On the Use of Horizontal Reinforcement in Masonry lintels

by - Ir. L. Van 8iervliet

ABSTRACT

Technical Director of Leuven Research and Development, Belgium

- Pref .Dr. Ir. F. Mortelmans Professor at the K.U.Leuven, Belgium

The paper describes the results of an investigation carried cut on reinforced single- ar double-leaf masonry lintels. The type of bricks and the martar mix were kept cons tant throughout the te sting. The t es ts have proved that a reinforced masonry lintel has good performances and can be used to replace the classical concrete and/or iron lintels . A design method i5 being developed by Ir. Smits, IWONL-Researcher.

719

1. INTRODUCTION

The common practice in constructing lintels is to put either a concrete lintel (precast or poured in situ) ar an iron lintel. Both solutions have severa! disadvantages, as there are:

- corrosion (iron lintels) - necessity of formwork (in situ poured lintels) - a linear thermal expansion modulus thatdiffers tram the masonry

modulus (this leads to the classical 45° cracks starting fram the contact surface lintel-masonry)

- thermal bridges when the in situ poured linte! is in contact with the outer leaf of a two-leaf wall.

Several tests have been carried aut during a research program sponsored by the "Belgische groepering van de kleinijverheid" - "Fabrimetal" -"FeBe" and the Belgian government. One of the purposes of this research program was to investigate the behaviour of reinforced masonry lintels.

2. PROPERTIES OF MATERIALS ANO TEST PROGRAM

720

The test program consisted of six lintel constructions. AlI lintels had an overall length of 3800 mm, a height of 600 mm and a free span of 3000 mm.

- single leaf - bricks: perforated clay bricks 290 x 140 x 140 mm'

average compressive strength = 10,1 N/mm2

- mortar: C300 (i.e. 300 kg P30

cement pro m' of sand)

average flexural tensile strength = 1,8 N/mm2

average compressive strength = 9,4 N/mm2

- reinforcement: 0 4 mm - width 100 mm in the lower two mortar joints

Thís lintel is identical to V, A except for:

- mortar: C300

average compressive strength: 10,2 N/mm2

average flexural tensile strength: 2,4 N/mm2

- double leaf - bricks

- inner leaf: perforated clay bricks 290 x 140 x 140 mm' average compressive strength: 10,1 N/mm2

T outer leaf: clay bricks 190 x 90 x 65 mm' average compressive strength: 16,0 N/mm2

- mortar C300

average compressive strength: 10,4 N/mm2

average flexural tensile strength: ~,o N/mm2

- reinforcement: 0 4,75 mm - width 250 mm in the lower ·two martar joints

This lintel is identical to VSA

2.5. Lintel Vs (see figo 3) ---------- single leaf - bricks: perforated clay bricks 290 x 190 x 140 mm'

average compressive strength: 15,7 N/mm2

- mortar: C300

average compressive strength: 11,0 N/mm2

average flexural tensile strength: 2,5 N/mm2 - reinforcement: ~ 10 x 7,1 mm2 - width 150 mm

in each martar joint

- douhle-!eaf lintel constructed in such a way that we had immediately the classical rabbet for incorporating a window

- bricks: inner leaf: perforated clay bricks: 290 x 190 x 140 mm'

average compressive strength: 15,7 N/mm2

outer leaf: clay bricks: 190 x 90 x 65 mm' average compressive strength: 16,0 N/mm2

- mortar: C300 average compressive strength: 11,7 N/mm2

average flexural tensile strength: 2,1 N/mm2

- reinforcement: ~ 10 x 7,1 mm2 - width 250 mm in each mortar joint

AlI lintels were loaded by a local charge at mid-span. The double-leaf lintels V5A an V

5B were loaded at both leaves, the

double-leaf lintel V9 ~as loaded at the inner leaf only.

The reinforcement used was of the MURFOR type (see figo 5). The characteristics are given in figo 6 for the 0 4 mm and in figo 7 for the ~ 10 x 7,1 mm' .

721

3. INSTRUMENTATION ANO TESTING

The local load was applied at mid-span by a flexure machine of a maximum capacity of 200 kN.

The strain in the masonry was measured by a 8" mechanícal extensometer. (See figo 1- 4, Demec points indicated by ~).

The strain in the reinforcement was measured by electrical strain gages (see figo 1-4, straín gages indicated by.). For measuríng the deflection of t he lintel dial gauges were used (see figo 1-4, dial gauges indicated by t) . The load was applied incrementally from zero load to a 5mall load and back to zero loading and then from zero load to failure. The load at which first crack(s) appeared was noted.

4. SOME TEST RESULTS

(The complete detailed test results are obtainable from the authors).

Lintel Load at first Ultimate crack

p c

kN

5 7,5

8 8 8

12

(1) compression of masonry (2) fracture of the reinforcement (3) shear failure

p u

kN

15 15 22,5 22,7 48 48

load Mode of failure

(1) (1) (2) (2) (3) (3)

Fig. 8, 9 and 10 give the crack pattern of respectively V1B, the inner and the outer leaf of V

5B and V

8•

4.3. ~~~1~~!~~~~ The measured deflections as a function of load are given in the

722

figo 11, 12 and 13 for respectively V1

B, V5

B, V9

(inner and outer leaf).

The figures 14, 15 and 16 give the strain distribution for the mid-section at crack load for respectively V

1A, V

5A (inner and outer

leaf) and Vg (inner and outer leaf).

Some typical strain evolutions are given in the figures 17, 18 and 19 for the lintels V1B, V5A and Va respectively.

5. DISCUSSION OF TEST RESULTS

The failure load of the normally reinforced lintels is about three times the first - crack Ioad. Taking the tensiIe strength of the masonry as a design strength for the reinforced lintel gives a security against faiIure of about 3 for normally reinforced lintels and even 4 for heavily reinforced lintels.

The strain and deflection measurements on lintel V9 show that 70% of the applied load is taken by the inner (loaded) leaf and 30% is taken by the outer Ieaf. This means that the zig-zag wire of the reinforcement really transfers load from the inner leaf to the outer leaf.

The comparison of lintel VR and V9

shows that the first crack moments are in proportion to the tnicknesses of the lintel. This leads to the same statement as mentioned above. The dotted lines in the figures 17, 18 and 19 give the calculated strain in the reinforcement. The design methode used is the same as the one normally appIied for the elastic design of reinforced concrete. AlI this goes to show that this design method can be an approximation for designing reinforced masonry lintels.

6. CONCLUSIONS

Given the results of these tests and the wellknown disadvantages of normally used lintels we can conclude that reinforced masonry lintels offer a much better solution for this ~roblem.

723

~I

724

1

3800

I I 1 + • + I I

6

========~--~>=~~~~~~~~~~2--=========n 1, . --- ............--- "1" --- .... _-I ~

':' p ... ~f 1

lOjA

~ I I I I I I

I I I I I I I I I I I I I I

I I I 128 I I I 1 1il ~ 400 ~ 3000

: 3800

16 3

I I 1 -I I I 120 I

I I I I

I I 1 1

.,

1 I I

I I I

I ~I,OO

n I

------~"'~//~1t+,:~//L-'-',------n ~/sf=s// r

p

f· II

--;;~e~_n , 1 I

<:' lIj ~ p~ .,

~ "1 ,r t -' I I I 1 I 3 I 1 1 1 1 1

I I I I I r r I I I I I I I I I I T I ,I 1 1 1 1

I I 1 I 1. T T , I I 20 J I I I • .00 ":' 13000 ~.oo

• j

21 23

________________ ~,~~~:~-,,-------------------n ____________ ~,~~~' -----------------I

p

p

725

Fig . 5

Lintel a b C d nnn nnn o (nnn)

VIA 100 406 0 4mm 3,75 V1B 100 406 0 4nnn 3,75 V5A 250 406 0 4,75 mm 3,75 V

5B 250 406 0 4 ,7 5 mm 3,75

Va 150 406 .10 x 7,1 mm' 3,75 V9 250 406 .1 0 x 7,1 nnn' 3 , 75

726

kN

kN

40

20

4

o 2

F i 9 7

4

MURFOR </>4mm Ilailur. laad'12, 4kNI

6

MURFOR iIl10x7,1mm

Ilailur. laad , 44kNI

6 E {%ol

727

Fig . S

Vs 8 inner leDf

\\;8 outer leDf

I I I I I

11 , r I

728

lOQd I kN)

729

40

20

730

loadlkNI

100

I I , ,

I I

• , I

I

I I

I

I , , I ,

I

, , ,

500

inner lea f

outer leaf

de fi. c! ion (10-2 mml

40.10- 5 strain

77.10- 5 strain

inner lea f

60.10- 5 strain

Fig 15

31.10- 5 strain

cutor l.af

62.10-5 strain

731

19.10.5 strain

inner ltaf

Fig.16

17.10' 5 strain

ouhr lea f

30.10'5 stra in

732

load (kN)

15

10

. ' . ' . 5 .' . .. '

. ' . ' .' .' .. '

O O 200 1000

(oad (kNJ

18

16

14

12

11 ...... .... ;'

;' .... 10

8 ./ .... -,'" .. ' '" .' '" .' , .' 6 ,," .... ... ..' 4 ( ••••••

2 .. ' ..... .

.. '

Fig 17

... .. ' o·····

2000

.....

.' .'

V1 B

- stra in gago 1

strain gago 3

calculatod strain

E ( fL strain)

2800

.'

0. ° ..

.;.. ......

....... -:-.. '" .'

.........

strain gago 1 autor loa! •

strain gago 2 innor loa!

calculoted stroin

E ()l. strain) ...... O~-r--__ ~--~~--~--__ -r--__ ~--~~--~--__ --~

400 2000 4000

Fi g. 18

733

lo ad IkN)

40

20

4

200

734

00

00" . .;.

"' , '.0

/.0° /.0

' o' , .' ; .. ' , .'

/0.° / 0. ° " .' ; ... , .' , .' , . '

; .' " o' ,,' ....

." • V ." o o ~ 8

1000

Fig . 19

st r a in gage

st r a in gage 3

ca(cutot ed stra in

2000 E I fJ. strain )