CHURCH ROOF STEEL STRUCTURE

More pictures, less words.
Below, spatial modelling using AutoCAD.


















AutoCAD 2006 in 3D. I used its 3D function to analyse a steel roof truss. It gets much easier to model using AutoCAD for linear spatial structures and transferring them to you analysis program of your choice.

Remember: That modeling needs not to be near perfect, but every inch perfect…that is if it is to be transferred to an analysis software. Perfectness is when all lines intersect exactly on the joints, not near intersection but exact intersection. If there’s a difference of less than a centimeter, the analysis software will call them two un-joined joints.


Then analysis using AVWin98. Figure below shows, truss system doing the calculation of moments after doing the complex modelling stance done in AutoCAD and transfering it to AVwin98 threw the import dxf file method, and defining each member(tricky one) together with the loads they carry(more tricky)

Thus some serious work was done here.
The elements had been defined and loads put in. There is a new version to this software, simply do not remember the name....probably something close to AVANSE or something. Will find out later.

The results to be presented at a future date. Most likely to forget.

NATHAN.
Kampala Uganda, East Africa
BSc. Civil Engineering (Makerere, Kampala)
MSc. Structural Engineering (Tongji, Shanghai)
Member_IABSE (International Association of Bridge and Structural Engineers)
Member_UIPE (Uganda Institution of Professional Engineers)
Also on the SEAINT list (Structural Engineers Association International_US)
Also on the CR4 list (Conference Room 4)

REINFORCEMENT BARS: TO BEND OR NOT TO BEND

We have to know that when bending occurs, the steel bar is strained within a certain section. The strain increases along the cross-section of the bar with the least strain closer to the direction
of bend and the maximum strain away from the direction of bend.

Some people have seen these strains to be about 0.02.

In bar bending schedules (BBS), this strain item is dealt with when dealing with bend. The bending in this case is for anchorage. In whichever case, I intuit, the strength in the area of bending may be reduced but not enough to be weaker than the bond between the concrete and steel bar surface. That may be the reason why bending in codes continue to be.

Caption from a dissertation chapter 4 "CONCRETE-STEEL BOND MODEL”

Well, in Uganda we mostly use twisted bars.
But there is another scenario at play: working with bars already embedded in concrete. What does this mean to the concrete already cast,
- will concrete cracks/spills be induced when the bars protruding out of the concrete are bent close to the concrete
- will concrete strength be affected by heat when heat, say heat from welding, onto the bars extends to the concrete sections nearby.
Does any of these loopholes affect the composite strength? It is indeed a real question to ponder on when considering that during design, so many factors of safeties are included in various stages of the calculations which I intuit should be enough to take care of these ‘minor’ holdbacks. But I still reserve the comment to certain concrete sections where there is a high degree of shear stress such areas as where concrete staircases begin/end, where concrete domes shoot off from a ring beam. But for areas such as starter bars, this really should be safe.

What do the others say:
One article (ref 1) states: that cold bending and straightening of rebar embedded in concrete does not weaken the bars if they are not bent through an angle greater than 90 degrees and the procedure is used only once. This was after conducting 56 tensile or stress tests, on rebar manufactured in New Zealand.

In ACI 318-77: 7.3.1: All reinforcement shall be bent cold, unless otherwise permitted by the Engineer.
In ACI 318-77 7.3.2 : Reinforcement partially embedded in concrete shall not be field bent, except as shown on the design drawing or permitted by the Engineer.
In ACI 318R-77, 7.3.2 "...The inspecting engineer must determine whether the bars can be bent cold without damage, or if heating is necessary. If heating is required it must be controlled to avoid splitting of the concrete or damage to the bars."

In an ACI Journal (ref 3): it recommends pre-heating rebar before gently and gradually bending it.
In a concrete international journal (ref 4): it was not necessary to use heat before bending the rebar. However, they only tested #8 (1" diameter) bars.

In a rebuttal to Ref 3 to Lalik and Cusick, again in a concrete international journal (ref 5): that it was hazardous to restraighten cold rebar.

Based on these and other studies, the Concrete Reinforcing Steel Institute concluded that: reworking bars entails some risk; #8 or smaller bars can be successfully field bent/straightened at temperatures above 32° F; rebars #9, #10 and #11 have a better chance of being successfully bent/straightened when preheated to 1400° or 1500° F and carefully manipulated; these conclusions do not apply to #14 or larger rebar.

After reviewing the three articles summarized here, as well as other studies, Monolithic's President David B. South said, "All of these studies show that care must be taken, but the smaller rebar sizes #3, #4, #5 and #6 usually can be bent cold as needed. However, care should be taken to make smooth bends, not kinks. Generally, the rebar is sufficiently forgiving -- especially #3 and #4. Larger bars (larger than #6) take special handling and may require heating."
So well, that does it.

REFERENCES:
Ref.[1]:If you want to know more, check the article "How Harmful is Cold Bending/Straightening of Reinforcing Bars?", by Authors José I. Restrepo, Senior Lecturer in Civil Engineering at the University of Canterbury, New Zealand and Member of the ACI (American Concrete Institute); Francisco J. Crisafulli, Senior Lecturer at the National University of Cuyo, Mendoza, Argentina and ACI Member; Robert Park, Professor of Civil Engineering, Deputy Vice Chancellor at the University of Canterbury, New Zealand and ACI Honorary Member.
Ref.[2]: ACI-318 is American Concrete Institute’s Building Code Requirements for Reinforced Concrete Structures
Ref.[3]: William C. Black in "Field Corrections to Partially Embedded Reinforcing Bars," ACI Journal, October 1973, pages 690-691
Ref.[4]: J.R. Lalik and R. L. Cusick in "Cold Straightening of Partially Embedded Reinforcing Bars," Concrete International, July 1979, pages 26-30
Ref [5]: L. A. Erasmus in "Cold Straightening of Partially Embedded Reinforcing Bars--A Different View," Concrete International, June 1981, pages 47-52.

INTUITIVE ABILITIES IN CONSTRUCTION

And while discussing with the foreman, it had been seen that there was a general lackof insight in doing finishes. It had been sited that most of the finishes done by this foreman was good but then he would spoil the work done in a particular sectionwith works unfinished in other sections. He knew it, I knew it. So then, it was notnecessary to tell the foreman what to do! All he could have done was to figure outways of protecting the earlier finished works from getting spoilt. Likewise the column pedestals had not been done at the same level by the previousforeman. The current foreman was to do a good finish on the pedestals taking intoaccount the levels but he did not do it. And so he had taken it for granted. Afterdoing the finishes did he realise that all the pedestals were not level. Then did I come in to tell him why he had not seen the problem, then I had to go through asmall lecture to teach him about intuitive abilities. I feel it's much my responsibility to teach these guys and not to simply assume thatthey know.Hey do you get the picture?

Nathan.

AVWIN98 & SAP SOFTWARE COMPARISONS

Quick answers took me to trying out a few design software we normally use in civil/structural engineering. Not that I can not use my head to do the designs but because the computer does it faster without losing accuracy if well commanded. So here I was trying to pick out the best software to use in my daily earning business. On the table was SAP2000 and AVwin98. Unfortunately, my STAAD Pro is expired and my other design software are just demos.
In the calibration, I tried out a simple beam of 300mm x 550mm to be reinforced in bending to support its own weight together with other dead loads of 40kN/m and a live load of 12kN/m. Concrete to be of class 30N/mm2 and Steel of 460N/mm2.
So, three options were tried out; (1) hand calculations, (ii) sap2000 nonlinear (iii) AVWin98
Aaand here we go:

Hand Calculations in steps:
Get ultimate design load basing on BS8110 = 75.2kN/m
Get ultimate design moment = 338kNm
Using formular such as K=M/bd2fcu and As=M/0.95fyz, the area of reinforcement = 1696mm2.

SAP2000 Nonlinear in steps:
Draw the frame or File->New model from template
Go to Define->Material and choose Concrete, then ensure correct properties are filled in.
Define->Frame sections
Select Member and go to Assign->Frame Static Load-> Point and Uniform
Go to Analyse->Set Options
Go to Options->Preferences and chose concrete tab.
Press F5 to run analysis
Press Ctrl+F5 to display the area of the reinforcement on screen. The value I got was 2324mm2

AVWin98 in steps:
Using on-screen menu,
Click Nodes tab and input structure node coordinates
Click Beams tab and join the nodes.
Click Nodes tab, then its 2nd button and select the nodes to have restraints
Click Beams tab and describe the beam
On beams tab, 2nd button to choose section properties
On beams tab 3rd button to chose which material properties, select Aconc
On beams tab, 8th button to input dead and live loads, then combination load, i.e 1.4dl+1.6ll for BS8110.
To analyse structure, click Calculation menu -> Analyse Structure.
Below were the results.
Moment = 338.23kNm
Click Print Menu->Reinforced Concrete Design, which gives Rebar area = 1770mm2 as shown below:

AVwin 98
------------------------------------------------------------------------------
AVWIN REINFORCED CONCRETE ELEMENTS DESIGN
Constructors format
------------------------------------------------------------------------------
******************************************************************************
Notes
------------------------------------------------------------------------------
Only graphically selected elements are printed
Deflection should be checked by user
V3, M22 and Axial actions are ignored in beam design
Reinforcement ratio producing balancing condition is ignored in columns.
Shear and torsion is ignored in column design
Rectangular diagram of concrete is used for beam and column design
Only rectangular beams are printed
Only rectangular columns are printed
Two axis bending moment in columns is solved amplifying larger bending moment
Recommendable only for rectangular columns with symmetric reinforcement
"Type": U= open stirrup []= closed stirrup >[]<= reinforcement in compression "Left" is J node of beam. "Right" is K node of beam "IP left" (inflection point) is the distance from J node to inflection point "IP right" (inflection point) is the distance from K node to inflection point Stirrups separation is given for ranges: 0%-25%, 25%-75%, and 75%-100% of beam length M min/max are maximum and minimum bending moments found for this beam "Skin" es required reinforcement to resist torsion. Half the area should lie at each face Columns are verified at 5 stations: 2 end stations and 3 middle stations Slendernes is ignored in the design of columns. Analyze with P-Delta to account for it. Design code: ACI-318 DESIGN STATUS ****************************************************************************** DESIGN LOADS.- LOAD 1 : c1=1.4dl+1.6LL ============================================================================================ B E A M S ============================================================================================ BEAM A.left A.cent A.right I.P.lef I.P.rig SKIN STIRRUP SEP. [cm] Mmin/max V[KN] Long Num [cm2] [cm2] [cm2] [M] [M] [cm2] LEFT CENT. RIGHT TYPE [KN*M] T[KN*M] [M] 1 ---------------------------------------------------------------------------------------------------------- RcBeamR 30x55x3.5 TOP: 0.0 0.0 0.0 0.0 0.0 0.0 6mm: 10.8 24.7 10.8 U 338.4 225.6 6 BOT: 0.0 17.7 0.0 0.0 0.0 8mm: 19.3 25.8 19.3 U 0.0 0.0 ============================================================================================ C O L U M N S ============================================================================================ COL Faxial M33 M22 Load A.calc A.min A.max Length TIE SEPARATION[cm] B x H Reduction Numb.[KN] [KN*M] [KN*M] id [cm2] [cm2] [cm2] [M] 6mm 8mm [cm] of inertia ============================================================================================

So well in brief the results were as follows:
6m simple beam design, 300mmX550mm,

fcu=30N/mm2, fy=460N/mm2

dl=40kN/m, ll=12kN/m
Code used
Moments, kNm
Below: Area of reinforcement, mm2
Hand Calculations [BS8110 ; 338 kNm ; 1696mm2]

SAP2000 Nonlinear [BS8110 ; 362.9 kNm ; 2324mm2]

AVWin98 [ACI-38 ; 338.23 kNm ;1770mm2]

Conclusion: SAP2000 Nonlinear results were off by 27%(less accurate), AVWin98 was off by 4%(more accurate)
But then again, it would not cause the structure to fail because the factor of safety is actually much higher but less economical
Which would be the only reason why the Software designers would have to be more exact.
Next, lets wait and see what the SAP software designers say of this. They can be contacted at http://www.csiberkeley.com/ / info@csi....


Nathan.

MORE UPDATES

update 1

CASTING THE GROUND SLAB [being edited]

So how is it done?

Materials: You need a well compacted base, hardcore, cement, aggregates and lake sand together with Mesh reinforcement popularly called BRC.

Mixing: You need cement, lake sand and aggregates.

Application: [coming soon]

Casting the ground slab. And everything now seems to be on course. The speed is cool...okay slow to the others but reasonable basing on the circumstances(no storage space yet)...hence whatever we bought had to be used up that very day.

Today we got the maxspans for the slab over the gulley. I changed the design a bit and now it is possible to use 5"maxspans and not 7"maxspans as long as that place will forever be used for toilet, bathroom, washroom, e.t.c

Today's work involved;

-timber shuttering for slab over the gulley,

- cutting, bending and fixing the bars

- putting in place the maxspans,

- adding on BRC.

I am out of money. Only remaining with 1m for the y25 bars.

More updates later on first floor works. Need to prepare for timber posts, more timber, more reinforcement, maxspans. This floor is going to be lightening speedy.

The building will be the best!

ciao!

Nathan.

INGENUITY, PART 1: [STRAP FOOTINGS]

INGENUITY, PART 1: [STRAP FOOTINGS]

Ingenuity was called into question when I faced a challenging task that I usually dont meet.The column to be built was too close to the boundary. Ofcourse my worry was its foundation which was to project into the foundation of boundary wall. I instructed the contractor to do that but later we found out that the boundary wall would fail and believe me, I was not interested in extra costs or neighbour interference. I later found out that the contractor had gone a step ahead to preserve the dimensions of the footing by re-positioning it so that it just ends at boundary wall footing. But he had not changed the column position and I could not have allowed it anyway because of it was locked in grid with other columns. And so that column was now closer to the footing edge and no longer in its middle.
I had not faced such a situation before. I had to delve into first-time principles. Creative engineers have used intuition when faced with new challenges, I thus followed suit:
- One was to internalise the problem
- Two was to internalise the solution.
- Three was to work out the calculations
- Four was to specify the materials to the contractor.

INTERNALISING THE PROBLEM: [MOVIE-1]
In my mind's vision, I saw a load path from the column and into the footing, creating an eccentric effect that led to overturning gestures thus creating weak points. It was like a movie playing in my mind. I had to concentrate my thought energies to keep the movie on. Next, I played and replayed movie clips in the internal vision taking into considerationthe nature of the surroundings close to column and its footing. In the replays, I saw two possible scenarios:

1. a hinge was created at the column-footing intersection and a crack in the floor next to the column ensued as a result. I saw why this had happened.
and/or
2. hogging moments created in the footing next to the column which would come first before any possible failure at the column-footing intersection.

This internalisation took about a minute. I was quite for that period while the contractor was there wondering what I am up to.
And so the weak points had been located. Solutions had to come in next.

INTERNALISING THE SOLUTION: [MOVIE -2]
In my mind's vision, I dared not view the load paths for all the 16 columns. It would get complex and the movie would get distorted. I concentrated on two columns. I thought of a base beam(or reinforced strip footing) which connected the eccentric column to the other column and made it to sit on top of the footings.

Now I replayed movie-1 in my previous internalisation. This time I saw the introduced beam at work, seriously reducing on the overturning effect. The more I made the beam rigid, the more I saw the residue load from the eccentric effect being transfered into the next column footing.

The problem had been solved. The possibility of a plastic hinge at the eccentric column-footing intersection was no more and the hogging moments in the footing was no more and likewise the floor was safe.

Now all the contractor needed was for me to specify the reinforcement and concrete grade for the new base beam. I had to do a bit more calculations for this.

To my surprise when I went into my reference books, in the foundations section, there I saw it; in the book, this kind of arrangement was called strap footings. In otherwords my challenge was not new, many others had faced this same problem and my work had been made even more easier at calculations. Whoa, talk about ingenuity. Engineers are never taught that in school, are they? If is the foundation for design and construction, isnt it?

ciao!

Nathan.
E.Africa(uganda).
MSCE, BSCE.
M_UIPE, M_IABSE.