Fixed Ladder Extension (ASCE 7 Section 4.5.4)

[271828]

I'm designing a steel ladder at a private school. Figure attached. The ladder is at the loading dock, hence the removable guard on the left.

The default rail size is PL3/8x2-1/2. The rails extend 42 in. above the top rung, and are unbraced above the top rung.

I'm having trouble with the 100 lb load requirement in ASCE 7-16 Section 4.5.4. If 100 lb is applied as shown in the figure, then the 3/8 in. bar is overstressed by about 2x and deflects over 9 in. If the rail is changed to 5/8 in., then it is strong enough and the deflection isn't huge.

Is this requirement appropriate for my ladder?

What do you usually see for the rail extensions in this situation?

Any other insight?

Thanks in advance for your time spent on this.

10000e

 

[r13]

The rail extended above roof shall be designed for a concentrate load of 100 lb in any direction. And the rung shall be designed for at least 200 lbs. See section 4.9 of the linked document. Link

 

[dik]

Not familiar with ASCE... but can you use 50 Grade steel and do you have to concern yourself with deflection?... BAR 1/2x2-1/2 yields Z= 2.5*(0.5)^2/4 = 0.15625 in^3 x 0.9 x 50 /12 = 0.586 K_ft. Moment is 3.5 x 0.1 x 1.5 = 0.525 K_ft... should be OK using LRFD... Because it's not on the exempt list... looks like you have to use it... sorry r13... LRFD again... I thought 300 lbs?

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[271828]

Thanks r13 and dik.

I'm also seeing 300 lb as the load on the rungs, but the rungs aren't the focus of my problem anyway.

About using Gr. 50 steel: That would help reduce the thickness for bending. However, I am also concerned about stiffness. With a 1/2 in. bar, the top of the rail would deflect over 1 in. with only a 25 lb lateral load. I'm wondering if the rail is so flexible that it's not useful to someone climbing the ladder. Opinions on this?

 

[JStephen]

On tanks (under different codes), the stringers are either attached to the handrail or they are attached to the ladder cage if there is a cage, normally not free-standing like that. If they do need to be free-standing, consider switching to angles for the top portion of the ladder.

 

[r13]

Do you have anything you can attach to, or do you have place for bracing?

Dik, Yes, 300# ASCE7. Below is from OSHA:

Each fixed ladder: At least two loads of 250 pounds (114 kg) each, concentrated between any two consecutive attachments (the number and position of additional concentrated loads of 250 pounds (114 kg) each, determined from anticipated usage of the ladder, shall also be included), plus anticipated loads caused by ice buildup, winds, rigging, and impact loads resulting from the use of ladder safety devices. Each step or rung shall be capable of supporting a single concentrated load of at least 250 pounds (114 kg) applied in the middle of the step or rung. Ladders built in conformance with the applicable provisions of appendix A will be deemed to meet this requirement.

 

[dik]

The 300lbs has no bearing on your problem... I'm not sure that 1" deflection is an issue unless you are limited to some other value. You will likely never apply a load of that magnitude, and it will still be safe... right up to 150 lbs. In these environs Grade 44 is common and the difference between it and Grade 50 is peanuts... I don't know about your locale. You might add a little stub so the parapet provides some passive resistance... it doen't have to be connected unless you are concerned about an 'inward' force. Put a chunk of 1/2" BAR in a vice that is 3'-6" and just see how stiff it is, if you're concerned...

Thanks r13... I have both the OSHA and ASCE documents... In Canada, we have slightly different forces.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[HTURKAK]

Mr. JStephen (Mechanical) has a valid point. If you do not have a cage, you should prefer angle or better use a pipe section (42 mm dia.).

 

[271828]

Regarding switching to angles: Is there a size standard for graspability?

 

[dik]

I'm not aware of any... just have to be graspable... BAR 3/8x2-1/2 are common in this area. Handrails have a minimum and maximum diameter.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[271828]

Thanks dik.