A review of conceptual transparency in US and Colombian seismic design building codes

Transcription

1 INGENIEÍA E INESTIGACIÓN OL. 33 No. 2, AUGUST (24-29) A review of conceptual transparency in US an Colombian seismic esign builing coes evisión e la transparencia conceptual en los reglamentos e iseño sísmico e eificios en EU y Colombia J. Carrillo 1, J. Blanón-alencia 2 an A. ubiano 3 ABSTACT The goal of re-examinination of seismic esign coes is aime at making their provisions as transparent as possible for users so that the esign woul be clear an enriching for structural engineers. This paper presents a transparency evaluation of the coes currently being use in the USA an Colombia for seismic esign of builings. It is emonstrate that the proceures use in most coes o not offer a clear view of builings seismic assessment. The Colombian coe shoul become as conceptually transparen t as possible when efining strength moification factors an assessing maximum lateral isplacement. In aition, at least tw o limit states (service an life safety) shoul be clearly efine, along with allowable story rift thereby better reflect expecte structural performance. Otherwise, using current proceures coul lea not only to interpretation errors but also inaequate estimation of seismic strength an eformation emans. eywors: seismic esign, strength reuction, over-strength, uctility, isplacement amplification, rift limit. ESUMEN La reexaminación e los reglamentos e iseño sísmico pretene que los requisitos sean tan transparentes como sea posible para los usuarios, e tal manera que el proceso e iseño sea claro y enriqueceor para los ingenieros estructurales. En este artículo se presenta una evaluación e la transparencia e los reglamentos utilizaos actualmente en Estaos Unios y Colombia para iseño sísmico e eificios. Se emuestra que los proceimientos utilizaos en la mayoría e los reglamentos e eificios no proporciona una visión clara para evaluar la respuesta sísmica. El reglam ento colombiano ebe llegar a ser tan conceptualmente transparente como sea posible, en cuanto a la efinición e los factores e moificación e resistencia y a la evaluación e los máximos esplazamientos laterales. Aicionalmente, se eben efinir claramente mínimo os estaos límite (servicio y seguria a la via), junto con erivas e piso permisible que reflejen mejor el comportamiento estructural esperao. De lo contrario, el uso e los proceimientos vigentes poría originar no solo errores e interpretación, sino estimación inaecuaa e las emanas e resistencia y eformación. Palabras clave: iseño sísmico, reucción e resistencia, sobrerresistencia, uctilia, amplificación e esplazamiento, límite e eriva. eceive: December 14th 2012 Accepte: July 16th 2013 Introuction 1 2 In the current seismic esign coes of many countries, base shear is calculate by elastic strength eman ivie by a strength reuction factor. This factor reflects the influence of the structure s elastic-plastic eformation an energy-issipating capacity (i.e. reuce forces ue to nonlinear hysteretic behaviour). A isplacement amplification factor is use to compute the expecte maximum inelastic isplacement from the elastic isplacement inuce by the esign seismic forces (Uang, 1989). 1 Julian Carrillo. PhD in Structural Engineering. Affiliation: Universia Militar Nueva Granaa, Colombia. wjcarrillo@gmail.com 2 John Blanón-alencia. PhD in Structural Engineering. Affiliation: Universia Nacional e Colombia, see Meellín, Colombia. jblanon@unal.eu.co 3 Astri ubiano. MSc in Engineering. Affiliation: Universia Militar Nueva Granaa, Colombia. astri.rubian@unimilitar.eu.co How to cite: Carrillo, J., Blanón-alencia, J., ubiano, A., A review of conceptual transparency in US an Colombian seismic esign builing coes., Ingeniería e Investigación. ol. 33, No. 2. August 2013, pp The assessment of the minimum lateral strength capacity resulting in suitable control of inelastic eformation uring strong earthquake groun motions requires a goo estimation of the strength reuction factors. These reuction factors have been the topic of several investigations over the last 40 years. However, many of these investigations finings so far have not been incorporate into builing coes. Several researches have expresse their concern about the lack of rationality regaring the reuction factors currently specifie in builing coes (ojahn, 1988). The improvement of reuction factors has been ientifie as a way of improving the reliability of current earthquakeresistant esign provisions (Mirana an Bertero, 1994). This paper was thus aime at showing US an Colombian earthquake-resistant coes as transparent as possible for the users, so that their esign will be clearer for structural engineers. This paper thus provies an overview of the evelopment an the 24

2 CAILLO, BLANDÓN-ALENCIA AND UBIANO most relevant changes in US an Colombian earthquake-resistant esign coes, compares an iscusses the seismic-esign approaches specifie by these coes an the challenges involve in improving coe compliance, particularly the Colombian coe. The stuy inclues a iscussion of the most important parameters for seismic esign, such as strength moification an isplacement amplification factors. The main components neee to calculate these factors, such as the structural over-strength factor an structural uctility ratio, are also iscusse. A comparison between coes concerning rift limit an reuction factors is also iscusse. Strength moification factors Design lateral strengths prescribe in earthquake-resistant esign provisions are typically lower, in some cases much lower, than the lateral strength require for maintaining a structure in an elastic range in the event of severe earthquake groun motions. Strength moifications from the elastic strength eman are commonly accounte for using both reuction an amplification factors (Mirana, 1997). euction factor ue to nonlinear hysteretic behaviour A typical iealisation of the structural is shown in Figure 1a. The level of inelastic eformation experience by the system experiencing a given groun motion is typically given by the isplacement uctility ratio (Priestley, 2000). Iealising the actual structural curve by the linear elastic-perfectly plastic curve in Figure 1a, then structural uctility ratio can be efine as the ratio of maximum relative isplacement to its yiel isplacement (Mirana an Bertero, 1994). max (1) y Figure 1a also shows the require elastic strength expresse in terms of maximum base shear eveloping in a structure if it was to remain in the elastic range e. Since a properly-esigne structure can usually provie a certain amount of uctility, then such structure is able to issipate hysteretic energy. Because of such energy issipation, a structure can be esigne economically an thus, elastic esign force e can be reuce to yiel strength level y, by factor (Figure 1a) (Moroni et al., 1996); the corresponing maximum eformation eman is max: e y Since calculating y an max involves nonlinear analysis, these quantities are not usually explicitly quantifie. Strength reuction factor (i.e., the reuction in strength eman ue to nonlinear hysteretic behaviour) is one of the first an most-stuie reuction factor components. Factor is efine as the ratio of elastic strength eman to inelastic strength eman (Mirana, 1997): e Fy ( 1) (3) y Fy ( i ) where F y(=1) is the lateral yieling strength require to keep the system elastic an F y(= i) is the lateral yieling strength require to keep isplacement uctility ratio eman, less than or equal to a preetermine maximum tolerable isplacement (target) uctility ratio i, when subjecte to the same groun motion (Mirana an Bertero, 1994). A 5% equivalent viscous (2) amping ratio is usually consiere when computing reuction factor (Uang, 1989). e y Elastic y = max/y max e y Elastic y Figure 1. Structural : (a) iealise, (b) overall C Iealize nonlinear C / e max Actual nonlinear Stuies reviewe by Mirana an Bertero (1994) agree that for a given groun motion (i.e. groun acceleration time history), reuction factor is primarily influence by the level of inelastic eformation (i.e. isplacement uctility ratio), the natural perio of the structure T, an soil conitions at the site. It is worth noting that strength reuction factor prescribe by US coes (NEHP-03, IBC-09, ASCE 7-10) an by some Latin-American coes, such as the Colombian coe (NS-10) isregar the perio of vibration, which is incorrect, an thus their use is not recommene (Mirana, 1997). Amplification factor ue to over-strength As well as, another strength moification can be consiere in the esign to take over-strength into account. Over-strength i not enter into the previous iscussion because structural was consiere to be an iealise system. There are several sources of structural over-strength. Most are relate to the sequential yieling of critical regions, internal force reistribution (reunancy), actual materials strength higher than those specifie in esign, strain harening of reinforcing steel, capacity reuction factors, member selection (member oversize), minimum requirements by coes regaring proportioning an etailing, multiple loaing combinations, eflection constraints on system performance, level of force reistribution taking place in the structure, the effect of non-structural elements an strain rate effect (Uang, 1989; FEMA-451). Figure 1b shows the typical overall structural. The actual structural, the iealise linear elastic-perfectly plastic an elastic are inclue in the figure. The structure remains essentially elastic until the first full plastic hinge forms. This level is commonly calle the first significant yiel, i.e. the level beyon which global structural starts to eviate significantly from elastic. The first significant yiel is the level of force that causes complete plastification of at least the most critical region of the structure (e.g. first plastic hinge formation). The formation of this first significant yiel occurs at a loa level referre to as system esign strength,. The reserve strength between actual structural yiel level an coe-prescribe first significant yiel, is usually efine in terms of the over-strength factor. As shown in Figure 1b, over-strength factor can be efine as the ratio between y an, the latter being the require strength prescribe by coes using a strength esign approach (Moroni et al., 1996). INGENIEÍA E INESTIGACIÓN OL. 33 No. 2, AUGUST (24-29) 25

3 A EIEW OF CONCEPT UAL TANSPAENCY IN US AND COLOMBIAN SEISMIC DESIGN BUILDING CODES y (4) For esign purposes, NEHP-03 reuces y level to level, the latter corresponing to first plastic hinge formation. The avantage of specifying as the esign level is that esigners nee only perform an elastic structural analysis. The first problem associate with this type of elastic esign proceure is that esigners o not know the true strength of the structure. If the reserve strength of a structure (the so-calle over-strength) beyon esign level is significantly less than that implicitly assume in the seismic provisions, then structure performance is not likely to be satisfactory uring severe earthquakes. The secon problem is that the maximum inelastic isplacements cannot be calculate from elastic analysis results (Uang, 1989). Deflection amplification factor The equal isplacement approach Parameter has been wiely accepte as a useful performance inicator because of its apparent relationship to strength reuction factor. The equal isplacement concept is the basis for iviing elastic force emans by a strength reuction factor. It is one of the most important concepts in earthquake-resistant esign. It implies that the isplacement of an inelastic system, having stiffness an strength y, subjecte to a particular groun motion, is approximately equal to the isplacement of the same system responing elastically (system isplacement is inepenent of system yiel strength) (FEMA-451). As shown in Figure 1a, the equal isplacement approach of seismic implies that (Priestley, 2000): (5) Builing coes consier elastic structural analysis base on applie forces becoming reuce to account for the presume uctility supplie by the structure (base on the level of etailing provie). Using reuce forces from elastic analysis will result in a significant unerestimate of isplacement emans. Therefore, isplacements arising from reuce-force elastic analysis must be multiplie by the uctility ratio to prouce true inelastic isplacements. It has been shown that an equal isplacement approach is nonconservative for short perio structures, roughly corresponing to the first region of the spectrum. Equal energy approach shoul be applie in this region. This reuction is lower an epens on both perio of vibration T an uctility capacity. The primary reason is that short perio systems ten to isplay significant resiual eformations. Thus, increases linearly in the first region of the spectrum from = 1 to a value close to uctility ratio. ASCE 7-10 effectively reuces the acceleration spectrum by a strength reuction factor in all perio ranges (FEMA-451). However, the ASCE 7-10 coe allows no reuction of peak groun acceleration in the very short perio region (acceleration spectrum having a constant plateau extening from T = 0 s) so this partially compensates for error in equal isplacement assumption at low perio values. In the spectrum s mi-region, is only slightly epenent on perio of vibration T. However, it is of oubtful valiity for meium perio structures when the hysteretic nature of the inelastic system eviates significantly from elastic-perfectly plastic. For very long perios, the factor maintains a constant value equal to prescribe uctility, an thus the equal isplacement approach can be applie ( = )(FEMA-451). Accoring to Newmark an Hall (1982), for structures having long, meium an short perios, =, = (2 1) 0.5, an = 1, respectively. These expressions inicate that / is not greater than 1. Moreover, this ratio is significantly less than 1 for structures having meium an short perios. Displacement amplification Displacements from elastic analysis involving reuce forces are amplifie by the isplacement amplification factor C to estimate the structure s maximum expecte isplacements, incluing effects cause by inelastic eformation. Factor C is efine as the ratio between maximum expecte nonlinear isplacement uring an earthquake max, an elastic isplacement inuce by reuce seismic forces (Moroni et al., 1996)(Figure 1b). C max (6) The isplacement amplification factor C can also be erive from Figure 1b as follows: C max max y (7) y where max/ y is, an y/ from Figure 1b is: y y (8) Therefore, (7) can be expresse as: C (9) From these erivations, it can be observe that the C factor is a function of structural over-strength factor, structural uctility ratio an amping ratio; amping effect is usually inclue in uctility reuction factor. Evaluating builing coes The evolution an practice of US an Colombian seismic coes is briefly escribe an iscusse in this section. US coes Figure 2a shows proceures for seismic esign prescribe by US builing coes, such as NEHP-03, IBC-09 an ASCE Strength moification factors: In US process, esign seismic forces are obtaine by reucing a linear elastic spectrum by moification factor an then member forces are etermine through linear elastic analysis. Hence, factor is efine as the ratio between base shear in the structure if it was to remain in the elastic range an the minimum base shear require to resist seismic action an to accommoate nonlinear isplacements without any risk to its stability (Moroni et al., 1996). Figure 2a shows that total strength moification factor can be consiere the prouct of uctility reuction factor an structural over-strength factor (arela et al., 2004): e (10) Equation 10 shows that it is misleaing to call the uctility reuction factor, because structural over-strength may play a role equally important than uctility in factor (Uang, 1989). Similarly to C, prescribe in seismic coes is primarily inten- 26 INGENIEÍA E INESTIGACIÓN OL. 33 No. 2, AUGUST (24-29)

4 CAILLO, BLANDÓN-ALENCIA AND UBIANO e to account for energy issipation capacity an over-strength; however, it also accounts for amping (if ifferent from 5% of critical amping) an reunancy. e e Constant an C factor values o not ensure the same level of safety against collapse for all structures. For builings having minimal reunancy, structural over-strength relie upon by current seismic esign provisions may be insufficient. There is a nee for incorporating a metho to quantify the structure s over-strength; such over-strength shoul not be less than that assume in establishing an C (Uang, 1989). Table 2. Design factors in ASCE-7-10 for steel structures Structural system 0 =/ 0 C y = y C C / e max y y e max' Figure 2. Proceures in builing coes: (a) US, (b) Colombia. Factors an C usually epen on the perio of the structure, structural system type an the structural uctility. However, an C prescribe in US seismic coes are primarily base on the observation of the performance of ifferent structural systems in previous strong earthquakes, consensus of engineering jugment, technical justification an traition. Strength reuction factors are one of the most controversial aspects of current builing coes. Several researchers (Uang, 1989; Mirana an Bertero, 1994) have expresse their concern about the lack of rationality in current factors an their improvement has been ientifie as a way to improve the reliability of present earthquake-resistant esign provision. For instance, most investigations reviewe by Mirana an Bertero (1994) recommen the use of perio-epenent strength reuction factors. Uang (1989) establishe basic formulas for evaluating an C from global structure characterise by the relationship between base shear ratio an storey rift. ariations in, with changes in perio of vibration, are not incorporate in current seismic provisions for builing structures in the US. The permissible levels of strength reuction are only base on the type of structural system. Current seismic esign provisions in the US o not require esigners to quantify an factors. ASCE 7-10 provies the an 0 factors for a large number of structural systems. Tables 1 an 2 show the esign coefficients for a few selecte concrete an steel systems, respectively. Table 1. Design factors in ASCE-7-10 for concrete structures Structural system 0 =/ 0 C Special moment frame Intermeiate moment frame Orinary moment frame Special reinforce shear wall Orinary reinforce shear wall Detaile plain concrete wall Orinary plain concrete wall It is very important to note that is uctility eman only if 0 is achieve an uctility eman m is minimum because 0 as liste in the tables is the maximum expecte over-strength. A uctility eman equal to one or less inicates that the expecte for these systems is essentially elastic. Special moment frame Intermeiate moment frame Orinary moment frame Eccentric brace frame Eccentric brace frame (pinne) Special concentrically brace frame Orinary concentric brace frame Not etaile Even though the equations presente by Mirana an Bertero (1994) seem reasonable an may be incorporate in future US seismic coes, toay (2013) single values for are still propose in such seismic coes for esigning ifferent structural systems. Displacement amplification factor: base on the equal isplacement approach, inelastic isplacement eman is the same as elastic isplacement eman. Figure 2a shows clearly that, isplacement preicte by this analysis woul be too low. Compute esign isplacement shoul be multiplie by isplacement moification factor C an thus to obtain an estimate of true maximum inelastic to correct for the too-low isplacement preicte by the reuce force elastic analysis. This factor is always less than because contains ingreients other than pure uctility. Similarly to an factors, ASCE 7-10 provies C (see Tables 1 an 2). It is interesting to examine the ratio C / in Figure 2a. It can be shown from (9) an (10) that: C (11) Equation 11 inicates that the C / ratio for a particular structural system is a function of structural uctility ratio only through an, an is inepenent of the structural over-strength factor (Uang, 1989). C / ratios specifie by US coes are constant an inepenent of the perio of vibration, thus estimating inelastic isplacements is not suitable for structures having a short perio an resting on rock or on firm soil. For structures resting on soft soils, the estimate is aequate only for structures having a very long perio ( = ). ASCE 7-10 also provies allowable story rift to be compare to true maximum inelastic rift. As shown in Table 3, allowable rift epens on a builing s importance. Colombian coe Many areas of South America are note for their high seismicity. ecognising the region s seismic activity, earthquake-resistant esign of structure is thus a requirement in these countries. Therefore, each country has evelope its own seismic coe base on their experience an laws. The coes also follow aspects of UBC-97an IBC-09. INGENIEÍA E INESTIGACIÓN OL. 33 No. 2, AUGUST (24-29) 27

5 A EIEW OF CONCEPT UAL TANSPAENCY IN US AND COLOMBIAN SEISMIC DESIGN BUILDING CODES Table 3. Story rift limits in ASCE 7-10 Structural system Structures, other than masonry wall structures, 4 stories or less above the base with partitions that have been esigne to accommoate story rift isk category 1 or % 2.0 % 1.5 % Masonry cantilever shear wall structures 1.0 % 1.0 % 1.0 % Other masonry shear wall structures 0.7 % 0.7 % 0.7 % All other structures 2.0 % 1.5 % 1.0 % Most South-American coes traitional esign philosophy is to maintain life safety by avoiing collapse uring severe earthquakes. Although ifferent activity levels may be use, the esign earthquake is typically an event having a 475-year return perio, as use in UBC-97. Strength moification factors: the overall seismic esign proceure prescribe by the 1998 eition (NS-98) an by the recently release NS-10 is shown in Figure 2b. Colombian coes have use the conventional forcebase/isplacement-check approach. The 1984 Colombian Seismic Coe use a moification factor that varie for each structural system, material an seismic risk (Garcia, 1996). is the reuction factor use by NS-98 an NS-10 coes to ecrease the elastic seismic forces. The reuction factor in NS has the same purpose as in the US coes (Eq.10), i.e. to account for the global uctility capacity of the lateral force resisting system, an the over-strength inherent in lateral force resisting system. is a function of the type of the system, perio of vibration, irregularity an a builing s expecte esign level or esign category. in NS-10 is also a function of structural system reunancy. The Colombian seismic coe uses reuction factors (always 1) to account for any irregularity an reunancy in the structure. In the 1984 eition, was a single an constant value use to constantly reuce elastic forces, regarless of a structure s perio of vibration (Chavez, 2012). prescribe by NS-98 an NS-10 coes oes not vary with perio of vibration when the coe spectrum is use, an it oes so only for the micro-zoning spectrum. The recently release NS-10 also explicitly specifies an overstrength factor 0 relate to the seismic-force-resisting system an is use for esigning certain fragile elements which are incapable of issipating energy in the non-lineal range, such as certain wall piers, anchors an collector elements, or where there are greater concerns about shear failure. For esigning such elements, the esign shear force nee not excee 0 times the factore shear etermine by analysing the structure for earthquake effects. Amplification factor 0 ranges from 1.5 an 3.0, epening on the type of seismic system. The approach involving using amplification factor to account for the seismic-force-resisting system s over-strength has been aapte from the ACI Builing Coe, where esign shear force is compute as 0 times the shear inuce uner esign isplacements. The effect of over-strength shoul be accounte for when evaluating a member s strength (as an amplification factor regaring strength). Because of the limitations of using avance non-linear analysis techniques by practicing engineers, most builing coes apply the effect of over-strength as a reuction factor to the loas instea of an amplification factor to the strength. However, the NS-10 approach coul be oubtful because it attempts to amplify earthquake forces by 0, instea of amplifying member strength or reucing earthquake loas. Displacement evaluation: in the 1984 Colombian Seismic Coe, rift was obtaine from elastic eflections amplifie by eflection amplification factor C. This factor also epene on the structural system, material an seismic risk level. The allowable rift limit was a single constant value equal to 1.5%. If rift was within the allowable limits, the esigner coul esign the ifferent elements using the requirements for each seismic risk level. If the rift requirements were not met, a structure ha to be stiffene an re-analyse (Garcia, 1996). Accoring to NS-10, structures can sustain extensive amage without collapsing when subjecte to the esign earthquake; this implie that a collapse prevention limit state was aopte. The NS-10 approach seems to assume the equal isplacement approach (Figure1a) because inelastic isplacement max is equal to the isplacement which woul occur uring elastic e (Figure 2b). Allowable rift for masonry structures controlle by shear eformations is 0.5%; for other structural systems the rift limit is 1.0%. A 1.0% rift limit is require when gross section stiffness is use in analysis; if cracke sections are use in analysis, calculate rift must be reuce by 30% before comparison (estrepo, 2008). Allowable rifts prescribe in NS-10 are very ifferent from the values prescribe in forming coes (see Table 3). When comparing a 1.0% rift limit in NS-10 with 2.0% or 2.5% for the collapse prevention limit state in ASCE 7-10, the rift limits presente in NS-10 are more relate to serviceability than collapse prevention limit state, regarless of the text within the coe referring to this scenario as a collapse prevention limit state. The 1984 seismic coe specifie a 1.5% rift limit; this value was later reuce to 1.0% in NS-98. The reason for such reuction was to prevent non-structural amage an to encourage the use of shear walls ue to the goo performance achieve uring an M w=7.8 earthquake in Chile in Some stuies were carrie out for making such change, but they were mostly base on financial loss (Garcia, 1996) without paying attention to changes in reuction factors that such moification woul have create. Trying to protect non-structural elements is a very important issue in a country like Colombia having limite economic means, as pointe in the above stuies. The root of the problem lies in trying to prevent non-structural amage an conceive a uctile structure in the same scenario. The serviceability limit state seems to be coherent with the rift limits prescribe within the coe an collapse prevention limit state seems to be coherent with reuction factor suggeste in the coe. Base on the roughly elastic behaviour foun with time history analysis (THA) of reinforce concrete frames, estrepo (2008) conclue that the way that seismic esign was being use in NS-98 (similar to NS-10) seeme to be inappropriate as it le to very expensive C frame structures an structures whose performance was quite beyon requirements. The main reason of such tren takes root in the single scenario that NS-98 (an NS-10) use to perform structural analysis. Member sections require to satisfy rift limits are immense, making very important the minimum steel requirements given by the coe, as this imposes high uctility etailing to a structure which will perform almost elastically. Such elastic performance raises large oubts about the reuction factors use in esign which are 28 INGENIEÍA E INESTIGACIÓN OL. 33 No. 2, AUGUST (24-29)

6 CAILLO, BLANDÓN-ALENCIA AND UBIANO mainly base on the suppose inelastic movements that the structure will unergo when subjecte to the esign earthquake. The Colombian coe has aapte the US coes without making significant changes relate to members esign, structural system, analysis methos an/or hazar analysis; however, a major change was mae in the analysis methoology since the Colombian coe uses only one scenario for esigning both structural an non-structural elements. This change to only a single esign scenario comes with rift limits which are inconsistent with both serviceability an ultimate limit states. The rift limits prescribe in NS-10 are very ifferent from the values prescribe in the US forming coes. An inconsistency arises by using a high reuction factor = 7.0 having a 1% restrictive rift limit for concrete frames. For instance, it oes not seem logical to use the rift values prescribe in NS-10 with some of the consierations for uctile structures within the coe (estrepo, 2008). This type of stagnation is not consistent with that foun in other coes aroun the worl. Conclusions This paper has summarise an iscusse the approach aopte in seismic esign provisions for builings in the USA an Colombia. The following conclusions can be rawn from this stuy: Strength moification an isplacement amplification factors (empirical to ate) have been mainly base on engineering jugment consensus an observe structural performance uring previous earthquakes. The only way to rationalize these factors is to quantify over-strength an structural uctility ratios by analytical stuies an experimental testing. The use of rational strength moification an isplacement amplification factors base on uctility, perio an soil conitions, together with estimates of the structure s over-strength an the relationship between global an local uctility emans are neee to establish a more rational an transparent seismic esign approach than that currently being use in the Colombian coe. In aition, more rational criteria nee to be stipulate in NS- 10 for computing lateral isplacements. It is recommene that a performance-base esign approach be inclue in the Colombian coe to inclue at least two limit states base on specific return perios. Each limit state shoul inclue specific rift limits consiering the type of non-structural elements attache to the structure an a particular structural system. Assessment of Colombian coes (estrepo, 2008) has shown iscrepancies regaring performance requirements an safety levels. A serious effort ought to be mae to improve such coes an their enforcement. For instance, the proceure prescribe in the next eition of the Colombian builing coe shoul allow etermining esign strengths an isplacements in a more rational way, more in accorance with the present state of knowlege an contemporary tenencies in builing coes. Despite these criticisms, it shoul be note than current forcebase seismic esign, when combine with capacity esign principles an careful etailing, usually prouces safe an satisfactory esigns. However, the egree of protection provie against amage uner given seismic intensity is non-uniform from structure to structure. eferences ACI Committee 318. Builing coe requirements for structural concrete (ACI -318) an commentary (ACI -318). American Concrete Institute, Farmington Hills, MI, ASCE Minimum esign loas for builing an other structures. American Society of Civil Engineers, ASCE, eston, irginia, USA, Chavez, J. Overview of the current seismic coes in Central an South America. Bulleting of IISEE, ol. 46, pp , FEMA-451. ecommene provisions: Design ex amples. Feeral Emergency Management Agency, FEMA, W ashington, USA, García, L. Economic consierations of isplacement -base seismic esign of structural concrete builings. Journal of Structural Engineering International - IABSE, ol. 6, No. 4, pp , IBC-09. International Builing Coe. International Coe Council, Falls Church, A, USA, Mirana, E. Strength reuction factors in performance-base esign. Proceeings of EEC-CUEe Sy mposium, Berkeley, CA, Mirana, E. an Bertero,. Evaluation of strength reuction factors for earthquake-resistant esign. Journal of Earthquake Spectra, ol. 10, No. 2, pp , Moroni M., Astroza M., Gómez J. an Guzmán,. Establishing an C factors for confine masonry builings. Journal of Structural Engineering ASCE, ol. 122, No. 10, pp , NEHP-03. ecommene provisions for seismic regulations for new builings an other structures (FEMA 450). Builing Seismic Safety Council, W ashington, D.C., USA, New mark, N. an Hall, W. Earthquake spectra an esign. Eart h- quake Engineering esearch Institute, EEI, El Cerrito, CA, USA, NS-10. Colombian Coe for earthquake-resistant construction. Colombian Association of Earthquake Engineering, AIS, Colombia, Priestley, M. Performance base seismic esign. Proceeings of 12th W orl Conference on Earthquake Engineering 12W CEE, Aucklan, New Zealan, paper 2831, estrepo, J. Concrete frames esigne w ith the Colombian seismic coe (NS-98) from a isplacement perspect ive. Master Thesis, Instituto Universitario i Stui Superiori i Pavia, ojahn, C. An investigation of structural moification factors. Proceeings of 9th W orl Conference on Earthquake Engineering 9W CEE, Toky o, Japan, ol. 5, , Uang, Ch. Establishing (or w ) an C factors for builing seismic provisions. Journal of Structural Engineering ASCE, ol. 117, No. 1, pp , UBC-97. Uniform Builing Coe. International Conference of Builing Officials, Falls W hittier, CA, USA, arela, J., Tanner, J. an lingner,. Development of moification coefficient an eflection amplification factors for esign of AAC structural sy stems. Proceeings of 13th W orl Conference on Earthquake Engineering 12W CEE, ancouver, Canaa, paper 1058, INGENIEÍA E INESTIGACIÓN OL. 33 No. 2, AUGUST (24-29) 29