What is the 75 plf AASHTO load on the edge of a bridge overhang construction bracket for? 2

[Ron247]

When checking a bridge overhang construction bracket (EFCO SA-80 or Dayton C49 for example) I see a 75 plf load requirement on the end of a bridge overhang. I know it is not related to the screed wheel loads but does anyone know what it is for? I am told the requirement is from AASHTO.

Seems high for a handrail, so there must be some other item it is modeling.

Thanks in advance.

 

[SlideRuleEra]

Forming for a bridge deck is required to include a 20 lb/ft[sup]2[/sup] construction load. However, the overhang, or most of it, is not considered to be part of the bridge deck. The deck is the roadway, which extends from curb to curb. The portion of the slab (with rebar projecting upward) that will support the parapet itself (and sidewalk / curb, if used) is not subject to the bridge deck construction load requirement.
The 75 lb/ft loading is the allowance for for a construction load for this parapet area.

Bridge deck falsework and overhang brackets are struck (removed) before parapet concrete is placed. The widely used New Jersey barrier bridge parapet weighs about 650 lb/ft... without sidewalk or curb.

http://www.SlideRuleEra.net

 

[Ron247]

Thanks SlideRuleEra,

Just to double check your posting:
[ul]
[li]The overhang bracket is usually removed when an overhang has cured enough to self-support. That has been my understanding.[/li]
[li]The overhang bracket generally supports the overhang slab, reinforcing, formwork, screed load and a minimum LL of 20 psf. I also noted that some bracket suppliers use a minimum floor live of 50 psf.[/li]
[li]The 75 plf would not be applied to an overhang bracket UNLESS for some reason the bracket was to remain in place to support the addition of a parapet.[/li]
[li]If it was to remain in place, It would also have to be able to support the weight of the parapet concrete (650 lb/ft) estimate.[/li]
[/ul]

 

[SlideRuleEra]

Ron247 - Bridge deck thickness varies with span length and design. We built bridges with decks from 8" to 24" thick. Overhangs are typically 8 or 9 inches thick. However, overhang brackets are not needed or used for construction of all bridge designs.

The 75 plf is really for the manufacturer's design of the brackets, Contractor does not need that info.

All forming for the bridge deck and the overhang brackets are removed before the parapet is placed. Pouring parapet is allowed to lag well behind slab placement. This is for several reasons:

1) The weight of the parapet is designed to be supported by the slab; brackets are not needed.

2) The brackets may be overloaded if they remained in place and the additional load / deflection could make removing them dicey.

3) Allowing the slab to fully cure means the concrete truck can drive right up to the parapet placement and pour concrete directly out of the truck (instead of using a crane / concrete bucket).

4) A bridge contractor probably has a limited number of brackets, he has to keep leap-frogging all the forming, including the brackets, to keep superstructure construction moving along.

5) A bridge contractor probably has a limited length of (steel) form for the face of New Jersey barrier. He keeps this set up, ready to go, and waits for an opportunity to make a parapet placement... say, with concrete "left over" after placing a slab. Divert that concrete to pour a limited length of parapet. Only takes 2 1/2 yd[sup]3[/sup] for a 15' length of parapet. Parapet is designed to be placed in segment with gaps at slab joints and perhaps in between. A 15' placement is just right for half of a 30' span... a popular length in our area.

http://www.SlideRuleEra.net

 

[Ron247]

Thanks again, SlideRule

 

[SlideRuleEra]

You are welcome, Ron247.

http://www.SlideRuleEra.net

 

[MIKE_311]

Per the commentary of AASHTO Temporary works, 2nd, C2.3.3.2.1 :

 

[BridgeSmith]

To hopefully add a little clarity to the commentary posted by MIKE_311 above, the 75plf accounts for the weight of the construction workers on the catwalk outside the formwork for the edge of the deck and the screed rail.

Notice the guy over on the right in this picture. (Assuming the link to the picture works)

Rod Smith, P.E., The artist formerly known as HotRod10

 

[r13]

Good picture, Rod. It's better than words.

 

[Ron247]

I have been told AASHTO requires a 20 psf construction load that I am going to call a floor live load. Slide's post above also noted the 20 psf floor live. For me, while 20 psf floor live is probably sufficient when you have plenty of "elbow" room, it seems to small for cramped quarters such as this.

I was also told that some brackets are designed for a 50 psf floor live load although the post above notes an AASHTO 20 psf construction load. I generally run the floor live load all the way to the edge of the support arm. Wouldn't the worker shown on the right side of that picture be included in the 20/50 psf floor live load or is the 20/50 psf floor live not intended to extend beyond the edge of concrete?

I guess I can see several possible design scenarios depending on the floor live you choose to meet AASHTO:
[ol 1]
[li]20 psf floor live load to the end of the overhang bracket arm and a 75 plf load at the end of the overhang bracket[/li]
[li]50 psf floor live load to the end of the overhang bracket arm and a 75 plf load at the end of the overhang bracket[/li]
[li]A 20 psf floor live load to the location of the edge of concrete and then a 75 plf load at the end of the overhang bracket; between those 2 loads there is only dead load[/li]
[li]A 50 psf floor live load to the location of the edge of concrete and then a 75 plf load at the end of the overhang bracket; between those 2 loads there is only dead load[/li]
[li]A 50 psf floor live load to the end of the overhang bracket and no other loading (while it has no end load, the extra 30 psf (50-20) over the entire arm length would seem to be similar to option 1 above[/li]
[/ol]

Which (if any) of the listed ones are the minimum to meet AASHTO for the overhang bracket. It is not that I am trying to design to the minimum but I like my formal calculations to show minimum code required loads. When a supplier prefers 50 psf, I would rather use their preference but note it is larger than code minimum.

 

[BridgeSmith]

Here at the DOT, we don't design the overhang brackets, but we design the girders for the torsional moment of 75plf at 1' past the edge of the deck.

The stay-in-place deck forms are designed for the 50psf construction live load.

According to the popular understanding of the bridge construction guide spec., there is supposed to be a 20 psf live load applied for 30' ahead and behind the screed machine during the deck placement. I've never applied it, since that would account for about 200 guys out there mucking around in the wet concrete, and I've never seen more than 5 in front of the screed, and nobody's walking around behind it.

Rod Smith, P.E., The artist formerly known as HotRod10