31 posts in this topic

I have observed few issues while getting the design results of reinforced concrete members from ETABS (v 9.7.4). I wanted to share this issues with you and get your input and observation on it.

  • ETABS always considers the minimum eccentricity for selecting the design moment of columns irrespective of the probable behavior of the column, whether short or long column. See section 10.10.6.5 and its commentary of ACI 318-08 which deals with minimum eccentricity of long columns. You should always check the design moments that ETABS uses for columns if you want to bring down the cost of construction.
  • ETABS always performs analysis of frame as if it is un-braced. You should investigate if the storey under consideration is braced, or un-braced (10.10.5.2), and decide appropriate design moments of column, if ETABS has included sway moments.
  • ETABS has a tendency to select a time period of the building that is considerably less than the value obtained by the approximate method, Method A, of the section 1630.2.2  of UBC 97. To quote the FEMA 451 document: ''Because this formula is based on lower bound regression analysis of measured building response in California, it will generally result in periods that are lower (hence, more conservative for use in predicting base shear) than those computed from a more rigorous mathematical model". So, there is no need to use the value of time period that is lot less than Ta. One should always check the time period used by the software; ETABS can overestimate the seismic force by more than 2 times.

    Method A gives lower T and higher V, so FEMA 451 has advised not to use the value of time period less than this value even if rigorous analysis gives a lower value.

    I have seen the results where Etabs have use the value of time period less than Ta; in-fact as low as 0.5Ta, which can increase the base shear two times.

Edited by Rana
Merged some posts to avoid confusion

Share this post


Link to post
Share on other sites

1

On 5/1/2016 at 2:26 PM, baz said:
  • ETABS always considers the minimum eccentricity for selecting the design moment of columns irrespective of the probable behavior of the column, whether short or long column. See section 10.10.6.5 and its commentary of ACI 318-08 which deals with minimum eccentricity of long columns. You should always check the design moments that ETABS uses for columns if you want to bring down the cost of construction.

Astounding observation. Just to add my 2 cents that in exterior columns, minimum e does not govern. For interior columns it is always good idea to design columns for minimum eccentricity. Yes this is conservative and depends on the choice of designer.

 

2

On 5/1/2016 at 2:26 PM, baz said:
  • ETABS always performs analysis of frame as if it is un-braced. You should investigate if the storey under consideration is braced, or un-braced (10.10.5.2), and decide appropriate design moments of column, if ETABS has included sway moments.

Even if the structure is braced, magnification factors will be so small that they will not matter much.

 

3

On 5/1/2016 at 2:26 PM, baz said:
  • ETABS has a tendency to select a time period of the building that is considerably less than the value obtained by the approximate method, Method A, of the section 1630.2.2  of UBC 97. To quote the FEMA 451 document: ''Because this formula is based on lower bound regression analysis of measured building response in California, it will generally result in periods that are lower (hence, more conservative for use in predicting base shear) than those computed from a more rigorous mathematical model". So, there is no need to use the value of time period that is lot less than Ta. One should always check the time period used by the software; ETABS can overestimate the seismic force by more than 2 times.
     

See discussion below

Share this post


Link to post
Share on other sites

ETABS 2015 have  option in design that whether you consider minimum eccentricity or not

 

Share this post


Link to post
Share on other sites

but in etabs 2015 it works.

 

Rana likes this

Share this post


Link to post
Share on other sites
On ‎7‎/‎27‎/‎2016 at 8:42 AM, Rana said:

Astounding observation. Just to add my 2 cents that in exterior columns, minimum e does not govern. For interior columns it is always good idea to design columns for minimum eccentricity. Yes this is conservative and depends on the choice of designer.

According to the section 10.10.6.5 and its commentary (of ACI 318-08), we do not need to use minimum eccentricity if there are negligible second order effects.   

Rana likes this

Share this post


Link to post
Share on other sites
On 5/1/2016 at 2:26 PM, BAZ said:

I have observed few issues while getting the design results of reinforced concrete members from ETABS (v 9.7.4). I wanted to share this issues with you and get your input and observation on it.

  • ETABS has a tendency to select a time period of the building that is considerably less than the value obtained by the approximate method, Method A, of the section 1630.2.2  of UBC 97. To quote the FEMA 451 document: ''Because this formula is based on lower bound regression analysis of measured building response in California, it will generally result in periods that are lower (hence, more conservative for use in predicting base shear) than those computed from a more rigorous mathematical model". So, there is no need to use the value of time period that is lot less than Ta. One should always check the time period used by the software; ETABS can overestimate the seismic force by more than 2 times.
     

Do you mean "There is no need to use the value of time period that is LESS than Ta" or "NOT LESS than Ta"?

 

As you quoted FEMA 451, it says

Ta is lower-bound limit (more conservative for base-shear as T-computed would be higher)

CuTa is the upper-bound limit (can be ignored for drift calculations)

 

But,

 

There is no guidance on if T-computed is less than Ta, not even in ETABS manuals. All the documents highlight the upper limit but not the lower one.

For example, consider a bearing wall building, (density of walls/floor is large), T-computed is ~ 0.10s, however Ta=0.5s; huge difference.

ETABS only checks the upper limit (lets say 1.0s) and not the lower one and selects 0.10 automatically and calculates base shear for this T.

 

However, if I am getting you right or as per; 2009 NEHRP Recommended Seismic Provisions; Chapter 4, page-41, it says;

if T-computed is less than Ta, then use Ta.

https://www.fema.gov/media-library-data/1393888487069-c071850c3050c34da135376baa478a1b/P-752_Unit4.pdf

Please shed some light on this issue.

 

UmarMakhzumi likes this

Share this post


Link to post
Share on other sites

There is no lower limit for T but there is an upper limit for Maximum V, so even if the time period calculated by ETABS is significantly lower than Ta, code provisions for maximum base shear would govern and the calculated value of T would be ignored.

Similarly, no vertical distribution of force would take place for a small time period hence your design would still be in reasonable FOS limits.

Share this post


Link to post
Share on other sites
4 hours ago, Saad Pervez said:

Similarly, no vertical distribution of force would take place for a small time period hence your design would still be in reasonable FOS limits.

Does vertical distribution depends on time period? It must depends on weight participation and stiffness of floors.

EngrJunaid likes this

Share this post


Link to post
Share on other sites
12 hours ago, Engr Waqas said:

Does vertical distribution depends on time period? It must depends on weight participation and stiffness of floors.

UBC97 1630.5: Concentrated force at top, Ft, becomes zero if time period is 0.7 seconds or less. My statement was not 100% correct. Apologies.

Waqas Haider likes this

Share this post


Link to post
Share on other sites

 

On ‎12‎/‎21‎/‎2016 at 11:27 AM, Rana said:

However, if I am getting you right or as per; 2009 NEHRP Recommended Seismic Provisions; Chapter 4, page-41, it says;

if T-computed is less than Ta, then use Ta.

I meant that.

Rana likes this

Share this post


Link to post
Share on other sites
1 hour ago, BAZ said:

 

I meant that.

Wow. But there is no clear reference for this in ubc or asce. I got the sense and could save some structures from over-design but would be very difficult to convince authorities without a clear overt, explicit reference to building codes.

Share this post


Link to post
Share on other sites

Code enforces minimum stiffness requirement through drift control and stability requirements by ensuring second order effects are not significant. I would personally be comfortable using Ta if "drift and second order effect are within limits". 

Thanks.

Share this post


Link to post
Share on other sites

I have another point,

 

Quote

The period of formulas in the 1997 Uniform Building Code (UBC1997), the 1996 Structural Engineering Association of California (SEAOC) recommendation...are derived from those developed in 1975 as part of the ATC3-06 project [Applied Technological Council (ATC) 1978] largely based on periods of buildings measured from their motions recorded during the 1971 San Fernando earthquake.

Goel and Chopra 1997 [1] developed improved empirical formulas to estimate the fundamental vibration period of RC moment resisting frames (MRF) buildings for use in equivalent lateral force analysis specified in building codes using motions of many buildings recorded during earthquakes.

The data used in [1] have been combined from the motions of buildings recorded during the 1971 San Fernando, 1984 Morgan Hill, 1986 Mt. Lewis and Palm Spring, 1987 Whitter, 1989 Loma Prieta, 1990 Upland, 1991 Sierra Madre and 1994 Northridge earthquakes

(Source: http://www.tdmd.org.tr/TR/Genel/pdf/TDMSK010.pdf)

Lower bound and upper bound limits of regression analysis are given as in building codes.

 

More exact solutions (such as in analytical models) give higher T than Ta due to absence of partition and infill walls etc. But my concern is for buildings having large shear walls areas such as tunnel construction, very very dynamically stiff (e.g. 0.1s). In such cases (T-computed < Ta) and as mentioned by following paper, we should not use Ta (as per discussion in posts above) and should use T-computed.

Paper can be found here: http://erolkalkan.com/Pubs/12.pdf

Quote

As is observed from those figures, the obtained finite-element analysis results significantly differ from the code-referred values...The significantcant deviation between current code given formulas and finite-element analysis leads to intolerable errors for the dynamic parameters and corresponding design loads.

Also see this: http://www.sciencedirect.com/science/article/pii/S1687404815000565

Share this post


Link to post
Share on other sites

I think both ETABS and UBC 97 provisions are correct.

As ETABS completely follows UBC97 therefore only UBC's provisions are elaborated below.

UBC seismic design philosophy limits the minimum time period (in the form of maximum base shear ) to Ts (UBC response spectrum) which is equivalent to Cv/2.5Ca.

For elaboration ,

V=CV.I.W/R.T

Substitutue T=Ts=Cv/2.5Ca in above eqn

therefore 

V=(CV.I.W/R)X(2.5Ca/Cv)

i.e V=2.5Ca.I/w (the maximum base shear that ETABS and UBC97 uses)

Therefore time period regardless of its shorter value below Ta cannot be lesser than Ts which is in contrast with UBC97 provisions that are inherent in ETABS.

However, analysis indicating time period lesser than Ta indicates a over stiff/over design structure, therefore it is recommended to maintain a natural period closer to 0.1x (no of floors).

 

 

 

 

BAZ and UmarMakhzumi like this

Share this post


Link to post
Share on other sites
1 hour ago, Syed Umair Haider said:

I think both ETABS and UBC 97 provisions are correct.

As ETABS completely follows UBC97 therefore only UBC's provisions are elaborated below.

UBC seismic design philosophy limits the minimum time period (in the form of maximum base shear ) to Ts (UBC response spectrum) which is equivalent to Cv/2.5Ca.

For elaboration ,

V=CV.I.W/R.T

Substitutue T=Ts=Cv/2.5Ca in above eqn

therefore 

V=(CV.I.W/R)X(2.5Ca/Cv)

i.e V=2.5Ca.I/w (the maximum base shear that ETABS and UBC97 uses)

Therefore time period regardless of its shorter value below Ta cannot be lesser than Ts which is in contrast with UBC97 provisions that are inherent in ETABS.

Dear Umair, I understand what you said, but, that is only one of the condition I asked about (see below). Ok let me put this in following way;

lets call T-computed simply as T, there are following 4 cases;

a. T < Ts < Ta

b. T > Ts but < Ta

c. T > Ta but < CuTa {not my concern}

d. T > CuTa {not my concern}

Now lets focus on condition a & b; Let's say; Ts=0.3s and Ta=0.7s

Condition a

Assume T=0.1s which is less than both Ts and Ta

In order to jump to V, we need to first set the time period.

a1. If we use Ts, then obviously maximum base shear would govern. This is what you are referring to.

a2. if we use Ta (which is higher than Ts), base shear would be less than Max. V so we get some saving.

Now the question is what T you'd use? Ts or Ta?

As per FEMA (moment frame example) use Ta.

As per research papers I have mentioned for tunnel construction you should use actual T (or in other words Ts as Max V would govern, as you rightly pointed out).

 

Condition b

Assume T=0.5s which is more than Ts but less than Ta.

Still, if we go with FEMA approach, we should use Ta which will give us lesser base shear. And use of actual T would give us higher base shear.

 

So the question remains, what value of T to be used?

Quote

However, analysis indicating time period lesser than Ta indicates a over stiff/over design structure, therefore it is recommended to maintain a natural period closer to 0.1x (no of floors).

0.1 x (no of floors) is for certain moment frame buildings as per ASCE 7-10 seismic design examples book (example 17), that might not be correct for buildings with large no/area of shear walls. Anyway, the intention, as i see, is to use approximate method (Ta) if analysis T is less than Ta, which is in-line with what Baz said. However, after going through the above research papers, I am again inclined towards using T from analysis instead of Ta for such buildings. For moment frames, yes we could use Ta.

Share this post


Link to post
Share on other sites

My initial post was to point to fellow engineers that one must look at the value of the time period used by the ETABS for calculating the base shear.

For normal construction in Pakistan, the construction in which the area of shear walls seldom exceeds 1% of the floor area, the time period of the building should not be significantly less than the value obtained by empirical formulae of UBC 97. If ETABS is using the value which is, say 60%, of that value, then there should be some mistake in modeling, and the design engineer should be aware of that.

UmarMakhzumi likes this

Share this post


Link to post
Share on other sites
10 hours ago, BAZ said:

My initial post was to point to fellow engineers that one must look at the value of the time period used by the ETABS for calculating the base shear.

For normal construction in Pakistan, the construction in which the area of shear walls seldom exceeds 1% of the floor area, the time period of the building should not be significantly less than the value obtained by empirical formulae of UBC 97. If ETABS is using the value which is, say 60%, of that value, then there should be some mistake in modeling, and the design engineer should be aware of that.

100% agreed. Usually shearwall/floor area is ~ 1%

BAZ likes this

Share this post


Link to post
Share on other sites

 

Dear Waseem

Your concern is valid but the lower limit given in FEMA example doesn't make sense to me as in a general the usage of time period in seismic analysis is not more than the evaluation of ground acceleration that will be imparted in structure in accordance with specific structural characteristics.

Similarly if the specific structural characteristics of a building reveals a time period lesser than generalized approximate time period , then it doesn't make sense to skip the accurate time period and to use approximate one when code itself defines approximate time period as a basis to start analysis for actual T.

More interesting is to note that indicated FEMA example is based on 2009 NEHRP seismic provisions , whereas in a separate document issued by NEHRP afterwards (named expanded seismic commentary to ACE-10) it is recommended to use "Tcomp" if "Tcomp < Ta".

Moreover, i didn't find any lower bound on time period in UBC97 and even in ASCE 7 which is based on same ELF procedure as given in FEMA example.

Therefore, it seems that indicated provisions in FEMA example are overlooked that they have fixed later and usage of Tcomputed if "Tcomputed < Tapproximate" seems valid.

 

 

 

BAZ, Ayesha and UmarMakhzumi like this

Share this post


Link to post
Share on other sites

ETABS can provide fairly accurate time periods if modelling assumption, soil structure interaction is accounted well. See attachment. T computed should be used if a person knows what he is doing. That is a big "IF.:D

Thanks.

 

ETABS vs Field Time Periods.pdf

Share this post


Link to post
Share on other sites
13 hours ago, Syed Umair Haider said:

 

Dear Waseem

Your concern is valid but the lower limit given in FEMA example doesn't make sense to me as in a general the usage of time period in seismic analysis is not more than the evaluation of ground acceleration that will be imparted in structure in accordance with specific structural characteristics.

Similarly if the specific structural characteristics of a building reveals a time period lesser than generalized approximate time period , then it doesn't make sense to skip the accurate time period and to use approximate one when code itself defines approximate time period as a basis to start analysis for actual T.

More interesting is to note that indicated FEMA example is based on 2009 NEHRP seismic provisions , whereas in a separate document issued by NEHRP afterwards (named expanded seismic commentary to ACE-10) it is recommended to use "Tcomp" if "Tcomp < Ta".

Moreover, i didn't find any lower bound on time period in UBC97 and even in ASCE 7 which is based on same ELF procedure as given in FEMA example.

Therefore, it seems that indicated provisions in FEMA example are overlooked that they have fixed later and usage of Tcomputed if "Tcomputed < Tapproximate" seems valid.

 

 

 

Ok, what about these;

 

#1

2015 NEHRP Recommended Seismic Provisions: Design Examples FEMA P-1051/July 2016

https://www.fema.gov/media-library-data/1474320077368-125c7a1d1a3b864648554198526d671f/FEMA_P-1051.pdf

"Calculation of a period based on an analytical model of the structure is encouraged, but limits are placed on the results of such calculations. These limits prevent the
use of a very flexible model in order to obtain a large period and correspondingly low acceleration."

Page: 2-17

and

"A lower limit to the base shear determined from the modal analysis procedure is specified based on the static procedure, and the approximate periods specified in the static procedure."

Page: 2-18

Also consider the example in section 13.2.6 and I have attached the sketch explaining that example for easy understanding here; (Ta was used instead of Tc).

dyn.jpg

 

#2

See ASCE publication seismic design guidelines based on ASCE 7-10, Example 17 on page; 117 & 118;

a.png

and

b.png

 

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!


Register a new account

Sign in

Already have an account? Sign in here.


Sign In Now

  • Similar Content

    • By BAZ
      I want to comment on some modelling issues in ETABS. Though some of these things are discussed elsewhere in the forum, I hope to extract some more useful conclusions.
      First thing is related to modelling the bending stiffness of flexural members, for strength level loads, that is representative of their condition near failure.
      The ACI code specifies the modifier of 0.35 on gross moment of inertia to represent its condition at yielding.
      Some people say that the factor should be multiplied by 2 to represent the stiffness of T-beam. This approach would be justified if you are not taking into the account the out of plan bending stiffness of slab.
      But, ETABS does include the out of plane bending stiffness if you have modelled the slab by using shell elements. So, a factor of 0.7 would overestimate the stiffness of your structure in this case, and will lead to under-design.
      If one has used the modifier of 0.35 in ETABS for beams in beam-slab floor system, then what value should be adopted for slab? It should not be 0.25, as this value has been specified for flat plates and flat sab floor system.
      If one is using some value of modifier for out of plane bending stiffness on shells, then the share of the bending moment in beams will be reduced accordingly. This approach is correct if one will be providing the reinforcement in column strips of slab. But, if you are providing reinforcement in slab in the direction perpendicular to supports only, i.e. beams, as is the general practice in Pakistan, then you are under-estimating the flexural demand in beams.
      Now, there is also a question of factors to be used while deciding the amount of reinforcement required in beams, columns and shear walls.
      If you are using factors 0.35 for beams and shear walls, and 0.7 for columns, then you are finding out the demand in members at the point of yielding, and this conforms to the code. But, this also means that the structure might experience unacceptable cracks widths. So, if you are using 0.35 for calculating the demand at strength-level forces, then you should also perform crack-control-check at service-level loads by using the factor of 1.
      If you are calculating the strength-level demand with a modifier of 1 for all structural members, after you have decided the location and the number of shear walls with modifier of 0.35, then you are overestimating seismic forces, as you are underestimating the time-period. But, the structural performance will improve.