load for a surface (kg/m^2 for example)into a uniform load for a frame (kg/m).i used shell elements with very small bending (0,00001 for examle)so that the moments will go directly to the frames.is this correct?

 

[ledi101]

what program are u using?

The Straus7 does it because is one element that you define and has no property, it only transform uniform surface load into uniform beam loads.
Maybe i have seen it in etabs, i will respond you soon if such a thing is possible with etabs.

 

[nikgr82]

i use sap2000 but i think it doesnt matter which program i use.its rather a static question.using a very small - almost zero - bending thickness for a shell element "transfers" the whole load of the shell to the rest of the structure?is it distributed correctly?

p.s sorry for my english!

 

[DENSAL55]

it's depend on the meshing of your shell element.If the number of elemnts shell is high the distrubution is more correct.You must divide your beam into the same number of elements as your shell elements.So it work together.

 

[Diquan]

yes, and also select a NULL section, the null section will only transmit the applied pressure to the frame

Diquan

 

[masomenos]

Load distribution should be different, depending on whether the area finite element is defined as 'shell' or 'membrane'. Membrane should distribute the surface area load along the length of the frame. Otherwise, you will need to follow DENSAL55's advice and add joints to your frames, then mesh your area finite element to connect to those special joints on the frame. Note that you don't have to divide the frame, you can draw special joints using the 'snap to line' or offset options. For design, it's usually easier to add special joints rather than dividing the frame. Otherwise, you're having to manager so many moment diagram output.

 

[DENSAL55]

masomenos Say
"Note that you don't have to divide the frame, you can draw special joints using the 'snap to line' or offset options"
I do not understand ?

 

[masomenos]

DENSAL, you do not have to divide frames in SAP to create joints for load distribution. You can draw special joints on existing frames to do this. When drawing joints on frames, the snap tools can be useful in helping you make sure the special joints are "locked on" to the frame. When you draw special joints, a properties box appears. You can type in offsets in this box in current units which draws the special joint a specified offset distance from wherever you mouse click. Try it. Also, you use the snap tools to draw special joints on frames approximately where you want them, then right mouse click the joints to modify their X,Y,Z coordinates to place more precisely. SAP will internally divide the frame at those joints, but then re-assemble it for reporting output results.

Why would you want to draw special joints instead of dividing the frame? Imagine 8 meter long girders intersected with secondary beams (or intersected by floor system mesh modeled with shells or whatever). You can draw special joints on girders then use those joints to draw and connect those secondary beams. SAP will internally divide the girders to handle load transfer between the girder and secondary beams, but it will report results and design the girders using their true 8 meter length. Whereas if you 'divide' the girders to do this, you would have maybe 4 or 5 or more different moment diagrams representing 1 girder, making design more difficult.

 

[masomenos]

After posting, I experimented a little with another method within SAP2000 that you can use to distribute loads to frames. In SAP version 10 (and 9?), you can make an area load assignment>Uniform to frame type load, and if your areas are shell type, when choosing this type of area load assignment, SAP will distribute the loads along the length of the frames the same way as it distrutes loads when the area is 'membrane' type. Unfortunately, SAP doesn't appear to automatically distribute the self-weight along the length of the frame using this option, only the load assignment, so if you want SAP to distribute self weight along the length of the frames, you would have to go to Define>Load cases and change DEAD self weight multiplier to 0, then go back and assign the self-weight to the areas using this 'Uniform to frame' area load assignment.

 

[DENSAL55]

masomenos
Using special joint such we have describe by giving the offsets for joints one by one, for this purpose need a lot of time. Especialy when we have a complex or a big structure.

 

[stressed]

Densal55, you don't have to use special joints like the way masomenos suggested, drawing one at a time. You can draw one or however many you want, then select them and use the Edit menu to replicate. Also, if it's applicable to the type of structures you are working with, the 'quick draw secondary beams' automatically connects and creates special joints. If you've got complex geometry, you also can import special joints from Excel if it makes sense to do so. Either way, you are absolutely right about using whatever method works fastest and best for what you're trying to accomplish. Special joints can be useful, but in some cases, they can take too long to model. They're like one more tool in the SAP toolbox, that's all.

 

[nikgr82]

anyway i use shell elements,i mesh them in many parts,devide the frames in the same number of frames and use almost zero thickness...the results seem good.adding special joints is a bit risky becouse i cant be sure if they are realy locked on the frame as you say.thanks for the help i didnt expect so many repliies....!

 

[edferlac]

Well, after some test, using differents shell elements, making dummy materials (very low mass, modify elesticity molulous...) seems that isn't and adecuate protocol to distibute surface loads to frames.

Any kind os shells introduces in the model restictions, and the displacements of the structure is't no happens if you put load as de usual method.

I'm still working.....

 

[te15]

Hi,
I have a question about slab-beam connections.
I am trying to model a slab using shell elements and the interior and edge beams using frame elements with rectangular sections. This is what I have tried-
1) Offset the beam neutral axis.
2) Then, divide the slab into smaller slabs (1/4 x1/4.
3) Create joints in the beams by replicating the slab joints (-dz).
4) Select individual slab edge joints and corresponding individual beam joints, and apply body constraint (x,y,z,r1,r2,r3).
Questions:
1) Is this a correct procedure?
2) I had to create individual body constraints for each of the joints (12 in number for a 1story 1 bay x 1 bay RC slab-frame str.). I had to do this, otherwise the beam displacements will be the same along the length. How do I do it for a model with a large number of elements?
3) How do I create points/joints on the edges of the slab without having to divide the slab into smaller slabs?
Please let me know if more info is needed to answer these questions.
Thank you