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I am attaching a steel rail and post guardrail to the coping of an existing parapet. The actions produced by the guardrail on the coping incur at point A as shown below.

elevation of parapet showing actions at A due to guardrail

However, how do I find out if the cement mortar connecting the coping to the parapet wall provides enough resistance against the forces at A due to the addition of the guardrail?

The length of the coping is 0.995m and the width is 0.44m as shown. The action A is applied at the centre of the coping 0.175m from the inside face.

Any help or advice would be muchly appreciated.

Thanks.

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  • $\begingroup$ Your description of the setup is quite confusing. Where in the picture are the steel post and guard rail? Is the coping made of stone? $\endgroup$
    – r13
    Dec 3, 2022 at 2:35
  • $\begingroup$ Apologies, the baseplate of the guardrail is at Point A. The coping is mass concrete. $\endgroup$ Dec 3, 2022 at 22:07
  • $\begingroup$ For the most part, when it comes to structural strength of concrete in tension, it is usually taken as 0. Mortar is weaker than concrete, so you can assume its tensile strength is also 0. You will need to rely on checking against the dead weight of the cap stone which odds are its far from adequate. $\endgroup$
    – Forward Ed
    Dec 23, 2022 at 15:12

1 Answer 1

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After a quick first look and ignoring the details of the wall:

One safe bet is to assume the coping is just sitting on the wall, totally cracked free from the mortar and the normal stresses are just not significant compared to what the wall can bear.

The lateral forces and shear are resisted by the friction between the coping and the mortar with a friction index of $\mu =70-80% \ $ The latest edition of PCA's Concrete Masonry Handbook, Appendix A, gives a precast concrete-to-concrete masonry friction coefficient of 0.4 based on a safety factor of two

The overturning moment $M_X$ is resisted by the moment of the unit length of the coping about the opposite edge of the wall.

Say the resisting moment of on meter length of the coping counterclockwise on your sketch is:

$M = 2500kg/m^3( 0.35*0.14*0.175-0.10*0.13*0.05)=2500*(0.00857-0.00065)=2500*0.00792=19.8kg/m$

If we use a safety factor of 2 we have a 9.9kg/m length of coping to resit the railing moment which is not nearly adequate. Code requires 75kg/m distributed and a concentrated load of 100kg if I remember correctly.

So we need to assess the strength of the wall and use anchors that penetrate the wall! For that, we need the rebar layout and details of the wall.

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