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Active Member
Engineerjbird
Posts: 7
Registered: ‎12-14-2012

Steel design

572 Views, 10 Replies
12-14-2012 08:59 AM

Is it possible to check "Both" live load deflection and total load deflection on a single steel beam?  I only see this for timber design and not for steel beam designs. 

 

Thanks

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Artur.Kosakowski
Posts: 4,947
Registered: ‎12-17-2010

Re: Steel design

12-14-2012 09:05 AM in reply to: Engineerjbird

This depends on the selected design code. On the other hand you can run a separate SLS check for live load cases selection having the corresponding deflection limit set in bar type definitions.

 

If you find your post answered press the Accept as Solution button please. This will help other users to find solutions much faster. Thank you.



Artur Kosakowski
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Active Member
Engineerjbird
Posts: 7
Registered: ‎12-14-2012

Re: Steel design

12-19-2012 07:49 AM in reply to: Engineerjbird

What you are saying is that the Live load case can be checked with a Live load deflection set in the Bar type definition, then, run another Total load case with the Total load deflection set (Changed) in the Bar type definition?

 

For at large model with many bar members, the defections can only be checked one limit at a time?  And this change in the bar type definition to the defection limits would have to be made to the entire model (all bars) to check multiple limits.

 

Am I interpreting this correctly?  Is there an easier way then having to change an entire model’s deflection limits in order to check total load and live load deflections at the same time?

 

When it comes to the code selected, I have the LRFD ASCE 7-10 and AISC 13.2 (ANSI/AISC 360-05) selected.

 

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Artur.Kosakowski
Posts: 4,947
Registered: ‎12-17-2010

Re: Steel design

12-19-2012 08:10 AM in reply to: Engineerjbird

Does LRFD ASCE 7-10 provisions require check of  deflection to be done separately :

 

1. For live load cases only with the 1st limit

2. For 'total' load with the 2nd limit ?

 

or it is some additional check you need to make?

 

If the prior, in which part of the code these provisions can be found?



Artur Kosakowski
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Active Member
Engineerjbird
Posts: 7
Registered: ‎12-14-2012

Re: Steel design

12-19-2012 11:33 AM in reply to: Artur.Kosakowski

I am referring to checking a steel beam for Live Load Deflection limit (L/360) and Total Load Deflection limit (L/240) at the same time along with the Ultimate checks like 1.2D + 0.5Lr + 1.6L and other Ultimate combinations.    When I run the program for different design load cases for ULS and the two service load cases for SLS, I am hoping I can see all the results with one run.  Along with the Ultimate verification I can then determine if my beam meets the Live Load Deflection limit check and Total Load Deflection limit check without having to run the program again.

 

Hope this makes sense. 

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Artur.Kosakowski
Posts: 4,947
Registered: ‎12-17-2010

Re: Steel design

12-20-2012 01:05 AM in reply to: Engineerjbird

The possibility of setting two different deflection limits is currently not for all steel codes implemented in Robot. It can be done e.g. for EC3 but not for the US codes. I'm trying to find the best solution therefore I would like to understand the LRFD code provisions for calculations of deflections. My problem is that not being familiar with this code I cannot find the part where it is described. In particular I'd like to understand what the live load term actually means. Is it that you create two set of SLS combinations:

1. Dead loads + Live loads and they are used for live load deflection check

2. Dead Loads + Live Loads + Other Loads and also Dead Loads + Other (excluding Live) Loads for total load deflection

Do you know which chapter of LRFD describes this part and where the deflection limits are listed?

 

I have also tried to search for some references but all examples I found use one deflection limit or are not clear. E.g. in the attached one you can find Live load and Snow and Wind limits which are the same.

http://faculty.delhi.edu/hultendc/AECT250-Lecture%205.pdf

 

Using such terms (Live load and Snow and Wind load); does it mean that the combinations are:

1. Dead loads + Live loads and they are used for live load deflection check

2. Dead Loads + Other (excluding Live) loads for total deflection check?



Artur Kosakowski
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Active Member
Engineerjbird
Posts: 7
Registered: ‎12-14-2012

Re: Steel design

12-20-2012 11:01 AM in reply to: Artur.Kosakowski

The checks are from the 2012 International Building Code, Table 1604.3 (Deflection Limits)

 

These are separate checks than the LRFD ULS design.

 

The checks are:

1.  L/360 for L (Unfactored).  No other loads applied.  Only Live Load.

2.  L/240 for D + L (Unfactored).  No other loads applied.  Only Dead + Live Load.

 

I was hoping these could be checked on the same run, but it looks as though this is not possible for the US code.

Can this be added in future updates?

 

 

 

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Artur.Kosakowski
Posts: 4,947
Registered: ‎12-17-2010

Re: Steel design

12-21-2012 02:17 AM in reply to: Engineerjbird

No, currently this cannot be done in one run as you cannot set two different deflection limits.

 

I will create a development request for such functionality but I would like to make sure I fully understand the code provisions. Looking at the table you mentioned I can see 3 columns L / S or W / D + L

 

Assuming that we have the ULS combination 1.2D + 0.5Lr + 1.6L)the corresponding SLS combination would be D + Lr + L (Unfactored).

 

My assumption is that the following deflection checks should be done:

 

1. L (l/360)

Live load only (meaning only value of deflection caused by live load from the definition of the above SLS combination is used) or shall Lr also be included?.  In the code L stands for live load plus Roof live load over 20 psf (mind that the value of roof live load acting on a particular bar will be rather difficult to determine automatically).

 

2. S or W (l/360)

ULS: 1.2D + 0.5S or 0.5W + 1.6L

SLS: D = L + W or S + L

Only value of deflection caused by snow or wind case is used from the above SLS combination is used.

 

3. D + L (l/240)

SLS: D + Lr + L - again live load or roof live load too?

In addition it is mentioned that for steel structural members (meaning these you actually design in Robot ?) the dead load should be assumed as 0. Does it mean that actually you have the same situation as in the point 1 above with different deflection limit?

 

Thank you for comments on the above points.

 



Artur Kosakowski
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Active Member
Engineerjbird
Posts: 7
Registered: ‎12-14-2012

Re: Steel design

12-21-2012 10:13 AM in reply to: Artur.Kosakowski

The table is broken down into the construction type; Roof members, Floor members, Wall members, etc. and the requirements of deflection limits for each Load;  L (Live), Lr (Live Roof members), D+L, D+Lr, W (Wind)  or S (Snow) or Loads.

 

 

  1. All live loads (Lr and L) would be checked together for either L/360, L/240 or L/180 depending on the material attached to it.  Sometimes we will want to satisfy L/480 or L/600, so it would be nice to have the capability to specify the limits in the SLS load combination. 

The (L+Lr) or pure live load deflection check would be for floor or roof members only.

The (D+L+Lr) load deflection check would be for floor or roof members only.

  1. The W and S load deflection checks are also dependent on the material attachment to them for either L/600, L/480, L/360, L/240 or L/180.  W and S deflections will be checked for roof or exterior wall members.   The engineer may want to create a SLS combination including a portion of D or L with W or S for a column member.    Most engineers will not use  D=0 according to footnote g.  You should ignore this note.

 

ULS cases (like 1.2D + 0.5LR +1.6L) do not include deflection limit checks, only strength is analyzed.

Note that the L in the L/360, L/240 and L/180 stands for member length not live load.  The table shoes lower case L’s. 

For this development request it is important for ROBOT to provide the capability of creating the SLS combination and specifying the deflection limit for that combination directly - instead of the member definition parameters table.  This way the different limits could be set for each SLS load case we create.

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Artur.Kosakowski
Posts: 4,947
Registered: ‎12-17-2010

Re: Steel design

01-03-2013 02:41 AM in reply to: Engineerjbird

Thank you for providing this information. I have added this point to the wish list.



Artur Kosakowski
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