My questions are:

1. When using the AREA LOAD or FLOOR LOAD to represent the load beeing carried by the slab does STAAD automatically convert those loads to equivalent loads and then automatically impose those equivalent loads to the beams bordering the slab? or do i have to manually solve for the equivalent load then manually impose those loads (from the slab) to the beams. Actually thats what our instructor taught us, manually solve the equivalent load then put in on the beams supporting the slab. but doubt it[*-)]

2. On the command  "selfweight -1" does that selfweight include the weight of the slabs even if the slabs were simply modeled as plates?

ex: ....

meter kn

Element Incidences

1 1 2 3 4

Element Property

1 thickness 0.15

3. Attached file shows how to correct improperly connected plates. We added nodes 9 and 10 to beam 1 4 supporting plate 1 to correct the improper connection of plates 2 and 1. Just for discussion purposes lets us consider PLATE 1 before we break it into 3 plates as a TWO WAY slab. After breaking it to 3 slabs, the resulting plates from plate 1 are plates 3 4 and 5. Sir/Ma'am if i'm gonna load these slabs using FLOOR LOAD / AREA LOAD. Do i have to load them individually as one way for plates 3 and 5 then two way for plate 4? or should i just apply the floor load over these plates?

Thanks a lot

### 26 Replies (Most Recent Reply)

• Hi,

1.

When you apply a floor load to a slab pannel, the programme calculates the equivalent load and apply the load to the beams bordering the slab as a uniformly distributed load. You do not have to calculate the equivalent load manually and apply them to the beams. Like in the attached figure a floor load of 10kN in the –GY direction is applied on the slab and the programme converts the load to equivalent distributed load and apply them to the beams.

Basically, the commands like Floor load or Area load are used when the entity transmitting the load, such as a slab, is not part of the structural model. The Area Load or One way Load may be used for modeling one-way distribution and the Floor Load may be used for modeling two-way distribution. For details please go through the Technical Referrence Mannual section 5.32.4.

2.

If you want to include the self weight of the plates then you have to assign the plates to the Self Weight command. The self weight command include the self weight of every active member and applied as a uniformly distributed member load.

The self weight of a plate is placed at the joints, regardless of plate releases. If you want to transfer the load from the plate to the connecting beam as a uniformly distributed load then you have to mesh the plate.

3.

As in your case you use plates in your model. So it is preferable to use “Plate Load” command in place of Floor Load or Area load command.

It is preferable to assign an area load for a one way slab. But there is no strict rule about this. You can assign a two way load also for those pannels.

If in your structure there are a number of one way slab, then you can create a Floor group considering those slabs and assign a one way load distribution for them.

Good Afternoon Ruddranil.

I modeled a Mat foundation using plate elements. And I have an Equipment Foundation on top of the Mat Foundation. Can you teach me which kind of load is much better to use Area Load, Surface Load or Plate Load. Because I cant find any explanation in the Help menu about the difference of this load type. And since I used Plate elements in my mat fdn i just used the Plate loads command.

In the Plate Load command menu, STAAD ask for the local and global direction of the loads. SInce my load is just the weight of the foundation I have the Y-Direction only, but Im confuse with the Local Y and GY(i suppose this is Global Y) directions. Which one should I use?

Thanks.

• In reply to Kiel Supertramp:

For specifying loads on elements , use the PLATE LOAD option . The area load command is now not used any more, it is superseded by the Floor load command , which is a better method of distributing the slab load to the beams .

Every member and plate has its own set of axis . Now the load may be applied to the member / element as per the Global Axis ( denoted by GY GX GZ) or the local axis of the individual member .

Right click on the GUI, go to Labels and check in the option to see the Beam Orientation and Plate Orientation

Thank you Shayan

• In reply to Kiel Supertramp:

Greetings Shany.

Is there a provision in STAAD to apply an Area Load on the Structure by simply specifying the bounding plane within which it must be applied?

Please see the attached file for RSTAB.

I just specify the magnitude of the load, the direction of the load and specify which Area the load has to act.

If there is a way in which I can simplify this, I will appreciate it if you can tell me.

Cheers.

• In reply to Rama Krishnan:

How far accurate an approximation would it be if I specify thin surface elements where I need my area loads and apply those loads on those surfaces as surface loads rather than Area loads?

• In reply to Rama Krishnan:

Hello RAM

I am not sure why you are finding the FLOOR LOAD command complicated, but I guess its very easy to use. You just need to specify a closed panel using the X Y and Z Range and give in the load intensity, the rest is done by the program.

Shany,

The objective is to work around the tedium of calculating individual member loads (based on tributary widths)  for each and every member subjected to a pressure load.

Please see the attachment. As per the STAAD convention, I need to specify the co-ordinate values of each and every of those rectangles (in colour) and later on specify the direction in which the load should be considered (one way loading - 'towards'). Honestly, that is mind boggling when considering I have quite a lot of those closed panels you mention. And the program requires me to enter the corner co-ordinates of each and every of those rectangles for it to apply the corresponding member load?

You might have noticed the delay in my reply. That's because I have been calculating those member forces manually!

Seriously, I believe there is another way around it that I have missed.

In RSTAB, it is possible for me to specify a bounding plane with four nodes, specify the load magnitude and specify the members that are associated with the loading. I can simply copy paste those nodes without calculating their co-ordinate values!

Can I do something similar in STAAD?

cheers.

• In reply to Rama Krishnan:

I suppose it will no do harm to explain the design environment.

The objective is to impose access loads, loads of chillers etc. etc. on say gratings laid over steel joists/girders as area loads. The design situation is thereby different from the slab loads you had in mind. There are designated areas meant for storage, designated areas meant for superimposed loads, areas where we have a combination of both etc.

The loading is therefore not uniform! Not in magnitude nor in the size of area to which it is confined.

It is not a complicated thing by any means, but if one has to calculate those forces manually after determining the tributary widths of individual members, it becomes very tedious.

I tried to work by determining the co-ordinate values and entering them. But STAAD repeatedly gives me the message "Load cannot be generated".

The way I see it, STAAD requires me to enter the ranges or corner co-ordinates of those rectangles, the magnitude and the direction towards which the force should act. Despite doing all this, I am still stuck at the Load cannot be generated stage.

• In reply to Rama Krishnan:

Just a typical design scenario for me Shany.

Can you please suggest a way to achieve it?

In connection with this topic which you have raised where you state that programs like RSTAB allow you to place a load over any region you wish, I have one question.

The figure below shows the plan view of a floor grid. The individual member numbers are labeled alongside the lines.

The red rectangle shows a part of the floor over which a uniform pressure acts. As you can see, this rectangle spans across part of the span of the following members - 6, 7, 8, 9, 21, 23 and 24.

The nature of this loaded area is that there is no closed loop made up of members contained inside this rectangle. Hence, if you apply a FLOOR LOAD or a ONEWAY LOAD in STAAD, you will get a message that no load has been generated.

So, if you had to manually calculate the load on each member, how would you do it? What method will you use to determine how much load goes on the individual members that are partly spanning within this loaded region?

• In reply to Kris Sathia:

Hello Kris.

First things first. I do not have decades of experience of modelling in STAAD. Yes, my experience has been with Dlubal. But the underlying principles on which the whole process is based cannot be any different.

I have no intention of putting forward RSTAB as the best solution around as your post seems to suggest. In  fact, if you were patient enough to go through what I had written, I kept mentioning missing out on some parameter. Something I must have overlooked. My intention is to find a solution for my problem using the best package I have at my disposal. Irrespective of whether it is STAAD or SAP or SCIA Engg.

I really don't want to continue my rant. Nor act like a kid acting out of spite.

Are you suggesting that I cannot manually compute load for this case?

• In reply to Rama Krishnan:

Ram_STAAD - not sure what method you are using to define floor load, but you may try to create groups, use dummy beams to enclose the areas if necessary, and apply  floor load on defined groups. Make sure that you have "SET FLOOR LOAD TOLERANCE" command at the beginning of input file (you may have to play with values, say 2-5mm to get correct load generation). Also check that floor is in the same plane, sometimes staad rounds coordinates making  some nodes of the plane.

In the past I had numerous issues with Floor/Define Range, so I am not using this option any more.

We appreciate your bringing attention to this topic because if there is a way to make it easier to use, we certainly wish to do that.

My previous question was with the intent of obtaining your suggestion on how an engineer would want the load distribution to be calculated on the individual members of a panel for the situation that I highlighted.

FLOOR LOAD and ONEWAY LOAD are based on widely accepted principles of load distribution. Those principles are based on the load being applied on a zone in which the members that are present create closed boundaries.

This is the answer to your question on whether there is something you may have overlooked:

For FLOOR LOAD, there are 2 methods:

1) The region over which the load acts is defined by XRANGE, YRANGE and ZRANGE each of which takes on 2 numbers - a min value and a max value. Those 6 numbers form the edges of a box inside which the loaded zone lies. Additionally, one has to specify the load intensity (pressure) and the direction in which the load acts - global X, global Y or global Z

2) The members which receive the load are clustered into a group called a FLOOR GROUP as sasa3k mentioned. There is less of a chance of any member going unloaded using this method. For this method too, one has to specify the load intensity (pressure) and the direction in which the load acts - global X, global Y or global Z

If you have specified all these required values, your input is correct, provided that the members inside the region form closed boundaries.

• In reply to Kris Sathia:

Hello Kris.

The technique that I employ is to calculate the tributary width for each beam on which the loading area spans and multiply the same with the applied pressure. Then I have on my hand a Uniformly Distributed Load applied on the members that support the load area. I am not sure why STAAD has set limitations on the applicability of area loads on closed panels alone. In the end, I am certain the program does the same thing that I do. determine the influence width of beams, multiply it with the applied pressure.

The corner co-ordinates of the rectangle specified essentially demarcates the boundary with in which the load acts. The geometry of loaded area is therefore very clear. Please note that I am applying neither on a inclined surface nor on a skewed geometry. I am essentially applying my loads on regular rectangles.

As I understand it, utilising the One Way load distribution is the more practical way of simulating the situation.

The manual on this very clear, I will read it again and again if need be. What I do not understand is why should the specified rectangle be within a closed panel.

If the process involved is determining the tributary width of the beams on which the area spans, multiplying it with the applied pressure and applying the associated UDL on the beams, what is the factor that necessitates the use of closed panels?

I am by no means stubborn and if there is a way for me to work around this issue, I will. I wanted to implement sass3k's suggestion. The only thing I am worried about is that it will seriously screw up my model if I introduce dummy beams now. (I have beams that are on top of each other and hence specified with definite member offsets). Still, it's better than calculating the member loads manually.

I will post a pic early tomorrow showing how I calculate the tributary width.

• In reply to Rama Krishnan:

Hello again Kris.

Just wanted to post the info right away.

The concept we use to determine the tributary widths of beams on which we have loaded areas is as shown below.

It's not very pretty, but it's the best I can manage right now. The concept is to define a simple beam element with support points that correspond to the centre lines of our loaded members. i simply load it with a unit load, run the analysis and obtain the reaction loads. The reaction loads at the support points essentially signify the influence widths of individual members in appropriate length units.

It might not be a false proof method of working it out. But it falls within the acceptable range of tolerance and is a simple way of doing it.

It's a easy matter then to multiply the influence width with pressure and arrive at UDLs on the beams.

• In reply to Rama Krishnan:

Sincere apologies Shany.

I am no longer authorised to access RSTAB at work. So I have to reach home to do what I promised. And my wife hates me for taking work home. :(

This is what I had in mind.

Can we do the same with STAAD?

What am I missing here?

For all its flaws, STAAD is such a robust performer. I haven't had a single program crash since I began using it. Not a single one. (OK except for cases when I would mess around with mesh refinement). I downloaded RSTAB, just did that simple model and the display driver crashed. :(

cheers.

Thanks for sending the details. We have passed on the suggestions to the software developers.

• In reply to Kris Sathia:

Many thanks for the effort Kris.

As I mentioned before, you guys do a top job at support.

Respect.

• In reply to Rama Krishnan:

Greetings Kris.

Any developments in this matter?

Regards.

• In reply to Rama Krishnan:

The AREA LOAD is one of the features that has been earmarked for improvement in a version of STAAD to be released later this year. Improvements that you and others like sasa3k asked for - selecting the region by mouse clicks instead of typing numerical values for ranges, enabling the loaded region to be viewed in a distinct color, etc. are being considered. I can speak only in generalities at this point because I don't have the details. Once the specific details are available, I will be in a better position to pass them on to you.

• In reply to Kris Sathia:

Greetings Kris.

Many thanks for the update.

• In reply to Kris Sathia:

Something?

Anything?
• In reply to Kris Sathia: