Bentley Success Factors Webinar: Code Requirements for the Analysis of Concrete Structures
Webinar Q&A
Webinar Date: October 2016
Webinar Description
While much effort and attention is paid to the calculation of concrete member capacities and reinforcement selection in the design of concrete structures, the analysis requirements are often less well understood, improperly applied, or neglected entirely. Code provisions that provide methods for determining the capacity of members are based on an assumption that the analysis used to obtain the member design forces considered specific issues of stability, 2^{nd}-order effects, and stiffness. What are the Code requirements for Analysis? What constitutes a valid analysis? Strategies and techniques for addressing the analysis requirements will be presented. Analysis requirements in ACI 318 and Eurocode EN 1992-1-1 are highlighted and compared. It is demonstrated how software can be used efficiently and productively to consider and satisfy these analysis requirements.
In this webinar:
Learn about issues of stability such as member and structural imperfections, 2^{nd}-order effects such as P-delta, and lateral and vertical stiffness, and how they affect structural behavior and concrete member design forces.
Join in a discussion of how these issues are addressed in the Analysis requirements of the building codes, when their effects must be considered and when they can be ignored.
See a comparison of the requirements of the Analysis sections of ACI 318 and Eurocode EN 1992-1-1. Understanding the philosophy behind other codes helps us better understand our own.
See a brief demonstration of how software can be utilized productively to achieve the analysis required by these building codes.
The recording of this webinar is available at: Code Requirements for the Analysis of Concrete Structures
The Webinar Q&A and the recording of a related webinar, Fundamentals of Analysis and Design for Stability, is available at: Fundamentals of Analysis and Design for Stability
During the webinar several questions were received. Time did not permit answering all of the questions nor of providing detailed answers. Compiled here are the questions that were received, with more thorough answers.
Analysis Requirements
Q: When using the Amplified 1^{st}-order Elastic Analysis method the Code indicates that the moments are to be amplified. What about drift?
A: Although the code is silent on this, it would seem reasonable and necessary to amplify the drift values. The P-delta effects certainly impact the drifts, potentially significantly, and failure to consider those effects could result in serviceability problems. Note that those usually aren’t life safety concerns, but rather impact the performance of finishes, windows, doors, and so forth. An advantage of performing a 2^{nd}-order analysis rather than amplifying the results of a 1^{st}-order analysis is that these 2^{nd}-order effects are automatically included in all of the design forces and the drifts, simplifying the design process.
Q: Why does the code permit small P-delta effects to be ignored when the column has a low slenderness value? Isn’t that potentially unconservative?
A: Unlike steel columns, the dimensions of concrete columns tend to be quite large, especially compared to the potential value of the small-delta displacement. The resulting small p-delta moment will therefore tend to be quite small, and so while ignoring that moment is unconservative, it is unconservative to a very small degree. So it is permitted in order to simplify the design process. The ACI Commentary indicates that it is assumed that it is acceptable to ignore a 5% increase in Moments due to the 2^{nd}-order effects (ACI 318-14 Section 6.2.5)
Q: What is meant by “sway frames” and “nonsway frames”?
A: If a frame is necessary to provide lateral stability to the structure, it is considered to be a “sway frame”. For example, moment frames; the stability of the building is dependent on the stiffness of the beams and columns in the moment frames. If a member is providing virtually no lateral stiffness compared to the stiffness of walls or other frames, it is considered a “nonsway frame”. For example, columns in a building with shear walls. The stability of the structure is dependent on the stiffness of the walls; the beams and columns essentially lean on the walls for their lateral support. In ACI 318-14 Section 6.2.5 it indicates that if the bracing elements are at least 12 times greater than the stiffness of the columns, the columns ae considered braced against sidesway.
Q: Why doesn’t ACI 318 require the consideration of notional loads to account for out-of-plumbness of construction?
A: I don’t know. Perhaps the feeling is that these out-of-plumbness values are small compared to the dimensions of the columns and so have little effect. Furthermore, if the out-of-plumbness is random (that is, some columns are out-of-plumb in one direction and some in another, these would tend to cancel out the effects. So perhaps it was felt that the effects of out-of-plumbness are well within the factor of safety.
Q: What ‘cracking’ factors are to be used in the Eurocode?
A: The factors that include cracking and creep are found in EC 2, Clause 5.8.7.2. They are a function of the axial force in the member. This may require that the analysis be performed iteratively, modifying the cracking factor based on each previous analysis. As a simplification, in statically indeterminate structures fully cracked sections can be assumed, to simplify the analysis process.
Q: Is there a code analysis guideline(s) for temperature analysis of concrete slabs?
A: I haven’t found any analysis requirements related to temperature in ACI 318-14. That doesn’t necessarily mean, however, that temperature need not be considered. It can often be ignored, but if for a given structure it has a significant adverse impact it must be accommodated regardless of the lack of requirements in the code. Eurocode EN 1992-1-1 Clause 2.3.3(1) says, “The consequences of deformation due to temperature, creep and shrinkage shall be considered in design.” Clause 2.3.3(2) goes on to say that these effects are normally accommodated by complying with the requirements of the Standard. And then Clause 2.3.3(3) says, “In building structures, temperature and shrinkage effects may be omitted in the global analysis provided joints are incorporated at every distance d_{joint} to accommodate resulting deformations.” The recommended value for d_{joint} is 30m but is subject to the National Annex. So this is probably true for most Codes, accommodate the effects of temperature and then you generally don’t need to include it in the analysis. Note that this may not be sufficient in regions with extreme temperatures, where it may be more common to perform temperature analyses.
Q: What percentage of dead load should be taken for notional loads? What does the code specify about notional loads?
A: This depends on the Code. ACI 318-14 has no requirements for Notional loads. Eurocode EN 1992-1-1 has a fraction given by Eq. (5.1) which is a function of height and number of columns. Notional loads for Dead and Live (Imposed) loads are required. As a comparison with steel, for the Direct Analysis Method and the Effective Length Method in AISC 360-10, Notional loads of 0.2% of the Dead and Live (Imposed) loads are required. To simplify the methodology for the Direct Analysis Method, the stiffness reduction factor taub can be set to 1.0 if an additional Notional load of 0.1% is applied. For the First-Order Analysis Method the Notional load is variable, with a minimum of at least 0.42%.
Q: You mentioned that notional loads could be applied to gravity-only load combinations or applied to all lateral and gravity load combinations. Where is it specified which to use?
A: This was in reference to the AISC 360-10 steel specification. Section C2.2b(4) states, “For structures in which the ratio of maximum second-order drift to maximum first-order drift… in all stories is equal to or less than 1.7, it is permissible to apply the notional load, Ni, only in gravity-only load combinations and not in combinations that include other lateral loads.” This only applies to steel structures.
Q: How is the stiffness adjusted in a model of a steel structure?
A: In AISC 360-10 Section C2.3, the Direct Analysis Method requires that a factor of 0.80 be applied to “all stiffnesses that are considered to contribute to the stability of the structure.” Section C2.3(2) requires that an addition factor, taub, be applied to the “flexural stiffnesses of all members whose flexural stiffnesses are considered to contribute to the stability of the structure.” In the RAM Structural System, for example, this is accomplished by factoring the steel section properties Ix, Area, etc., used in analysis.
Q: Can you explain more about the skip load? The term is a little unfamiliar to me.
A: Another term for this is “pattern loading”. This has to do with the code requirements related to the arrangement of live loads on alternate spans and adjacent spans to produce the worst design moments and shears. In ACI 318-14 this is given in Section 6.4.2 for one-way slabs and beams and in Section 6.4.3 for two-way slabs. In EN 1992-1-1:2004 this is given in Clause 5.1.3(1). As explained in the presentation the RAM Structural System automatically performs an exhaustive skip, or pattern, loading analysis of the Live loads for one-way systems, and allows the user to define the pattern loads for two-way systems.
Q: Is it advisable to use slender columns in design just by considering the additional moment amplification given by the code?
A: Slender columns may not be the most economical due to the greater reinforcement that is often required. However, column size may be driven by architectural requirements or constraints so it is often unavoidable. From a safety standpoint I assume that the Codes provide adequate factors of safety in the analysis and design requirements to confidently allow for the use of slender columns.
Q: How is cracking of concrete to be modelled?
A: This is a much broader topic than can be covered here, but briefly, the requirements for using cracked section properties are given in Section 6.6.3.1.1 of ACI 318-14 and in Clause 5.8.7.2 of EN 1992-1-1:2004. In the RAM Structural System crack factors can be specified for each member, and then the program applies those factors to the member stiffnesses which are then used in the analysis. Many programs offer this capability, otherwise you must manually determine and specify reduced section properties in your model.
Q: In ACI is it ever permitted to perform an analysis without the "cracked" factors? For example, is it acceptable to not apply the 0.25 factor for post-tensioned slabs?
A: ACI 318-14 Section 6.6.3.1.1 requires that the cracked section properties be used “unless a more rigorous analysis is used.” I can see nowhere in the code that otherwise allows you to perform the analysis without the cracked factors. A more rigorous analysis is permitted to be performed, but that analysis method must somehow account for the effects of cracking. For example, RAM Concept’s load history analysis has the ability to account for cracked deflections due to gravity loads for both post-tensioned and reinforced concrete slabs.
Q: When do you use the stiffness modifier for cracked and uncracked walls? How do you determine since in ACI there are 2 stiffness modifier factors for walls?
A: The ACI 318-14 Commentary to Section 6.6.3.1.1 states: “If the factored moments and shears from an analysis based on the moment of inertia of a wall, taken equal to 0.70Ig, indicate that the wall will crack in flexure, based on the modulus of rupture, the analysis should be repeated with I = 0.35Ig in those stories where cracking is predicted using factored loads.” So this requires performing an analysis with the 0.70 crack factor, looking at the stresses, and then if necessary assign the 0.35 crack factor to those walls that would crack, and analyzing again. It isn’t stated but it seems that you would then need to look at the stress in the walls that previously didn’t crack to see if they now crack, and if so use the 0.35 crack factor for them and analyze yet again. This then becomes an iterative process.
Q: Do we need to use cracked section in drift check?
A: ACI 318-14 Section 6.6.3.2 addresses the service load analysis, which would seem to include drift, although it doesn’t say that explicitly. Section 6.6.3.2.2 states that for such analyses it is permitted to use moments of inertia that are 1.4 times those defined in 6.6.3.1, which is the section that requires the use of cracked moments of inertia. The Commentary to Section 6.6.3.2.2 does refer to using this greater value in the investigation of deflections, vibrations, and building periods, so it seems reasonable to also use this greater value when investigating drifts.
Q: If the core wall length is too long, it fails due to out-of-plant bending, so in that case what should be done to improve the modelling?
A: Be certain that the proper out-of-plane properties are specified so that the program is using a realistic stiffness. Also be certain that the supporting effects of the diaphragm are appropriately modeled. It is possible that the wall truly is failing due to out-of-plant bending; that possibility needs to be considered and dealt with, not by changing the model, but by modifying and adding stiffening elements to the structure itself.
RAM Structural System
In the actual construction, presence of opening in the beams is usually unavoidable. How can we ensure accurate estimation of stiffness reduction? How can it be modeled in RAM Structural System?
Q: Will RAM Concrete give warning if instability occurs due to 2^{nd}-order effects?
A: The analysis of the structure is performed in RAM Frame. If you use the option to include P-delta you will get a warning if there are instabilities. RAM Concrete Column module assumes that the analysis in RAM Frame included P-delta, it does not apply the moment magnifier for sway frames. It does however apply the nonsway moment magnifier of Section 6.6.4.5.1 to nonsway frames. If Pu exceeds 0.75Pc in equation (6.6.4.5.2) the program will give a warning.
Q: Can you customize the reinforcement in the beams?
A: Yes. There are a number of criteria that can be set to customize the steel selection. Once the design has been performed using those criteria, you also have the ability to modify the reinforcement selected by the program. In this way you can customize the reinforcement in any beam, column or wall.
Q: I believe ACI allows skip live loads to be applied per building bay and does not necessarily require skip loading every single beam in the structure. Does RAM have an option to only skip load the building bays?
Can live load be skipped in two way slabs?
A: ACI 318-14 Section 6.4.2 gives the requirements for arranging the Live loads on one-way systems, requiring arranging loads on alternate spans and adjacent spans. Section 6.4.3 gives the requirements for arranging the Live loads on two-way systems, requiring arranging loads based on bays. For one-way systems RAM Concrete automatically performs the skip loading. For two-way systems the user can create whatever patterns of loads they want, by laying down individual load polygons in the Modeler, and then specifying that each load polygon is a distinct load case using the option in the Criteria – Analysis command.
Q: As indicated per ACI 318-14, will RAM determine if the 12x stiffness provision in Section 6.2.5 is satisfied in order to consider columns to be braced against sidesway?
A: No, currently the program does not perform that test automatically.
Q: When performing dynamic analysis there are two inputs in RAM frame called scale factor. What are they for?
A: Many codes require that when a response spectra analysis is performed the resulting base shears must be within some fraction of the base shears determined from the equivalent lateral force method. For example, in Section 12.9.4.1 of ASCE 7-10 it requires that if the modal base shear (Vt) is less than 85% of the calculated base shear (V) using the equivalent lateral force procedure, the forces are to be multiplied by 0.85V/Vt. Rather than multiplying all of the forces by this factor it is easier to scale the response spectra analysis. This can be done using these scale factors. By specifying the correct scale factor the modal base shear can be made to equal 85% of the base shear from the equivalent lateral force procedure, indicating that the forces can be used for design without further scaling.
The Webinar Q&A and the recording of a webinar previously presented, A Practical Approach to Using the Response Spectra Analysis Method, is available at:
A Practical Approach to Using the Response Spectra Analysis Method
Q: Can cold form hot rolled tubes be designed in RAM?
A: Yes. The program includes tables of round, square and rectangular HSS shapes, including those that conform to the A1085 specification.
RAM Concept
Q: How can support column shortening be modeled properly in RAM Concept?
A: RAM Concept analyzes elastic axial deformations on walls and columns. These elastic deformations can also be used to estimate subsequent creep deformations and differential column shortening, although RAM Concept does not automatically calculate the long-term column and wall deformations. This option to consider elastic axial deformations is controlled via the “compressible” check box on the column and wall properties. You can see the values of these deformations on a vertical deflection plan, using the plot distribution tool.
Q: Will RAM Concept consider the added stiffness in a cracked beam due to additional reinforcement?
A: Yes, RAM Concept considers the stiffness due to reinforcement in cracked sections in the load history deflection analysis.
Q: How does RAM Concept address the requirement to place reinforcement for unbalanced moment within a smaller slab effective width?
A: RAM Concept does not design or check this reinforcement directly, but reports via the punch check audit tool the portion of unbalanced moment to be transferred by flexure so that this clause can be readily checked manually.
Q: Is there a reason RAM Concept does not use Direct Design Method?
A: Direct Design Method is an approximate approach that only applies to a set of very specific conditions. There would be no benefit to implementing this in RAM Concept. RAM Concept’s analysis is more accurate and is not bound by the constraints of Direct Design Method.
Q: For the long-term deflection, is the effect of creep and shrinkage included?
A: Yes, the effects of cracking, creep, shrinkage, and load history are all considered in the load history deflection calculations.
STAAD
There were several questions specific to STAAD that would require responses longer than could be accommodated here. Information on STAAD is available elsewhere on Bentley Communities. Recordings of past STAAD webinars is available here: http://pages.info.bentley.com/videos/
Information on STAAD training courses is available here:
https://www.bentley.com/en/learn/for-users/training-programs
Q: What is the difference between results of STAAD and RAM?
A: The RAM Structural System is specifically for building structures. This allows for faster modeling and more specialized analysis, design, and reporting. Generally the RAM Structural System is preferred for buildings. STAAD has a wider selection of building codes, some of which aren’t available in the RAM Structural System. It is also general purpose, suitable for any structure, building, plant, tank or frame of virtually any configuration. Both programs have their strengths, so it depends on what is being analyzed and designed. With the Structural Enterprise License, a bundled license of STAAD and the RAM line of products, you can have both:
https://www.bentley.com/en/products/product-line/structural-analysis-software/structural-enterprise
https://www.bentley.com/~/asset/14/2716.ashx
Q: Does STAAD.Pro perform P-small delta analysis?
A: Yes. The PDELTA ANALYSIS in STAAD.Pro includes both large and small p-delta effects by default. If one does not want to include the SMALLDELTA, one has to specifically use the command PDELTA nn ANALYSIS LARGEDELTA where nn stands for the number of iterations.
Q: Are these increased loads due to stability accounted for in STAAD.Pro or we will have to input this as part of the loads in load cases?
A: When generating the automatic load combinations, notional loads could be automatically included as part of the load generation process itself. One can also manually add the notional loads to any load case if so desired. The following wiki has more information on the topic:
http://communities.bentley.com/products/structural/structural_analysis___design/w/structural_analysis_and_design__wiki/20964.does-specifying-the-notional-load-factor-as-0-002-automatically-define-notional-loads
Q: In addition to using the additional moments provided by the code, what other stability checks should be performed by STAAD.Pro? How do you detect the stability effects and whether the structure is safe or not from STAAD output?
A: Apart from including large and small p-delta effects, one can include notional loads to account for imperfections/out of plumbness and stiffness reductions ( both axial and flexural ) to account for material yielding, inelastic softening etc. The Direct Analysis method for steel structures which takes into account all of these, is supported in STAAD.Pro. To ensure that the structure is stable, one should always pay attention to the maximum deflections and ensure that those are realistic values. One should also check the equilibrium from the static check results. If the total applied load matches the summation of reactions, the force equilibrium is guaranteed. If there are instabilities or zero stiffness encountered at certain nodes during analysis, the software would generate warning messages. So one should always watch out for zero stiffness or instability or convergence failure messages which are indications that the structure may be unstable.