Loading on a Garage Roof Located Under a Street

[ron9876]

I have a condition where a portion of a garage extends under a street. There is about 4' of soil between the street and the top of the roof slab.

I need to be sure that I meet the requirements for AASHTO loading. What manual do I need?

 

[msquared48]

I would first recommend a geotech.

The normal 250 psf vehicle loading may be able to be reduced due to the 4 feet of overburden.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[JAE]

Uh, you need the AASHTO manual?

 

[apsix]

Except there is no single 'the AASHTO manual'.

 

[COEngineeer]

ask the building department. Either HS20 loading (LFD) or truck loading (LRFD). The 4 ft of soil will help you reduce the LL impact.

Never, but never question engineer's judgment

 

[COEngineeer]

And the wheel load will be distributed to a bigger area.

Never, but never question engineer's judgment

 

[miecz]

Contact your state DOT and get the State Bridge Manual. Find out which AASHTO Code the State Bridge Manual Requires. It's either the Standard Specification (17 Ed.) or the AASHTO LRFD Bridge Design Specification, 4th Edition. Find the Section for Culverts (Standard Spec) or Buried Structures (LRFD Spec).

 

[ron9876]

It looks like the AASHTO manual costs around $350. Is that right? Can I order the section for buried structures only?

 

[JedClampett]

I don't see anything wrong with using the loadings defined in ASTM C857, "Minimum Structural Design Loading for Underground Precast Concrete Utility Structures" if you can get your hands on a copy.

 

[wiktor]

The portion of the garage extending under the street is legally a bridge, with all implications for the design standards and requirements. The design shall be according to your state DOT and AASHTO. The structure will be subject to biennial bridge inspections in the future.
As far 250 PSF mentioned somewhere above - it does not apply. Follow AASHTO "Distribution of wheel loads through earth fills" guide.

 

[ron9876]

Is there a manual for culvert design only or something to that effect so I can get the price down?

This will be more like a privately owned bridge but it will still need to meet FDOT requierments. I have some bridge design experience from years ago so I understand the basics but
I will certainly need a code to follow.

 

[FFENGINEER]

what is your span? is the slab simply supported?

 

[ron9876]

It is a standard garage layout--3 span--14'-32'-14' in the short direction and about 28' spans in the long direction. It will be a post tensioned slab on columns.

 

[bridgebuster]

Attached is the CalTrans culvert design manual, which illustrates how to apply wheel loads.

AASHTO Standard Specs:
6.4 DISTRIBUTION OF WHEEL LOADS THROUGH EARTH FILLS
6.4.1 When the depth of fill is 2 feet or more, concentrated
loads shall be considered as uniformly distributed over a square with sides equal to 1-314 times the depth of
fill.

6.4.2 When such areas from several concentrations overlap, the total load shall be uniformly distributed over the
area defined by the outside limits of the individual areas,
but the total width of distribution shall not exceed the total width of the supporting slab. For single spans, the effect of live load may be neglected when the depth of fill is more than 8 feet and exceeds the span length; for multiple spans it may be neglected when the depth of fill exceeds the distance between faces of end supports or abutments. When the depth of fill is less than 2 feet the wheel load shall be distributed as in slabs with concentrated loads. When thecalculated live load and impact moment in concrete slabs,based on the distribution of the wheel load through earthfills, exceeds the live load and impact moment calculated according to Article 3.24, the latter moment shall be used.

The AASHTO HS-20 truck is three axles spaced at 14' - 4T-16T-16T; the wheels are 6' apart. I'll post the eauations from 3.24 shortly.

 

[bridgebuster]

3.24 DISTRIBUTION OF LOADS AND DESIGN OF CONCRETE SLABS
3.24.1 Span Lengths (See Article 8.8)

3.24.1.1 For simple spans the span length shall be the distance center to center of supports but need not exceed clear span plus thickness of slab.

3.24.1.2 The following effective span lengths shall be used in calculating the distribution of loads and bending moments for slabs continuous over more than two supports:
(a) Slabs monolithic with beams or slabs monolithic with walls without haunches and rigid top flange prestressed beams with top flange width to minimum thickness ratio less than 4.0. "S" shall be the clear span.

(b) Slabs supported on steel stringers, or slabs supported on thin top flange prestressed beams with top flange width to minimum thickness ratio equal to or greater than 4.0. "S" shall be the distance between edges of top flange plus one-half of stringer top flange width.

(c) Slabs supported on timber stringers. S shall be the
clear span plus one-half thickness of stringer.

3.24.2 Edge Distance of Wheel Loads
3.24.2.1 In designing slabs, the center line of the
wheel load shall be 1 foot from the face of the curb. If
curbs or sidewalks are not used, the wheel load shall be 1
foot from the face of the rail.

3.24.2.2 In designing sidewalks, slabs and supporting
members, a wheel load located on the sidewalk shall
be 1 foot from the face of the rail. In service load design,
the combined dead, live, and impact stresses for this loading shall be not greater than 150 percent of the allowable stresses. In load factor design, 1.0 may be used as the beta factor in place of 1.67 for the design of deck slabs. Wheel loads shall not be applied on sidewalks protected by a traffic barrier.

3.24.3 Bending Moment
The bending moment per foot width of slab shall be
calculated according to methods given under Cases A and
B, unless more exact methods are used considering tire
contact area. The tire contact area needed for exact methods is given in Article 3.30.

In Cases A and B:
S = effective span length, in feet, as defined under
"Span Lengths" Articles 3.24. I and 8.8;
E = width of slab in feet over which a wheel load is
distributed;
P = load on one rear wheel of truck
P = 12,000 pounds for H 1 5 loading;
P = 16,000 pounds for H 20 loading.

3.24.3.1 Case A-Main Reinforcement
Perpendicular to Traffic (Spans 2 to 24 Feet Inclusive)
The live load moment for simple spans shall be determined
by the following formulas (impact not included):

HS 20 Loading:
(S+2)/32*P = Moment in Foot - pounds per foot - width of slab
HS 15 Loading:
(S+2)/32*P, = Moment in foot - pounds per foot - width of slab
In slabs continuous over three or more supports, a continuity factor of 0.8 shall be applied to the above formulas for both positive and negative moment.

3.24.3.2 Case B-Main Reinforcement Parallel
to Traffic
For wheel loads, the distribution width, E, shall be
(4 + 0.06s) but shall not exceed 7.0 feet. Lane loads are
distributed over a width of 2E. Longitudinally reinforced
slabs shall be designed for the appropriate HS loading.
For simple spans, the maximum live load moment per
foot width of slab, without impact, is closely approximated
by the following formulas:

HS 20 Loading:
Spans up to and including 50 feet: LLM = 900S foot-pounds
Spans 50 feet to I00 feet: LLM = 1,000*( 1.30S-20.0) foot-pounds

HS 15 Loading:
Use 3/4 of the values obtained from the formulas for
HS 20 loading
Moments in continuous spans shall be determined by
suitable analysis using the truck or appropriate lane loading

 

[ron9876]

bridgebuster thank you fpr your help.

 

[bridgebuster]

no problem