DESIGN OF MASONRY STRUCTURES Part 9 doc

We have Using ground roughness category 3, Class B, with height of thebuilding=21.0m, from Table 3, CP3, Chapter V: Part 2 Therefore design wind speed is and dynamic wind pressure is Fro

Table 12.1 Loading on wall A per metre run

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Table 12.1 (Contd)

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Table 12.2 Loading on wall B per metre run; inner leaf

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walls will provide the resistance to wind loading In an actual design, thedesigner must of course check that the structure is safe for wind blowingeast-west and vice versa.

In the calculation below it has further been assumed that the walls act

as independent cantilevers; and hence moments or forces areapportioned according to their stiffness

12.5.2 Wind loads

These are calculated according to CP 3, Chapter V: Part 2 We have

Using ground roughness category 3, Class B, with height of thebuilding=21.0m, from Table 3, CP3, Chapter V: Part 2

Therefore design wind speed is

and dynamic wind pressure is

From Clause 7.3, CP3, Chapter V: Part 2, total wind force

The total maximum bending moment is

total max BM=F×h/2 where h is the height under consideration Total BM just above floor level

is given for each floor by:

Fig 12.3 The variation of the factor S2 and the wind velocity along the height of the building (Assumptions made in the design shown in full lines.)

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12.5.3 Assumed section of wall resisting the wind moment

The flange which acts together with the web of I-section is the lesser of

• 12 times thickness of flange+thickness of web

• centre line to centre line of walls

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Table 12.3 Distribution of bending moment stresses and shear force in walls

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12.6.2 Selection of brick and mortar combinations for wall

(Table 7 of BS 5628),
m=3.5 (see section 12.3) The design loads from theprevious subsection and the characteristic strengths are shown in Table12.4 along with the suitable brick/mortar combinations.

Check for shear stress: design characteristic shear fv=
f
mv (shear force/

area) < 0.35 +0.6gA (clause 25),
f=1.4 and
mv=2.5 (12.3) The value ofshear force is taken from Table 12.3 For the sixth floor

For the ground floor

There is no need to check at any other level, since shear is not a problemfor this type of structure

The BS 5628 recommends gA as the design vertical load per unit area ofwall cross-section due to vertical load calculated from the appropriateloading condition specified in clause 22 The critical condition of shearwill be with no imposed load just after and during the construction

12.6.3 Load combination, wall B

The design principle has been covered in great detail for wall A; hencefor wall B this will be limited to the ground floor level to explain furthersalient points

Inner leaf wall B –ground floor level

(i) Dead and imposed loads

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Table 12.4 Design load and characteristic brickwork strength

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The worst combination for this wall just above ground level also isdead+wind, and the design load is (1.96×102.5×103)/103=201kN/m.

12.6.4 Selection of brick and mortar for inner leaf of wall B

The design vertical load resistance of the wall is (ßtfk)/
m (clause 32.2.1)

The value of ß depends on the eccentricity of loading; hence the value of

e needs to be evaluated before design can be completed.

12.6.5 Calculation of eccentricity

The worst combination of loading for obtaining the value of e at top of

the wall is shown in Fig 12.6 Axial load

P=(0.9×78.54+1.6×7.29) (Gk and Qk from Table 12.2)

BM at centre of the panel=627.8×(Cpe+CPi)h×0.104×1.4

=627.8×(1.1+0.2)×(2.85)2×0.104×1.4

=964.6Nm/m (Cpe and Cpi from CP3, Chapter V: Part 2)

(BM coefficient for four-sided simply supported panel is 0.104; table 3.1,

BS 8110)

(since both leaves are of same stiffness)

where

Resultant

(b) Wind blowing west-east direction

The panel B is not only subjected to dead and imposed loads, but alsosubjected to wind loading from west to east direction Then

(the bending moment induced due to wind loading acts against thosedue to the vertical load)

Since resultant eccentricity of case (b) is greater than case (a), case (b)eccentricity is considered in the design

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12.6.6 Calculation of characteristic compressive stress fk for wall B

• Leeward side

The design is similar to the inner leaf and will not be considered anyfurther The slight tension which is developing is of no consequence,since 6 to 10% of the dead and imposed load will be transferred to theouter leaf even in cases where the slab is supported on the inner skin

The bending stress caused by the wind will be smaller if S2 factor isassumed variable as explained in section 12.5.2: the staircase and lift wellwill also provide the stability against the wind which has been neglected.However, any facing brick having water absorption between 7 and 12%

in 1:¼:3 mortar may be used, provided that it satisfies the lateral loaddesign The grade of mortar is kept the same as for the inner leaf.Characteristic flexural strength:

(Table 3)

Design characteristic shear as in inner leaf:

Instead of the conventional design calculations described in this chapter

a more sophisticated analysis of the structure is possible by idealizing it

as a frame with vertical loading as shown in Fig 12.7 Similarly, thestructure can be idealized and replaced by a two-dimensional frame (Fig.12.8) and analysed as discussed in Chapter 6 for wind loading

12.7 DESIGN CALCULATION ACCORDING TO EC6 PART 1–1(ENV 1996–1:1995)

To demonstrate the principle of design according to EC6, the wall A inthe ground floor will be redesigned The dead and live loading is taken

as calculated before and as in Table 12.1 The bending moments andshear forces due to wind loading are given in Table 12.3 The category ofmanufacturing and execution controls are assumed to be II and Crespectively; thus ␥m=3 as given in Table 4.6

Load combination for ultimate limit state:

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Design vertical load resistance of wall , where depends

Fig 12.9 Calculation of eccentricity of the loading (not to scale).

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EC6 allows the use of the value from the national code Hence 50N/mm2brick in mortar will be sufficient.

In the absence of test data a formula as given below is suggested foruse:

or

Therefore, 100N/mm2 bricks are required which is much higher than theprevious case It would be better and economical to do tests on prisms toobtain the characteristic strength

For the ground floor

G has been taken as 1 for favourable effect

The allowable shear due to precompression in BS 5628 is higher than

in the Eurocode, but it does not make much difference to the design

12.8 DESIGN OF PANEL FOR LATERAL LOADING: BS 5628

(LIMIT STATE)

To explain the principle of the design only panel B between sixth floorand roof will be considered The low precompression on the inner leaf isignored in this design Assume:

• Inner leaf 102.5mm brickwork in 1:1:6 mortar

• Outer leaf 102.5mm brickwork with facing brick in 1:1:6 mortar

• Boundary conditions: two sides simply supported and two sides fixed

as shown in Fig 12.10

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