Interstorey Drift Limits, Moment Releases And Base Fixity

[Yasir Saleem]

Hello everyone,

 

I am looking for some answers to simple questions like

 

1.  What is the inter story drift limit for steel columns , and what is the maximum deflection allowed at the mid span of the column which has axial load and lateral loading as well ? 

 

2.  How we decide the end at which moment need to be released and which one like M22 or M33? Does this depends upon the support end and type of support?

 

3.  What is the difference between the pin support and fix support in terms of its application to the model? As lateral meaning is not required its the sense that on which basis we decide that oke we donot model pin support and assign fix support to the model. and then when we are designing base connection like base plate and anchor bolt and position of anchors on the base plate. Does this all only gives us freedom in the fixity of structure at the base like we are free on locating the suitable position for anchors on the base plate or it has to do something else as well ?

 

4. What about the wind loading? if we calculate the wind load manually and then apply it as a lateral load to the whole structure is ok or we just go as making the areas around the nodes and apply the load only to the nodes and then chk the deformation. What about the wind analysis we do by the help of software how accurate are they as the deformation due to wind is nothing in many cases if you see the analysis report or table... 

[abdulqadeer29]

i am anxiously waiting for someone to comment on above questions,

[UmarMakhzumi]

Hello everyone,

 

I am looking for some answers to simple questions like

 

1.  What is the inter story drift limit for steel columns , and what is the maximum deflection allowed at the mid span of the column which has axial load and lateral loading as well ? 

 

2.  How we decide the end at which moment need to be released and which one like M22 or M33? Does this depends upon the support end and type of support?

 

3.  What is the difference between the pin support and fix support in terms of its application to the model? As lateral meaning is not required its the sense that on which basis we decide that oke we donot model pin support and assign fix support to the model. and then when we are designing base connection like base plate and anchor bolt and position of anchors on the base plate. Does this all only gives us freedom in the fixity of structure at the base like we are free on locating the suitable position for anchors on the base plate or it has to do something else as well ?

 

4. What about the wind loading? if we calculate the wind load manually and then apply it as a lateral load to the whole structure is ok or we just go as making the areas around the nodes and apply the load only to the nodes and then chk the deformation. What about the wind analysis we do by the help of software how accurate are they as the deformation due to wind is nothing in many cases if you see the analysis report or table... 

 

1) Inter-story drift limit depends upon type of lateral loading, i.e., wind and seismic. The following two articles explain how to check them for both kind of lateral loading.

http://www.sepakistan.com/topic/1341-ubc-seismic-drift-limits/

http://www.sepakistan.com/topic/109-building-drifts-in-etabs/

Maximum deflection are allowed based on codes too. A Column with axial load and later loading is a beam-column. You can use a limit of L/360 as per UBC for that. This is for local member. For overall structure, you need to satisfy wind and seismic limits. See the links above.

 

2) Yes it does and no it doesn't. There is a trade off between pinned and fixed support and both have reasons for that e.g., impact on foundation. You as a engineer should value reasons, then analyze the same type in the model and finally make sure your connection is fabricated so that what you idealized is real too. Also, you need to see your framing to see what kind of connection would serve best.

 

3) To some extent it is what you are saying. If you assume pin, your anchor bolts need to be inside flanges along the webs as you wouldn't develop any moment in base plate. So your base plate wont be that thick. If you go other way, your base plate would be thick with stiffeners and anchor bolts located outside to make that happen. But, assuming pin or fix will have an impact on frame member sizes. It is not just location of anchor bolts. You quantity of steel may bump up if you move from fix to pin and vice versa depending upon framing.

 

4) Wind should be same for both types. I have done both and my drifts were same. Check your calculation and do a comparison of shears and drifts to see you calculated forces correctly.

[Yasir Saleem]

sir agree with you about the first point and i have already gone through these pages but my concern is still about the deflection of a certian member in the mid span. I say it like this that you have a steel column which is fixed at both ends and its height is say 5 meters how much the allowed deflection for that at midspan. column is loaded by uniform lateral load. As if we go to that l/360 crityeria then the deflection is too much i have seen in BS that for curtian walls and mullions this is l/175 or l/250 my question is this deflection is at the end again what about the mid span. Oke one thing we chk the slenderness of the section and it should be less then 200. what if this ration is less like 60 or 160 and the deflectionn of our coulmn is like 100 mm to 300mm? What is the criteria for that. I chkd this by considering the coulmn as a simply supported beam and calculated the deflection manually at mid span . but again what are the upper limits to confine it.    

 

About wind what about the wind results from the analysis as in many cases no value any reason for it or i am doing something wrong.

[UmarMakhzumi]

sir agree with you about the first point and i have already gone through these pages but my concern is still about the deflection of a certian member in the mid span. I say it like this that you have a steel column which is fixed at both ends and its height is say 5 meters how much the allowed deflection for that at midspan. column is loaded by uniform lateral load. As if we go to that l/360 crityeria then the deflection is too much i have seen in BS that for curtian walls and mullions this is l/175 or l/250 my question is this deflection is at the end again what about the mid span. Oke one thing we chk the slenderness of the section and it should be less then 200. what if this ration is less like 60 or 160 and the deflectionn of our coulmn is like 100 mm to 300mm? What is the criteria for that. I chkd this by considering the coulmn as a simply supported beam and calculated the deflection manually at mid span . but again what are the upper limits to confine it.    

 

About wind what about the wind results from the analysis as in many cases no value any reason for it or i am doing something wrong.

Yasir, post a image of your complete frame, identifying the column you are talking about. What kind of lateral force resisting system you have? Generally, for columns, drift is checked at the story of interest rather than deflection at midspan; so you can not do a correlation by modelling it as a beam. Anyhow, what is your utilization of the column? And are there any beams framing in to the column at different levels? Post images so that situation can be assessed. Also post member sizes.

About the wind: what is the difference of your manual calculations from software generated total wind load? Post calculations.

[Yasir Saleem]

Sir g if you look at the drawing and first section at the top left side these columns the consultant has asked for inter story deflection for that section. so how will i chk that inter story deflection. i am attaching the commented copy as well from the consultant. The engineer who was working before me in this office calculated the deflection for that section busing L/200 and submitted that in the report which got rejected. So look at the drawing i think you will understand what i am saying. You will see the comment in the same first section top left side of drawing.

NYU-ATA-DRG-C1-AR-00003-00.dwg

NYU-ATA-DRG-C1-AR-00003-00 REV.02_STATUS_B_iss1_revA.pdf

[Badar (BAZ)]

This is what I understood from the drawings: non structural members, about 5 m in length, are called upon to support the lauver. These nonstructural members are supported at bottom, mid height and at top of panel by concrete beams.

Now the question is: what should be the 'serviceability requirement' for these non-structural members. According to IBC, floor beams supporting non-plaster ceiling should be limited to a max live load, as well as wind load, deflection of l/240. But I would say that this member should fall in the category of the one's not supporting any ceiling. In that case the deflection for both load cases is limited to l/180 (IBC).

[Yasir Saleem]

Yes sir you are right that these are nonstructural members and and the limits you mentioned i have gone through these, but my element is attached at the top as well and consultant is asking question of inter story drift here and these deflections are again at the top end as you mentioned (not supporting the ceiling) what about the mid span if the element is fixed at both ends. 

[Badar (BAZ)]

Checker has used the word 'interstory deflection' in his comments. I think he wants you to make sure that these members will not deflect to a extent that will damage those lauvers. So, the values that I have mentioned above are for beam or flexural members. These nonstructural members need to be checked, against wind loads, for mid span deflection. You don't have to model its connects that will resist end moments, it seems unneccesary: model these members as having simple supports, and consequently design the bolts at mid and at top for shear and tension. You can do the same for the bottom connection.

[UmarMakhzumi]

Yasir, see the attached file. 

• Make sure, your wind & settlement are in one direction. In the attached sketch, I didn't have enough space to draw everything systematically, so I just randomly drew stuff.
• Your settlement at C should be Delta 1 + Delta 2
• Make sure you run all your combos for wind

 

@Sir Zeeshan, I think his checker wants him to calculate the effect of story drifts on the member itself. Same as we used to talk about designing members that not part of lateral force resisting system but are attached to lateral force resisting system, and will undergo a delta equal to story drift that will induce moment & shear in them.

SEFP yasir.pdf

[Yasir Saleem]

Sir g please have look at these what is wrong and where...all the deflection criteria which are available or i have seen give us or tell us about the end deflection only. Slenderness and buickling is only one thing which t=is there to chk the element but what about this what i as asking, and if i am wrong please do correct me.

[UmarMakhzumi]

Sir g please have look at these what is wrong and where...all the deflection criteria which are available or i have seen give us or tell us about the end deflection only. Slenderness and buickling is only one thing which t=is there to chk the element but what about this what i as asking, and if i am wrong please do correct me.

Yasir, like posted above the max midspan for element is controlled by the maximum deflection acceptable at that point by attached non structural member. What I cann't understand is that why are you checking max deflection at midspan when your checker is asking you to satisfy interms of story drifts?

[UmarMakhzumi]

You need to see the vendor catalogue for the attached aluminium louvers to see what displacements do they specify. In absence of that, you can use wind inter-story drift limits for cladding attached to columns.

[Yasir Saleem]

sir does that means that this we should not consider even if our deflection is like that much we just go with the delta if its oke then leave whatever the model show. and what about report when we print the report for certain load cases and the values are high does that will not annoy the checker ?

[UmarMakhzumi]

sir does that means that this we should not consider even if our deflection is like that much we just go with the delta if its oke then leave whatever the model show. and what about report when we print the report for certain load cases and the values are high does that will not annoy the checker ?

For a non-structural member attached as cladding, unless limits or drifits are specified by vendor brochure, you are left with wind code provided limits only as your criteria. You also need to make sure that your wind loads are reasonable. Your loading looks high to me. Plus your are looking at displacements where as limits are imposed for drifts. As long as you are satisfying code specified criteria your checker cant say anything, i still think he wants you to do what I posted above in the pdf.

[UmarMakhzumi]

Check the attached snapshot!

[Badar (BAZ)]

@umar I don't think so that checker will ask him to ascertain the effect of story drift on nonstructural member. If his structural system,  which, I think, is comprised of RCC frame, satisfy the the drift limits that will limit, or avoid, the damage to cladding/non-structural-members, he does need to do anything else on these non-structural members. 

 

Yasir could inquire from him about that aspect, to know if he is worried about the damage to these vertical posts, and attached louvers, due to the drift experienced by RCC frame. In any case you can check the strength of these post against the moment and shear that will result from drift. 

 

I think that strength and serviceability limits should be satisfied for these non-structural members, independently of main structural system. It is the mid span deflection due to wind load that also needs attention.

 

The thing you have mentioned refers to the members which are part of a system resisting gravity loads. Code requires us to design these members, meant to resist only gravity loads, for drift inflicted by the members,  part of Lateral load resisting systems, on to them . The box-shaped posts, used in current discussion, only supports louvers.

[Badar (BAZ)]

@ Yasir, Is the height of your frame(RCC) about 10m ? I am not able to understand the arrangement made at about 5m; it does not look part of main frame, but an arrangement to support door opening.

[Yasir Saleem]

sir thanx for the detail comment i was going to write some of your concerns about the limits that sir umer shared above these are there when gravity load or point load is acting and drift the consultant concern for these posts is midspan deflection due to wind load how much they will deflect and what is limit.

 

Sir Zeeshan the height of total frame system is 10 m but there are two floors each with height of 5235 and at mid the RCC element is slab with beam just like at top.