Foundation Design For A Ware House (Steel Structure)

[zaib]

I have to design the foundation of a warehouse(Steel structure) on prokon. Reactions at nodes for different loads combination are available. soil bearing capacity is 180KN/m^2. Can anyone guide me how to design footing at one node. Reactions given are at base plate. I am attaching autocad drawings containing structure layout as well as reactions.PROPOSED WARE HOUSE GA DRGS 7-7-2014.dwg

[WR1]

Well Zaib it depends!

 

If you have assumed pin base then it means moment at base = 0 and you have only the vertical load reaction. If this is the case then the footing area required Areq = P/180 where P is kN and Areq is in m².

 

Then you can convert this area into a square footing or a rectangular footing as you need.

 

Or if you have moments things get more complicated so you can either do it manually or choose any software; like SAFE, PROKON or MIDAS.

 

I am not aware of PROKON, so I would tell you what to do in SAFE.

 

Start a new model and make isolated foundations (preliminary sizing) and put a node in the center of footing and put the vertical reaction on this node. Run the analysis check the bearing pressure and see if the sizing is ok. If not then change footing sizes and re-run.

 

After it design it on SAFE for reinforcements.

[zaib]

Steel structure is designed by an other consultant. i just got the reactions for footing design. From the reactions, it seems that support conditions are considered pin as Moments are zero in reactions. But there are three reactions along x,y and z directions respectively. Reaction in z direction is only due to wind load combination. I am unable to understand which reaction will be considered for footing design. If i considered the vertical component that is reaction in y-direction then what about the other reactions. Please review the reactions written on attach file and guide me. I am mentioning the reaction for one node for your consideration. at this node reactions are in x and y direction only.

Load Combination                                                       Fx(KN)            Fy(KN)             FZ(KN)

Dead                                                                           16                       45                   0

Live                                                                             31                         58                 0

Wind left(angle=0) with internal pressure                    -61                      -101                0

Wind left(angle=0) with internal suction                     -52                      -61                   0

Wind end(angle=90) with internal pressure                   -26                     -80                 0

Wind end(angle=90)with internal suction                   -17                       -40                  0

 

Base plate size is 230mm x 550 mm

[WR1]

From the table it sounds like this

 

X= x direction

Y= vertical direction

Z = y direction

 

that makes sense for wind loads as the vertical reactions (uplift due to wind) is -101, -61, -80 and -40 (units?)

 

and at the same time wind loads in horizontal x direction are: -61, -52, 26 and -17

 

but what doesnt make sense is why there are no reactions in y direction which is Z in your case??? there is no wind loading applied in that direction?

 

----------------

The next step is to combine these loads according to chapter 9 load combinations of ACI

 

remember here we will make both service combinations for bearing pressure check and ultimate combinations for reinforcement design of footings

 

then combine all these loads and check bearing pressure and reinforcement design by hand or by any software

[zaib]

In the reaction sheet provided to me Fx and Fz both are written as Horizontal and Fy as vertical

Reactions are in KN

The reactions i have mentioned are at a node of a frame whose vertical members are not braced with inclined members.

There are frames whose members are  braced with inclined members, reactions at the nodes of these frames have Fz components of magnitude 6-12KN. The values of Fz are only for the Wind load left(angle=0) with internal pressure.

 

I have the load combinations but before proceeding for design i must understands about these reactions so that i can put them rightly in the software.

For design of footing by prokon, i have to put the reaction in the following

 

P

Hx

Hy

Mx

My

 

I am confused about either vertical reaction Fy will be placed at the place of P or Hy?

Where i have to put the value of Fz?

[WR1]

You should check sign convention used in Prokon!

 

and sign convention of your system

 

-------

From the above discussion it looks like that in prokon

 

P = Fy (check + or - sign convention in Prokon)

 

Hx = Fx

 

Hy = Fz

 

Mx = 0

 

My = 0

 

Mz = 0

[zaib]

I am unable to check sign conventions used in Prokon. However i have designed the footing by taking Fy=P and Fx=Hx,

I have used 13 load cases for design of footing.

Reactions and load cases are as under

Load combinations Reactions Moments

Fx(Hz) Vt(Fy) Hz(Fz) Mx My Mz

KN KN KN KN-m KN-m KN-m

Dead Load 15.465 45.012 0 0 0 0

Live Load 30.93 57.864 0 0 0 0

Wind left (angle=0) with internal pressure -61.315 -100.846 0 0 0 0

Wind left (angle=0) with internal suction -51.87 -61.126 0 0 0 0

Wind end (angle=90) with internal pressure -25.917 -79.44 0 0 0 0

Wind end (angle=90) with internal suction -16.471 -39.72 0 0 0 0

Load cases P Hx

D 45 15

L 58 31

1 D+L 103 46

2 1.2D+L+1.6W1 -49 -49

3 1.2D+L+1.6W2 14 -34

4 1.2D+L+1.6W3 -15 8

5 1.2D+L+1.6W4 48 -14

6 1.2D+1.6L+0.8W1 66 19

7 1.2D+1.6L+0.8W2 98 27

8 1.2D+1.6L+0.8W3 83 47

9 1.2D+1.6L+0.8W4 115 55

10 0.9D+1.6W1 -121 -84

11 0.9D+1.6W2 -57 -69

12 0.9D+1.6W3 -87 -28

13 0.9D+1.6W4 -23 -12

Load case 10 seems to be most critical

I am attaching the design of footing considering the above mentioned data. Please review and advice

Design of footing at node-1.pdf

[WR1]

Well you must use service combinations for bearing pressure of foundations!

 

13 load combinations you mentioned are ultimate combinations for design of reinforcement!

 

You should really check with seniors as these are basic things which you must learn asap!

[zaib]

The first combination is D+L. Is it not the service load Combination?

My senior most of the time remains on site. He just asked me to design footing on Prokon and get help from net.

[WR1]

This is very weird way of seniors to teach their juniors. Well D+L is a service combo but not the only combinations. There a lot of other combinations like 1.0D+0.6W etc. Goto ASCE 2005 pdf page 34, chapter 2 for example.

 

You must first read basics before using prokon and other softwares; for example read Nilson's 14th edition book on Reinforced Concrete Design. Chapter 17 i think on foundations.

 

 

I am very upset to say that if that is the behaviour of your seniors in office, tomorrow they gonna say

 

"well you have done so much your self so you are now an expert. Come on, take this building and design everything because you did it before. Right?"

 

Talk to your seniors boss and dont worry, otherwise QUIT THIS JOB! A sincere but strange suggestion from my side!

[zaib]

Ok i will study the referred Chapters.

I have already discuss the matter with boss. He said that you will learn by consulting books and internet help. He said no one have time to teach the Juniors, you have to learn yourself and little bit guidance of seniors.

I have not designed this building(steel structure). Have to design foundation.

I will left this job when in will get any good opportunity. Your suggestion not seems strange to me.

[WR1]

thats good! read good books and use internet forums!