×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Concentrated load on structural slab

Concentrated load on structural slab

Concentrated load on structural slab

(OP)
Can anyone give me the ACI Code requirement for the maximum width of one way slab which can be considered effective in resisting a concentrated load?  Thanks.

RE: Concentrated load on structural slab

I don't believe that the ACI 318 ever directly addresses this.  They do deal with concentrated loads on top of walls (Chapter 14) but for slabs - no.

RE: Concentrated load on structural slab

ACI does not give any guidelines for this, but lucky for us Westegaard solved this problem back in the 30's when wheel loads on bridge slabs started becoming large.

Westergaard, a professor at the University of Illinois, issued a comprehensive report in March of 1930 to the Bureau of Public Roads "Computation of Stresses in Bridge Slabs due to Wheel Loads".  I believe Westergaard's report is the basis for the modern day AASTHO requirements. The AASTHO spec., unfortunately, has taken Westergaard's recommendations and converted them into engineering data that works only for truck loads and bridges. Building engineers must work with varying wheel bases, axle lengths and tire contact areas. You must go back to Westegaard's original report to find useful information.  You should be able to order this report thru your Public Library.

This report has charts that give coefficients both for principal and distribution reinforcements for one way slabs subjected to concentrated loads. It has charts to handle overlapping stresses, when wheel loads are parallel or perpendicular to the span. It even takes into account the contact area of the concentrated loads.

I hope this helps!
 

RE: Concentrated load on structural slab

A rough and dirty way of approaching the problem....

Sometimes what I do is find out the actual bearing area of the concentrated load (say it is distributed uniformly over a 2ft by 2ft area).  Treat the width of the loaded area as the width of a "beam" element within the slab (but the "beam" depth is the same thickness as the slab of course!)  

Then you calculate the effective overhanging flange width on either side of the width of the loaded area.

The total effective width in each direction is the width of the loaded area plus the width of the effective slab overhang.  This is what I use for the effective resisting width of point loads.

RE: Concentrated load on structural slab

Please note that Westegaard's method is only valid for solid, monolithic, poured in place, formed, concrete slabs. It does not apply to precast plank, poured in place slabs over composite metal deck, grating, floor plate, one way concrete joists, etc.  

The amount of reinforcement, placed at right angles to the main reinforcement (often called distribution steel), is very important and is generally more than the normal requirement for temperature and shrinkage steel. This steel is what helps the concentrated load(s) spread out over the effective width.

RE: Concentrated load on structural slab

P.S.
Sorry, but the MathCad file is for a slab on grade.

RE: Concentrated load on structural slab

Australian Standard AS 3600–2001 Concrete structures (Clause 9.6) calculates the moment resisting width for a one-way slab supporting concentrated loads thus:

   b_ef = b_load + 2.4*a*(1.0 – a/L_n)
      
   where:
   -  ‘b_ef’ is the effective width
   -  ‘b_load’ is the load width
   -  ‘a’ is the perpendicular distance from the nearer support to the section under consideration
   -  ‘L_n’ is the length of clear span

As ‘b_load’ approaches 0 and ‘a’ approaches L_n/2, ‘b_ef’ approaches 60% of L_n.

RE: Concentrated load on structural slab

I think for One Way Slab you should calculated and desinged as beam. But you must check that a slab can be resist a puching shear at concentrate load (It very serious for this case).

RE: Concentrated load on structural slab

Try my quick homeopathic formula. First, "ASSUME" a slab thickness that works for all the typical ACI Code criteria using a 12 inch trial width. Then, to be conservative I would use the width of the loaded area plus the 45 degree projection down and out from both sides of the actual loaded area and compare my results. Good Luck.

RE: Concentrated load on structural slab

The Australian code corresponds well with Westegaard's report. Does it also give a way to calculate the additional stresses due to multiple loads with overlapping stresses? Does it also give a way to calculate distribution steel?

RE: Concentrated load on structural slab

I have another formula for the case where the load is at the center of the span

EffWidth=
actual width of load
+0.6·(span - dimension of load parallel to span)

RE: Concentrated load on structural slab

AS 3600 doesn’t specifically address overlapping loads.  I guess it’s up to designers to calculate the unit moment (kNm/m) for each concentrated load and sum them where the effective widths overlap.

The design of flexural reinforcement is based on a rectangular stress block with a uniform compressive stress of 0.85*f_c (where f_c is the characteristic compressive cylinder strength of concrete at 28 days) and a maximum strain in the extreme compressive fibre of 0.003.

Cross-sectional steel area is calculated from the following equation for the moment capacity (kNm/m):

phi*m_u = (phi*f_c*q*(1 – q/1.7)*d^2)/1000 [kNm/m]

where:
- phi: strength reduction factor, equal to 0.8 for slabs [dimensionless]
- m_u: ultimate moment capacity [kNm/m]
- f_c: characteristic compressive cylinder strength of concrete at 28 days [MPa]
- q = (A_st*f_sy)/(1000*d*f_c)
  where:
   - A_st: cross-sectional area of tension reinforcement [mm^2]
   - f_sy: yield strength of reinforcing steel [MPa]
- d: effective depth of a cross-section, i.e. the distance fro extreme compressive fibre to the resultant tensile force in the reinforcing steel [mm]

RE: Concentrated load on structural slab

Based upon an effective width of .6 L, for a single concentrated load, the previously suggested 45 degree distribution may be unconservative. A 30 degree distribution away from the concentrated load in each direction corresponds more closely with the .6 L effective width.

RE: Concentrated load on structural slab

James G MacGregor's book "Reinforced Concrete" contains a section on Yield Line Theory for slabs.  One part of the section addresses analysis and design for concentrated loads.  I have used it from time to time for heavy concentrated loads in industrial facilities.

Good luck.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources