Guardrail System: The True Definition of Post Spacing

Warning:
This blogpost is exclusively written for structural engineers. Non-engineers may find this blogpost unentertaining.



A TYPICAL GUARDRAIL SHOWN WITH TYPICAL LOADINGS



The post spacing, L, is generally defined simply as the horizontal distance between adjacent rail posts regardless of whether the rail is sloping or horizontally oriented.

Unfortunately, engineer's obscure understanding of the 50-lb/ft uniform railing load also obscures the definition of post spacing that turns it into an inclined distance between adjacent posts -- an obscure idea that this blogpost is trying to disprove.
The analyis and design of post for guardrail are quite simple. Being a flexural member, a guardrail post is normally idealized as vertically oriented cantilever beam -- fixed at the bottom end and subjected to a concentrated load at the top free end.

The concentrated load at the top of rail post originates from a single 200-lb concentrated load and a 50-lb/ft uniform load specified for railing system by some building codes. The two loads, however, are not applied concurrently.

When the 200-lb concentrated load, P, is applied at the top of rail post, portion of the load is distributed to the adjacent posts on both sides thru the horizontal rails -- thereby reducing the effective load on the rail post. This reduction is dependent on the stiffness of posts and the stiffness of horizontal rails that comprised the guardrail system. Generally, this type of loading does not directly define the rail post spacing.

When the 50-lb/ft uniform load, w, is applied at the top rail, the amount of load that a rail post sustains is dependent on the spacing between posts -- which translates into load tributary length. The wider the post spacing, the greater the load tributary length will be and, subsequently, the greater the load that a rail post sustains. Hence, this is the type of loading that determines and defines the post spacing. The maximum spacing that rail post may attain is a function of its strength. Although the top rail may also control the spacing of rail post, this discussion only covers the rail post. Let us just assume here that the top rail is not the controlling guardrail member.

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BASIC HANDRAIL CALCULATOR: A JavaScript Application via blogpost

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360 Degree PANORAMA: Alona Beach, Panglao Island Bohol

Long before it became a famous tourist destination in the Philippines, the pristine island beach had been a site of a location shooting of one of FPJ’s (Fernando Poe Jr) films a couple of decades ago. The film, the locals recounted, was starred by no less than the ‘King of Philippine Movies’ himself with his favorite leading lady, the actress Alona Alegre. Later, the beach was named after the actress.

Alona Beach is a short stretch (about one and a half kilometers long) of white sand on Panglao Island, Bohol in the Philippines - a paradise. I was there for the first time last Holy Week, from April 21 to 23, 2011.

When a friend planned and arranged a trip to the place and invited me to join, I took the once in a lifetime occasion as an opportunity to escape from work -- to relax and unwind. So after more than a month of virtually non-stop working beating deadlines, I finally got a chance to escape from a very stressful work of ‘crunching numbers’. Working as a structural designer, I really crunch numbers -- dealing with mathematics in a daily basis -- on breakfast, lunch and supper. Now, it’s a chance for me to finally put my canon DSLR camera I bought less than a year ago into work. All I’ve wanted is to create a 360 degree viewer-interactive panoramic view of the beach.

I enjoyed my stay in the beach taking pictures of beautiful scenes in the surroundings. Mostly, my attention was focused on finding nice subjects for a photography lesson (I really want to learn photography) until I found something strange in the place; yes, really strange. Together with three other people, I saw a giant centipede about 2 feet in length. This unusual sighting took place in a place at the southern end of the beach. There is a small store in the area where we stood by for an hour savoring hot noodle soup. The time was almost 6 in the evening. We had an interesting conversation with the store owner when suddenly, coming from the grassy side of the store, a black and long fast moving creature moved towards our location. Thinking it was a snake, everybody panicked and struggled to find a safe place to avoid the moving creature. We can only guess, by its sudden appearance, that the creature might be in an attack mode. Once it was exposed to much illuminated portion of the ground as it moved toward us, we were sure what it was. We have a clear image of how it looked like; it has several feet. After a sort of short 'deliberation', we unanimously identified the creature as a giant centipede.

The next thing I knew was that I grabbed my camera for a shot. Unfortunately, my reflex was so slow: the centipede was nowhere to be found when I finally had my camera ready. It has already made its way towards a nearby pile of stones and wood. Had I made a successful shot, that could have been a shot of a lifetime. Imagine capturing a giant centipede on camera? Never did I find such a creature in books and in scientific television programs such as in National Geographic channel. A check with the internet, however, I found out that such a giant really exists. If that is the case, then I could be the one to first discover its existence in Alona Beach. There may be a lot of these giants hidden somewhere in the place underneath indisturbed by humans. That should remain undisturbed, anyway.

Shortly before leaving the beautiful Alona Beach after a three day stay, I was able to take a series of pictures with a vantage point in front of 'Sierra Madre Resort Hotel'. I setup my tripod at this vantage point and took pictures in clockwise direction. The results, below, are good but with a little problem: the camera was not properly aligned with the horizontal plane -- resulting to a sloping horizon. Bloggers who want to show pictures as viewer-interactive in their blogs may follow what I did with the pictures below:

Image Sequence# 01 Image Sequence# 02 Image Sequence# 03

Image Sequence# 04 Image Sequence# 05 Image Sequence# 06

Image Sequence# 07 Image Sequence# 08 Image Sequence# 09

Image Sequence# 10 Image Sequence# 11 Image Sequence# 12

Stitching the series of 12 images arranged in sequence above using the application 'ADG Panorama Tools' by Albatross Design Group, I finally created a simple yet astonishing Panoramic View of the Alona Beach Resort as viewed from my vantage point. The resulting panoramic image, however, has some noticeable image irregularities.

Irregularities that you may find in the resulting panoramic image include the following:
  1. The horizon appears to curve up and down as you pick and drag to navigate the 360 degree view of the panoramic image. This is the result when the camera mounted on a tripod is not properly aligned with the horizontal plane. ‘Image Sequence# 01’ to ‘Image Sequence# 04’ show sloping horizon gradually decreasing until it becomes totally horizontal in ‘Image Sequence# 05’.

  2. There is also a ghostly image of a woman that appears at the left side of the coconut tree. This is a problem when shooting moving subjects. The woman that appears at the right side of ‘Image Sequence# 06’ no longer exists at the left side of ‘Image Sequence# 07’ on the same location of each scene. When the program tries to stitch the two images, it has a hard time resolving pixels at the calculated stitch line.

A Panoramic View at ALONA BEACH, Panglao Island Bohol, Philippines

Pick and Drag to navigate through the Panorama

Note:

If you cannot view this content, please install Quick Time. And if you already have installed Quick Time and you are using Google Chrome and still cannot view this content, please right click on the image area. When a menu pops up, choose and click on 'Run this plugin' from the selection.

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