Design of One way Slab

Once thickness is settled. Its time to design slabs using spreadsheet. Lets focus on one way slab..

observe that all colored values except red are linked from the preceding tab, the design criteria tab and slab thickness determination tab. This is the beauty of the spreadsheet. You can linked values in the entire spreadsheet to assure uniformity. 

Loads from design criteria and design constants are displayed to be consistent with what is shown in design criteria. Input rebar diameter to be used which is normally 10mm and the spreadsheet will show required spacing.

Observed that governing spacing is 300mm because the load for residential is lighter compared to buildings. 

Slab Thickness Determination

After you input values in the design criteria tab. You will need to determine slab thickness using a separate tab in spreadsheet. A structural framing plan is required here. You will notice the rebar strength values from design criteria tab is linked here. Assume a beam width which is conservative to determine thickness. Remember that slab takes one of the most volume of concrete in building. Thus, it needs to be determine early on because having a large depth will cost a lot. Observe that just input type of span and a clear span will be determined and required thickness for one way slab. 

 

For two way slab, input long and short direction span then required thickness will be determined.

The Importance of Design Criteria

Design Criteria tab of the spreadsheet shows the parameters used in the whole design of the structure. What are these parameters? These are the following:

1. Reference Code
• In the Philippines, its NSCP which is the reference code but this is actually referenced from UBC 1997 from the USA. Modifications of UBC to adapt to local conditions is made however.
2. Loads
• Dead Loads
• These are the permanent loads in the structure. Values in red are input values. That means these will be referenced by other design sheets of the spreadsheet to guarantee uniformity of values.
• Live Loads
• For residential building, a typical floor has 1.90 kpa of live load.
• Seismic Load
• Engineer will need to refer to NSCP Chapter 2 to determine seismic parameters.   
• For seismic parameters, just input the distance of the site from the nearest seismic fault line, importance factor, ductility coefficient and soil profile, then the spreadsheet will come up with proper seismic values.
• 
• Wind Load
• For wind load, you need to choose the zone factor of the site location and corresponding wind velocity also the roughness exposure of the site.
3. Materials 
• Concrete Strength
• The red input for concrete strength will be linked to other design tabs of the spreadsheet like the slab design, stair and beam design. 
• Rebar strength
• 
  
• Structural Steel strength
4. Soil Data
• 
• Soil data will be linked to footing design tab. 
5. Design Constants
• Since you have inputted material values above, design constants will be automatically computed by the spreadsheet. These are important in design procedures of structural elements. Because you will need to determine the max and minimum reinforcement ratio, from these parameters.

Sample Structural Design Report

Note:Image by EMatias

Structural engineers designing residential buildings will need to compile their calculation to satisfy their client requirements. Thus, submission of Design Report is a must. This is a document showing that you followed the appropriate codes used in the country or state where you are designing.

In the USA, some of the governing codes are the following:
1. ASCE 7 - This code covers the provisions in seismic,wind and minimum loading condition and combintation to be used.

2. ACI - American Concrete Institute is the body publishing the concrete provision referred in IBC.

3. AISC - American Institute of Steel Construction.

In designing, an engineer may use one software alone to design the structure. However, some structural engineers still preferred to use both analysis software and spreadsheets. Design criteria is the most important sheet because parameters like rebar strength, concrete strength as shown below must be linked to all excel sheets containing the beam design, column and footing design.

Softwares tend to produce overdesign result or different sets of rebars in members in the same floor. Most expert or old engineers will obtain the maximum governing design forces in a certain set of beams say all seismic carrying members then design them in spreadsheet and implement result to all members under that set. Example, values in green below image i.e. rebar fy and fc',  is linked to all values under Design Criteria tab in spreadsheet.

Same manners of design in column and footing is followed. Lets say in all corner column member, we get the governing forces and design then apply rebar design to all corner columns.

For footing say in all corner footing, we get all governing design forces in all corner footings and design that then apply the rebars obtained to all corner footings. See below image.

The footing spreadsheet should have inputs for both unfactored and factored loads to check the footing size and to design for rebars.

Lastly, even the design of other members like stair can be done in spreadsheet. Again, values in red is linked to design criteria page in spreadsheet to retain uniformity in fy and fc values in all structural members.

Slab takes the major volume of concrete in structure. Hence, a spreadsheet for this member is really important to check if certain thickness at certain span is really capable before modeling in software like STAAD.

Buy Now to get this sample design report with architectural drawing..

Sample Structural Design Repor