Design Guide 2

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Guidance for vertical resistance This guide is the second in a series of three giving guidance on the design of masonry structures to Eurocode 61.  The first guide, Introduction to Eurocode 62 gives an introduction to design and assessment of actions using Eurocode 6 and also covers the specification and execution (workmanship) of masonry.  This guide explains how to design for vertical actions and determine vertical resistance. The third guide in the series3 covers the design of laterally loaded masonry panels. Throughout this guide the Nationally Determined Parameters (NDPs) from the UK National Annexes (NAs) have been used. These enable Eurocode 6 to be applied in the UK.Design procedureThis guide explains how to determine the design resistance for a vertically loaded wall. The first guide in the series, Introduction to Eurocode 6, should be referred to so that the design load can be determined. In essence, when using the Eurocodes the designer should check that the resistance is greater than or equal to the effect of the actions. A flow chart for the design of masonry walls to resist vertical actions is shown as Figure 1.Compressive strengthEurocode 6 introduces some new concepts when dealing with the design of masonry for vertical loads. The first of these relates to the way the compressive strength of the masonry units is expressed. For design purposes the normalized compressive strength, fb, of the masonry units is used. This is the compressive strength of the units converted to the air-dried compressive strength of an equivalent 100 mm wide by 100 mm high masonry unit. The detail is contained in Part 1 of BS EN 772, Methods of test for masonry units4. The advantage to the designer is that the normalized strength is independent of the size and shape of the units used in the final construction. Grouping of masonry unitsThe second change relates to the way in which masonry units are classified. This is dealt with by grouping masonry into one of four groups as shown in Table 3.1 of Eurocode 6. The group designation will normally be declared by the manufacturer. The designation depends upon the volume and direction of holes in the unit and the thickness of webs and shells. Historically only Group 1 and Group 2 units have been used in the UK, so only values for these groups are given in the UK NAs.How to design masonry structures using Eurocode 62. Vertical resistanceEur Ing, Prof. J J Roberts BSc(Eng), PhD, CEng, FIStructE, FICE, FIMS, FCMI, MICT     O Brooker BEng, CEng, MICE, MIStructEIntroductionThis publication is part of a series of three guides entitled How to design masonry structures using Eurocode 6. The aim is to make the use of Eurocode 6, Design of masonry structures as easy as possible by drawing together in one place key information and commentary required for the design of typical masonry elements.The Concrete Centre (and, originally, The Modern Masonry Alliance) recognised that effective guidance was required to ensure that the UK design profession was able to use Eurocode 6 quickly, effectively, efficiently and with confidence. Therefore a steering group, with members from across the masonry industry (see back cover for a list of members), was established to oversee the development and publication of the original guides.This second revision addresses the publication of PD6697 in 2010 and revised National Annex to BS EN 1996-1-1 in 2013. It was overseen by a reconstituted steering group from industry (see back cover).Revision 2
How to design  masonry structures using Eurocode 62For blocks laid flat, Table 8 of the National Annex to Eurocode 6,  Part 1–1 contains a specific value for K to be used in Equation (3.1)  of Eurocode 6, Part 1–1.The following limitations are placed on Equation (3.1): ¢The masonry is detailed and constructed in accordance with the requirements of BS EN 1996–1–1, section 8. ¢fb is taken to be not greater than 110 N/mm2 when units are laid in general purpose mortar and 50 N/mm2 when laid in thin layer mortar (fb is determined in the normal direction of loading). ¢fm is taken to be not greater than fb nor greater than 12 N/mm2 when units are laid in general purpose mortar or 10 N/mm2 when units are laid in lightweight mortar. ¢The coefficient of variation of the strength of the masonry unit is not more than 25%.For masonry made with general purpose mortar, adjustments are made to the value of K as shown in Figure 2.In addition the following points should be noted: ¢For masonry made of general purpose mortar where Group 2 and Group 3 aggregate concrete units are used with the vertical cavities filled completely with concrete, the value of fb should be obtained by considering the units to be Group 1 having a compressive strength corresponding to the compressive strength of the units or of the concrete infill, whichever is the lesser.The characteristic compressive strength of masonry The characteristic compressive strength of masonry (other than shell bedded masonry) is determined from the results of tests in accordance with BS EN 1052–15. The tests are carried out on small wallette specimens rather than the storey-height panels used in the past. The designer has the option of either testing the units intended to be used in a project or using the values determined from a database. Values from a large database are provided in the UK NA to Eurocode 6, Part 1–1 in the form of the constants to be used in the following equation:fk = K fba  fmb   [Equation (3.1) of Eurocode 6, Part 1–1]wherefk  = characteristic compressive strength of the masonry, in N/mm2K  = constant – see Table 1 and Figure 2a, b = constants – see Table 2fb    = normalized mean compressive strength of the units, in the       direction of the applied action effect, in N/mm2fm    = compressive strength of the mortar, in N/mm2Figure 1Flow chart for the design of masonry walls to resist vertical actions               .   Obtain gM from table 1 of Introduction to Eurocode 6Determine requirements for mortar strength and durability.  See tables 5 & 6 of Introduction to Eurocode 6Determine effective height, hef, of the wall (see page 4)Check slenderness ratio hef /tef ≤ 27Check area ≥ 0.04 m2Determine eccentricity (see page 5)Determine capacity reduction factors, Fm and Fi from (see page 6)Determine normalized compressive strength, fb. Characteristic vertical actions   Check Ed ≤ NRd  Check complete  Calculate design resistance (per unit length) from leastfavourable of:NRd = Fm t fk / gMandNRd = Fi t fk / gM  Where cross-sectional area, A < 0.1 m2, factor fk by (0.7 + 3A)Determine design value of vertical actions (per unit length), Ed, using Expression (6.10), (6.10a) or (6.10b) of Eurocode (see Introduction to Eurocode 6)Masonry unit properties •  Type and group •  Dimensions •  StrengthDetermine characteristic compressive strength of masonry, fk, from Equation (3.1) of Eurocode 6 and Tables 1 & 2Determine effective thickness, tef,of the wall (see page 4)How to design masonry structures using Eurocode 6

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