Guardrail Post Design 3

[KarlT]

I wanted to post a question about the design of guardrails. Here in Canada, the National Building Code specifies that the top of a guardrail (minimum 3'-6" high) needs to withstand a lateral horizontal service load of 50 lbs/ft or a point load of 225 pounds at any point along the guard, whichever governs.

The thing is the posts are usually spaced around 4'-0" o/c which means the 225 point load will govern the post design. Now, I usually design the post member and base connection for the full point load at the top of the guard, with no distribution of load laterally to the other posts. In reality there should be some kind of load sharing going on because the horizontal rail member along the top of the guard should be able to span across several posts with the point load applied at a single post.

How would I calculate the load distribution to the other guardrail posts from the point load on a single post?

 

[CTSeng]

Yes you should distribute the concentrated load based on the ratio of the siffness of the top rail to the stiffness of the post. Here are a couple of sites that explain it:

http://www.nomma.org/support/rail-engineering-data/MRM-Rail-Engineering-Data.htm

http://www.usace.army.mil/usace-docs/eng-tech-ltrs/etl1110-2-534/a-a.pdf

However the a free end post is always the one that drives the design.

I've always wondered how the concentrated design load was determined. Is it an impact load and if so can we increase allowable stress?

 

[KarlT]

Thanks a lot for the assistance. Those sites were both very helpful!

I kind of wonder about the load as well, if it can be considered an impact load then the material would be able to sustain more load than under static conditions. On the other side of the coin, if it is an impact load we usually have to factor it up in addition to the live load factor of 1.5 in our codes.

The current NBCC code treats the loads as "live". We use limit states design to check the ultimate strength, ignoring any service load deflections.

Karl T

 

[structuresguy]

Another place to get some info on the design of guardrails is from Hollaender. They make aluminum guardrails. well, in their catalog they list the different design philosophies, with example problems.

http://www.hollaender.com/catalog.htm

 

[KarlT]

Just a further thought....

It seems that these design charts for distributing point loads were developed for steel, aluminum, etc.
Can these design charts also be used for the design of wood guardrails? I just finished a job where we had to design a guardrail out of wood (2x4's on the flat for pickets and 2x4 for the top rail) and without distributing the loads you end up with really stupid numbers for the moments on the wood and forces on the lag screw connections.

I guess the load distribution to the pickets would would be a function of whether you had a continuous 2x4 top rail?

Karl T