SEISMIC CODE EVALUATION. NICARAGUA Evaluation conducted by Guillermo Santana

Transcription

1 SEISMIC CODE EVALUATION NICARAGUA Evaluaton conducted by Gullermo Santana NAME OF DOCUMENT: Reglamento de Construccón que regrá el Terrtoro Naconal (Constructon Regulaton that wll govern n the Natonal Terrtory) YEAR: 1983 GENERAL REMARKS: Document elaborated by a techncal commttee under the supervson of the Mnstry of Housng and Human Settlements of the Government of Natonal Reconstructon of the Republc of Ncaragua. SPECIFIC ITEMS: NOTE: Bracketed numbers refer to Code specfc chapters or artcles: [1.2.3] Parentheses numbers refer to Items of ths document: (see 2.2) 1. SCOPE 1.1 Explct Concepts. [Art. 1] The norm apples to the desgn and constructon of new buldngs, as well as to the repar and retrofttng of exstng facltes. It ncludes load prescrptons for earthquake, wnd and even volcanc ash deposton, wth the assocated zonng. 1.2 Performance Objectves. [Art. 1] The performance objectves are stated as a) to avod the loss of lves and to reduce the possblty of physcal damages to persons; b) to resst smaller earthquakes wthout damages; c) to resst moderate earthquakes wth mld structural damages and moderate non structural damages; d) to avod the collapse of buldngs due to large earthquake, reducng the damages at economcally admssble levels and e) to resst wnd effects and other accdental actons wthout damage. 2. SEISMIC ZONING AND SITE CHARACTERIZATION 2.1 Sesmc Zonng (Qualty of Data). [Art. 22] The country s dvded nto sx sesmc zones. The lowest level s assgned to the northeastern half of the country. Ths area, shown n the map below, covers 80 percent of the Carbbean coastlne and all the lowlands of that regon. The other half of the country s dvded nto the addtonal fve zones, whch ncrease n ntensty as t surrounds the Lake of Managua and the

2 captal cty of the same name. The map s a dervaton of the one proposed n A Study of Sesmc Rsk for Ncaragua conducted n 1975 by the John Blume Earthquake Center at Stanford Unversty, Calforna, USA. Ths study was contracted after the devastatng December 23, 1972 earthquake centered under Managua that klled over 8,000 people. Consderable attenton s pad to that event. It s of partcular nterest to note that the subducton zone s not deemed as a more severe earthquake source n ths proposed zonaton. Another mportant fact s that most of the populaton was located n zones 5 and 6 at the tme of publcaton of ths document. Other mportant events have taken place after the publcaton of ths document as s the case of the 1992 earthquake and tsunam that affected Rvas, near the Costa Rcan border, shown n the map. 2.2 Levels of Sesmc Intensty. [Art. 11] Three levels of sesmc ntensty are consdered. Although not explctly stated, a normal occupancy s assgned a group (sesmc ntensty) level II, specal occupancy s named group I and assgned a varable ncrease on the ntensty dependng on zonaton (21% to 45%, where the hgher the hazard the lower the ncrease). Group II ncludes essental and hazardous facltes. Fnally, group III ncludes solated structures, warehouses, barns, slos and smlar facltes. They are assgned a reducton factor that ranges between 2

3 85 and 91% of the value gven for group II. These factors are presented mplctly n [Tables 9 to 14]. 2.3 Near Fault consderatons. No near-fault consderatons are provded n ths document. 2.4 Ste Requrements. [Art. 23] Three sol types are establshed. Soft, medum and hard sol defntons are gven n terms of depth of strata and number of blows N. In locatons where the sol propertes are not known n suffcent detal to determne the sol profle type, medum sol profle s to be used. For stes prone to lquefacton, the use of the sol types defned n [Art. 23] n not permtted. The three sol types help defne the frequency content of the acceleraton response spectrum envelope. 2.5 Ste Classfcaton. [Art. 23] The ste defntons and assocated coeffcents are gven n the followng table Type Hard Medum Soft Table 1. Ste coeffcents Descrpton A sol profle wth ether: (a) Rock of any type, hard and sound or soft or meteorzed, or (b) stff or dense sol condton where the sol depth s less than 60 m, N > 50 for cohesonless sol and N > 30 for cohesve sol. A sol profle where the sol depth exceeds 60 m wth ether: (a) sand or gravel of medum to hgh compacton (21 > N > 50), or (b) slts and/or clay medum stff to stff (9 > N > 30). A sol profle contanng 10 m n thckness of ether: (a) soft- to medum-stff clays wth or wthout ntervenng layers of cohesonless or cohesve sols (2 < N < 8) (b) loose to frm cohesonless sols (0 < N < 20) 2.6 Peak Ground Acceleratons (Horzontal and Vertcal). [Art. 22] Horzontal peak ground acceleratons are defned n terms of the C coeffcent presented n [Tables 9 through 14], as a functon of type of structure, occupancy and qualty of constructon [Tables 1 & 2]. Peak acceleratons range from 0.026g n Zone 1 to 0.452g n Zone 6 for normal occupancy. The vertcal component of ground moton s not consdered n these regulatons. 3. PARAMETERS FOR STRUCTURAL CLASSIFICATION 3.1 Occupancy and Importance. [Art. 11] Three categores are defned. I Essental/Hazardous Facltes and Specal Occupancy (1.21 < I < 1.45); II Standard Occupancy Structures (I = 1.0); III Mscellaneous Occupancy Structures (0.84 < I < 0.91) that nclude the followng: 3

4 Occupancy Groups I Essental/Hazardous Facltes and Specal Occupancy Table 3. Occupancy Groups Occupancy Type or Functon of Structure Hosptals and other medcal facltes havng surgery, and emergency treatment areas or large medcal supples storage facltes; fre and polce statons; tanks or other structures contanng, housng, or supportng water or other fresuppresson materals or equpment requred for the protecton of essental or hazardous facltes, or specal occupancy structures; publc markets; waste water treatment plants; powergeneratng plants, ncludng transmsson lnes and substatons; local and natonal government buldngs; jals; stadums; structures and equpment n communcaton centers and other facltes requred for emergency response; rado broadcastng statons; structures housng, supportng or contanng suffcent quanttes of toxc, explosve or radoactve substances; hgh occupancy buldngs ntended for publc assembly; buldngs for schools or day-care centers; museums; ar and ground transportaton termnals; lbrares; structures housng especally costly tems. II Standard Occupancy Structures Hgh occupancy buldngs ntended for publc assembly (Churches, Move Cnemas; Audtorums; Marketplaces); Low frequency hgh occupancy buldngs lke: Hotels, Offce Buldngs; Factores; Banks, Commercal Buldngs; Restaurants; Dwellngs; Outpatent Health Facltes; Motor Vehcle Servce Statons. All other structures whose collapse may endanger structures lsted n groups I or II. III Mscellaneous Occupancy Structures All solated structures not classfed n any of the prevous groups such as warehouses, commercal structures under 100 m², repar shops, stables, slos, posts, fences. All other structures whose collapse wll not endanger structures lsted n groups I or II. 3.2 Structural Systems. [Art 12] Fve structural systems are defned and a K value s assgned to each. Ths K value s a system qualty factor that dentfes the acceptable level of nelastc deformaton demand. Also, heght lmtatons specfed as lmts on the number of stores are assgned to each system. Table 4. Structural Types Structural Type Lateral Force Resstng System Descrpton K 1 Only ductle moment resstng frames Combned ductle moment resstng frames and shear walls Combned non-ductle moment resstng frames and shear walls One or two-story structures made of shear walls Only shear walls or braced frames All structures not classfed as types 1 through Elevated tanks Structural Regularty: [Art 10, 14, 30 and 39] Artcle 10 states that, as a condton for mprovement of the sesmc performance of buldngs, symmetry n mass and stffness dstrbutons and avodance of abrupt changes n lateral resstance should be taken nto consderaton. Later on, n [Art. 14], symmetry requrements are used to assgn a grade to each structural system as defned n [Table 1]. Structural regularty s defned at the bottom of [Table 1] as follows: Symmetrc (sc) 4

5 when the eccentrcty s equal or less than 10%, Regular Symmetry (sc) when the eccentrcty s between 10 and 20%. [Art. 30] defnes eccentrcty lmts for the consderaton of torsonal effects when usng the statc equvalent method. [Art. 39] establshes a procedure for the consderaton of abrupt changes n lateral resstance only as a reducton n the allowable desgn stresses. 3.4 Structural Redundancy. Not explctly consdered. 3.5 Ductlty of elements and components. [Art. 10, 12, 35] [Art. 10] states as a conceptual gudelne that desgn should be based on ductlty consderatons n order to warrant better structural performance. Later on, [Art. 12] states that specal ductlty requrements apply to renforced concrete moment resstng frames for Zone 6. Also, [Art. 35] provdes gudelnes for the determnaton of desgn forces for elements and jonts of structural types 2 and 3 accordng to ultmate strength desgn. The document does not gve any reference for the ductlty requrements for specfc constructon materals. 4. SEISMIC ACTIONS 4.1 Elastc Response Spectra (Horzontal and Vertcal). The sesmc actons are defned n terms of the mass and stffness of the structure. Three methods are gven for the numercal calculatons. These are the smplfed method [Art. 29], the equvalent statc method [Art. 30] and the dynamc method [Art. 31]. Elastc response spectra are defned for both the equvalent statc and the dynamc methods of analyss. In the frst case, a reducton factor ( ) 0.5 D= λ T, where λ s 0.5 for medum and hard sols, for T > 0.5 s. and 0.8 for soft sols, for T > 0.8 s, s prescrbed as a modfcaton factor for the set of sesmc coeffcents C gven n [Tables 9 to 14]. For the second case, a further lnear reducton n the sesmc coeffcent C s appled to structures wth perod T less than 0.1 s. so that the coeffcent s ultmately reduced to one half for T = 0. These coeffcents represent the upper value for the sesmc forces as a functon of sesmc zone, occupancy, structural type and grade. 4.2 Desgn Spectra. [Art. 33] The Desgn Spectra are defned n terms of the value d T whch s a functon of coeffcents K and D defned n [Art. 12] and [Art. 23] respectvely. Ths relatonshp s gven as K = D dt and t follows the recommendatons gven n the sesmc rsk study for Ncaragua mentoned n (2.1). 4.3 Representaton of acceleraton tme hstores. Acceleraton tme hstores not explctly consdered. 5

6 4.4 Desgn Ground Dsplacement. The desgn ground dsplacement s not explctly consdered. 5. DESIGN FORCES, METHODS OF ANALYSIS AND DRIFT LIMITATIONS 5.1 Load Combnatons ncludng Orthogonal Sesmc Load Effects. Load Combnatons are gven n [Art. 32] as: a) Ultmate Strength Desgn. u C = 1.7 CM + CV 1 u 2 ( ) ( ) C = CM + CV + S o P u C3 = 0.8CM + S b) Allowable Stress Desgn. e C = CM + CV 1 e C = CM + CV S o P 2 C = 0.80CM S e 3 where CM = Dead load CV = Lve load S = Horzontal sesmc acton P = Wnd pressure or force Orthogonal Sesmc Load Effects are consdered n [Art. 25] where t s stated that for structural types 1, 2 and 3, vertcal elements and ts foundatons must be desgned for 100% of the effect n one drecton plus 30% of the vertcal load due to sesmc acton n the orthogonal drecton. For structural type 7 and all smlar structures must be desgned for 100% of the effect n one drecton plus 50% of the vertcal load due to sesmc acton n the orthogonal drecton. 5.2 Smplfed Analyss and Desgn Procedures. A smplfed analyss procedure s stated n [Art. 29] for buldngs of less than 12 m n heght. Also, the buldng must comply wth the followng: a) for each level, at least 75% of the vertcal loads shall be carred by walls joned together by rgd daphragms, where the walls may be bult of renforced concrete, confned masonry or renforced masonry, accordng to the correspondng constructon materal specfcatons; b) for each level and each drecton of analyss, there should exst at least two parallel or nearly parallel walls (formng an angle of less than 20 ), where the jont between each wall and the rgd daphragm should cover at least 50% of the length of the buldng n the drecton of the walls, also, these walls should be not dffer 6

7 more than 70% lengthwse, be made of the same materal and be located n opposng sdes; c) the heght to smallest base dmenson should not exceed 1.5; d) the base length to wdth rato should not be more than 2.0, except n cases when for sesmc purposes, the structure may be consdered as consttuted by separate modules that satsfy ths condton as well as the requrements of [Art. 29]; e) wood structures of up to 6 m n heght, wth flexble daphragms that comply wth the Techncal Gudelnes (Normas Técncas) ssued by the Mnstry of Housng and Human Settlements of the Government of Reconstructon of Ncaragua. 5.3 Statc Method Procedures. A Statc Equvalent Method s prescrbed n [Art. 30]. The total force s dstrbuted over the heght of the structure n conformance wth the followng relatons, for the th level for the top level F F = 1 = α Wh Wh n = 1 Wh S ( 1 α ) n n n = α + n α = 1 when T 0.5 s. α = 0.95 when 0.5 T 1.0 s. α = 0.90 when T 1.0 s. n S = Fj j= Wh where: α = Coeffcent for shear force dstrbuton along heght of the buldng. F = Horzontal force appled at the th level. h = Heght of the th level measured from the base. W = Weght of the th floor calculated accordng to [Art. 32]. 5.4 Mode Superposton Methods. [Art. 31] It s requred whenever the Statc Equvalent Method s not allowed. Number of modes should be at least three or all modes whose modal perod exceeds 0.4 s. Modal combnaton s to be performed usng the square root of the sum of the squares procedure (SRSS) n order to estmate resultant maxmum values. 5.5 Non-Lnear Methods. Non-Lnear Methods are not prescrbed n ths document. S S 7

8 5.6 Torsonal consderatons. [Art. 30] It s requred as part of the Statc Equvalent Method and t may not be less than an accdental torson rsng from uncertantes n locaton of loads (5 percent of buldng dmenson). 5.7 Drft Lmtatons. [Art. 34] Story drft s calculated as δ = dtδ where δ s the horzontal dsplacement of the center of mass obtaned by elastc analyss wthout consderaton of torson and usng the coeffcent d T assgned for each structural type lsted n Art. 34 and reproduced below Tpo K d t Calculated story drft s not to exceed the values gven n the followng table, under the provson that the wndows, façade and other fragle ornaments be placed so as to avod damage due to dstorton. Allowable Story Drft Type of Buldng Drft Lmtaton Masonry structure 0.003h Concrete structure 0.006h Steel structure 0.009h h s the nter-story heght 5.8 Sol-Structure Interacton Consderatons. No consderaton s made of sol-structure nteracton. 6. SAFETY VERIFICATIONS 6.1 Buldng Separaton. [Art. 38] All structures are to be separated from adjonng structures by a dstance equal to 5 cm at each level, but not less than 4δ calculated wth respect to the base and where δ s taken n accordance wth [Art. 34]. 6.2 Requrements for Horzontal Daphragms. [Art. 6 & 12] Floor and roof daphragms are requred to comply wth the followng condton: 6 w 10 F = qL where F s the stffness factor of the core of the daphragm, w s the deflecton due to shear appled on the core of the daphragm n cm, q s 8

9 the average shear on the daphragm n kg/m over a length L. For renforced concrete slabs t s possble to apply the followng formula: F = 1.0 t W 3 fc where t s the thckness of the slab n cm, W s the weght of the concrete n kg/m³ but larger than 1450 kg/m³, f c s the concrete compresson strength at 28 days n kg/cm², and F s the stffness factor n mcro-cm/m of clear length and for unt shear n kg/m. As stated n (3.2), [Art. 12] defnes the dfferent structural types allowed. In here the specfc recommendatons are made for the use of flexble versus rgd daphragms. Flexble daphragms are allowed for structural type 1 only for buldngs of up to 3 stores wth the provso that the ductle frames be desgned for lateral forces calculated usng trbutary wdth per frame. For structural type 2, flexble daphragms for buldngs or up to 3 stores s also permtted wth the provso that there be at least one wall connected to the frames n each axs of each floor of the buldng and also desgnng for lateral forces calculated consderng the trbutary wdth. Type 3 requres rgd daphragms, although t s not clear from the defnton whether for buldngs of up to 3 t s permtted to use flexble daphragms wth the above mentoned provso. Type 4 clearly allows the use of flexble daphragms and fnally, type 5 clearly states that the ths type of structure shall have rgd daphragms. 6.3 Requrements for Foundatons. [Art. 37] Ths artcle gves a recommendaton for the desgn of foundatons that s not substantated by any sort of numercal analyss. It smply states that the desgn should prevent dfferental settlements specally n sols of low bearng capacty. Reference s made to a companon document enttled Normas Técncas para Fundacones that ths revewer has not had access to for ths evaluaton. 6.4 P- Consderatons. P- effects are explctly consdered n ths norm. 6.5 Non-Structural Components. [Art. 36] Requrements are gven n the form of a smplfed desgn sesmc force for parts and portons of structures and ther attachments, permanent nonstructural components and ther attachments, and the attachments for permanent equpment supported by a structure. The total lateral desgn sesmc force, F P = C P W P where C p s defned n [Art. 36] and a table of mnmum values s also provded n the same artcle. 6.6 Provsons for Base Isolaton. No provsons are made for Base Isolaton. 9

10 7. SMALL RESIDENTIAL BUILDINGS Small resdental buldngs are consdered n ths document as those consstng of Structural Type 4 lateral resstng systems [Art. 12]. Accordng to [Art. 28] ths type of buldngs can be analyzed usng the smplfed method [Art. 29]. No consderaton s gven to torson, overturnng moment nor drft for ths method of analyss. 8. PROVISIONS FOR EXISTING BUILDINGS Provsons are gven for exstng buldngs only n as much as t requres that any retrofttng made should comply wth the same level of forces prescrbed for new buldngs whch s obvously too hgh for most exstng facltes. Also, provsons are made for the assgnment of lmted ductlty for exstng buldngs that are beng retroftted. RECOMMENDATIONS FOR CODE IMPROVEMENT Hgh prase should be gven to the code wrtng commttee n Ncaragua for producng ths document under very dffcult condtons back n However the present evaluaton has revealed that the current Ncaraguan provsons need to be updated quckly. No commentary sectons s ncluded and attempt to provde background nformaton s gven n the form of Observatons at the end of Chapter IV where an example of calculaton of lateral forces s presented. Ths clearly belongs n a Commentary secton that s unfortunately lackng. On the postve sde, one must take notce of the fact that ths Norm ncludes provsons for small resdental facltes and also consders loads due to ash depostons comng from volcanc actvty. Ths feature s lackng n all Central Amercan documents consulted. 10