Badar (BAZ)

Below is an excerpt from the document titled: Seismic Design of ReinforcedConcrete Mat Foundations. The document is prepared by NEHRP. Earthquake load effects, E, are defned in ASCE 7-10 §12.4.Inherent in the defnition of these seismic demand levels is thestructural response modifcation factor, R, which is dependenton the type of lateral force-resisting system considered for thesuperstructure. The same overturning that is determined at thebase of the superstructure is to be applied directly to the matfoundation. Consideration of higher foundation demand levelsis triggered only by unusual structural geometries, such as acantilever column system or a discontinuous lateral system,which require the consideration of a system overstrength factor,per ASCE 7-10 §12.4.3. By defining earthquake load effects with an inherent R-value, some inelastic behavior is assumed in the superstructure and therefore implies that a mat foundation also may experience inelastic demands. You can also study the document in this link: bulletin.nzsee.org.nz/13/2/0171

When you are analyzing RCC frame members, you are concerned with flexural and shear demands in most cases, if serviceability and stability concerns have been addressed. The choice of modifier for torsional stiffness does not effect those results. Choosing the modifier equal to 0.001 will lead to conservative results if your modelling the slab for serviceability and strength-checks. As far as designing the beam for Tcr, in case of compatibility torsion, the code clearly directs the engineer to do that, as you have mentioned in your post..

Looks like the accident during construction operation rather than the structural failure.

*Comments/Observations regarding modelling in ETABS* *Doc No: 10-00-CD-0006* *Date: May 06, 2017* Some of the observations made during extraction of results from ETABS (v 9.7.4), for design of reinforced concrete members, are being share in this article., 1) Minimum Eccentricity 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. 2) Unbraced/ Braced Preference If your model has lateral loads, ETABS will give you design moments in column irrespective of its status as braced or un-braced as per ACI 318 criteria. You should investigate if the storey under consideration is braced, or un-braced (10.10.5.2), and decide appropriate design moments of columns. 3) Time Period 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. (For a complete discussion on time period, please see the following this thread that complements this section). 4) Stiffness Modifiers 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. This article is based on my two separate posts regarding the subject matter. You can view the discussion on the items raised above by viewing the following links: 1) http://www.sepakistan.com/topic/2008-issues-in-etabs-results/ 2) http://www.sepakistan.com/topic/2290-modelling-issuesconsideration-in-etabs/ Thanks.

For newer version, go to Design --------> Concrete Frame design --------> View/Revise Preferences

Select the seismic design category by going to the "options" drop down menu.

What do you intend to include in additional live load?

Thanks. It can be done.

ACI Code has not specified any stain-range for under reinforced beams. They just say that the beam should fail in tension before failure of compression region. By that definition, the tensile strain in steel must have a value of more than 0.002 before the compression strain in concrete reaches 0.003 for under-reinforced section.

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.

After running the analysis, one way is to look for the symbol that is used for displaying the deformed shape of your structure. The deformed shape will be displayed against a load case, or load combination. Once you have deformed shape, right click on the node where you want displacements. Limits are already discussed in the forum. Use the search facility, either on google, or in the forum.

You are getting unexpected Shear force distribution because, I believe, you have assumed the uniform distribution of bearing pressures under the strip. Find out the resultant of series of point loads, and then provide the dimensions of bearing area in such a way the geometric centroid of the bearing area coincides with the resultant of series of point loads. If you can't do that, then do the finite-element-modeling.

Why are you modelling the structure in ETABS. If you are modelling to know the seismic demand on walls, then you should have included beams, because you can't neglect the weight of beams as the weight effect the seismic demand. The weight also effect the time-period. If you are modelling to find stresses in your bearing walls, you need to model these beams, as they will be inducing the point load on walls.

A two hour session will be held in the Wah campus of CIIT. The attendees will receive a certificate which can be helpful for engineers in matters of PEC-registration. The speaker will be discussing the topic of performance-based design and evaluation of tall buildings. It is good opportunity for structural engineers to equip themselves with recent advancements regarding the structural analysis and design of tall buildings. The speaker, Dr. NAVEED ANWAR, who is executive director at AIT Solutions, is not only a Structural engineer, but also has been actively involved in software-development of CSI products such as ETABS, SAP2000 and CsiCol. Please find the brochure in the attachment for further details. CPD_Dr Naveed_1.pdf

If the wall is receiving the slab loads by two-way action, then you need to check the adequacy of slab against punching.

What is the question?

One example is to transfer of diaphragm forces to shear wall, if the wall does not have collectors, or drag members. The flexural reinforcement of slab will transfer in-plane shear of slab.

It should be dubbed as building frame system. The relative stiffness of vertical members of lateral load system and their strength and ductility decides the category of structural system.

The factor of 0.35Ig is not applicable to beams with flanges. For the interior beam, the factor will be around 0.7Ig, and for the edge beam, it will be around 0.5Ig. Moreover, these factors are for un-braced frames; factors are different for braced frames. Please go through the commentary of the section 8.7.1 of ACI 318-08.

I think both will indicate the torsional irregularity at a level in the building. The ETABS gives those results in the summary report; the software reports in term of maximum and average displacements and their ratio at each story.

In calculating design forces, have you considered the application of point load on a panel, which will support lights, at the top of the pole, in addition to the UDL on the pole itself. Is it possible that only two bolts will be effective in resisting moment for a wind direction in which two bolts will lie on the neutral axis, and the moment arm will be sqrt(2) x 12.

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.

The design of tie beam will be governed by amount of differential settlement of the foundation selected for the job at hand. If delta is the differential settlement, then the fixed-end moment due to that value will be: (6EI/L*L)delta. Add the gravity load Fixed End Moment to it, and then distribute this moment among members of beam-column joint according to their flexural stiffness. That is a crude way of proportioning the tie beam.

I hold the same opinion on this topic.